1. A GOLDEN OPPORTUNITY straight through, and then … · Read the passage below on A GOLDEN...

moulsham high school 1

1. Read the passage below on A GOLDEN OPPORTUNITY straight through, and then more carefully, in order to answer the following questions.

A GOLDEN OPPORTUNITY

“THERE’S GOLD IN THEM THAR HILLS!”

And plenty of volunteers to help dig for it, it seems. Few people strike it lucky, however; more fortunes were lost than were ever won in the famous 19th century gold rushes of California, Ballarat and the Klondike.

Gold mining today may not be quite so frenetic, but it is still a risky business. Even after finding reliable deposits, extracting the gold from them is no mean feat. Between 15 and 30 per cent of the world’s gold reserves occur as refractory minerals – microscopic particles of gold encapsulated in a mineral matrix; well known examples include arsenopyrite (FeAsS),iron pyrites (FeS2) and chalcopyrite (FeCuS2).

Gold is usually obtained by crushing and grinding ore from the mine and separating these refractory minerals from the oxide ore and other non–metallic minerals by froth flotation to produce a sulphide concentrate. This is then roasted to liberate the gold, which is extracted by treating the resulting mixture with an aerated solution of sodium cyanide. The process is not without problems; roasting converts any sulphur in the refractory minerals to sulphur dioxide and any arsenic to arsenic(III) oxide, both of which have undesirable environmental and economic implications. In some cases, roasting traps the gold in fused silicate minerals and fails to liberate all the metal. Cyanidation is also difficult. The mineral matrix acts as an impervious physical barrier and shields most of the gold particles from attack by cyanide ions. Despite being used for over 100 years, cyanidation of refractory ores yields only a fraction of the contained gold.

And so it might have continued, if it had not been for a chance coincidence that resulted as part of the reorganisation of the University of London. At the time, a team of Chelsea undergraduates was carrying out a final–year project on the mineralogy of a refractory sulphide concentrate produced from a gold deposit at the Clogau St David’s mine in North Wales. The group quickly hit upon a problem. Since roasting the concentrate to liberate the gold was not permitted on the National Park property where the mine was located, they were only able to extract 10 per cent of the gold by conventional cyanidation. As an alternative to roasting, the undergraduates investigated various ways of solubilising the gold in the concentrate by using acidic solutions of thiourea with different oxidants. They showed that gold could be solubilised, but not to an extent that would be economic.

Instead, the answer to the problem arose from existing interests at Queen Elizabeth College on metal–microbe interactions. For the Chelsea researchers, this work suggested a possible solution and the two groups soon combined forces. The researchers treated the concentrate with the thermophilic (heat–loving) bacterium Sulpholobus acidocalderius.

This is a bacterium which catalyses two processes. It helps atmospheric oxygen to oxidize sulphide minerals and it helps to make the products of oxidation water soluble. Fresh water contains sufficient dissolved oxygen to carry out the oxidation. The bacteria, concentrate and fresh water were mixed together in stirred tanks. When the oxidation process was complete, the pH of the mixture was adjusted by the addition of lime. Sodium cyanide solution was now added. Gold was then extracted from the resulting solution by reaction with zinc shavings. The extract increased the gold recovery from 10 to 100 per cent. BacTech have patented the process and have been using it at the Youanmi Mine 500 km north–east of Perth in Western Australia.


The recovery of base metals during gold extraction is also high. BacTech researchers have

recovered between 95 to 99 per cent Cu and Ni, 89 to 99 per cent Co and 91 to 96 per cent Zn, all of which contribute to the profitability of plant operations. In fact, recent work has shown that bacterial recovery of the base metals alone, whether or not gold is present in the concentrate, is feasible and economically competitive with conventional processes.

BacTech has recently managed to raise more funds, which will help to accelerate the development of its technology and establish its presence in the US. The company has also joined forces with the South African company Mintek to pool and jointly market bacterial oxidation technology. Soon, BacTech will be floated on the Toronto stock exchange under the name BacTech Mineralogical Solutions.

(711 words)

Adapted from A Golden Opportunity by Jack Barrett and Martin Hughes, published in Chemistry in Britain Vol. 33, Number 6 June 1997, and the Royal Society of Chemistry.

(a) How does gold occur in refractory minerals? (1)

(b) Why is cyanidation difficult in the traditional process for extracting gold? (1)

(c) State TWO reasons why roasting concentrate is not permitted on National Park property. (2)

(d) Why is fresh water needed for the BacTech process for gold extraction? (1)

(e) What other factor contributes to the profitability of the BacTech operation, apart from the extraction of gold?

(1)

(f) Write a summary in continuous prose, in no more than 100 words, describing the BacTech process of extracting gold from its ore.

(9)

You are not asked to summarise the whole passage, nor to include equations in your summary. At the end of your summary state the number of words you have used.

Credit will be given for answers written in good English, using complete sentences and with correct use of technical words. Avoid copying long sections from the original text. Numbers count as one word, as do standard abbreviations and hyphenated words. Any title you give to your passage does not count in your word total.

There are penalties for the use of words in excess of 100. (Total 15 marks)


2. Read the passage below on COFFEE AND CHEMISTRY straight through, and then more

carefully, in order to answer the following questions.

COFFEE AND CHEMISTRY

In Britain people drink an average of 1.7 cups of coffee every day, yet few of us know what they contain or the interesting chemistry involved. This passage describes what happens when coffee beans are roasted, and during the actual process of brewing coffee.

How would you like to have only beer to drink at breakfast? Until some 300 years ago, people had little choice but to drink beer and cider throughout the day since water was of poor quality and milk (unpasteurised) went off. The arrival in Britain of tea, coffee and cocoa around 1650 thus gradually transformed people's lives. Nowadays over half the world's population regularly drink tea and one third of them drink coffee. The great popularity of these beverages depends on two opposing characteristics comfort and stimulation. They are comforting in providing a pleasant–tasting and warming drink (almost a minor food when milk and sugar are added). They are stimulating because they contain small amounts of the drug caffeine.

O

NN

NNH C

O

CH

CH

3

3

3

Caffeine

Tea leaves contain about 4% caffeine, coffee beans from I to 2% depending on the type of bean. However, the situation is reversed when the drink has been brewed. A typical cup of tea contains only around 40 mg caffeine compared with about 60 mg in a cup of instant coffee and more like 85 mg in one made from ground roast coffee. Thus you are more likely to be kept awake by an evening drink of a cup of coffee than by a cup of tea. Too much caffeine (like too much of almost anything) is bad for you and a few people are very sensitive even to small amounts. Athletes, in particular, need to be very careful since it is a restricted drug.

Anyone who wants to enjoy one of these beverages in a caffeine–free form can buy decaffeinated coffee and, in recent years, also decaffeinated tea. The most satisfactory way of selectively removing the caffeine is by the use of supercritical carbon dioxide.

Coffee

The coffee bush produces coloured berries or cherries which each contain two large grey–green seeds or beans.

These are removed and roasted by passing hot gases at around 400 ºC through a bed of tumbling beans. Roasting is a major chemical process.


Sucrose, about 7% by mass of the bean, decomposes to carbon dioxide and a

series of organic acids. These organic acids cause the acidity of the beans to rise during the initial period of roasting. On further heating the acidity falls as aliphatic acids of low molecular mass, like ethanoic acid, evaporate. The carbon dioxide formed by sugar decomposition causes the beans to swell to about twice their original volume. This carbon dioxide is then lost, as are water and other volatiles in the latter stages of heating. A little caffeine volatilises during roasting, but because of the other mass losses, its concentration actually increases slightly. Roasting causes the colour of the beans to darken, especially when the temperature rises above 200 ºC and pyrolysis (the breakdown of chemicals due to heating) starts.

The delicious aroma of roast coffee beans has been found to contain over 800 different compounds! This has been shown by combining gas chromatography (which separates the different volatile compounds) and mass spectrometry (which is used to identify them).

Some of these volatiles, and much larger amounts of non–volatile compounds such as caffeine, are extracted into the coffee brew. This includes minerals, a range of carbohydrates and proteins, and compounds formed by Maillard reactions (named after their French discoverer) between carbohydrates and proteins during the roasting process. Called melanoidins, these compounds are largely responsible for the unique flavour of a cup of coffee.

The infusion process–how fast?

When ground coffee beans or tea leaves are immersed in hot water, the concentration of caffeine (or any other soluble constituent) increases with time. The concentration, c, rises rapidly at the beginning but then increases more and more slowly until it reaches a constant or plateau value c.. By plotting a graph of (C, – C) against time it is possible to show that infusion is a first order process with rate equation.

rate = k1(c¥ – c)

Further research has shown that the rate constant, ki, does not depend on the degree of stirring, but that it does vary with the square of the radius of the ground coffee particles. The rate constant also increases 3% for every I ºC rise in temperature.

Some recent developments

The battle for supremacy between tea and coffee has grown fiercer with globalization of the world economy. In the USA, coffee supplanted tea following the Boston Tea Party in 1773, after which drinking tea became unpatriotic. Now tea is slowly making a comeback, especially with cold tea and fruit mixes in bottles and cans. In Britain, tea predominated until the arrival of coffee in its convenient instant form some 50 years ago. The tea trade eventually fought back with teabags containing smaller pieces of leaf, which infused more rapidly, and recently with fruit–flavoured tea drinks.


A major consideration these days is health. The coffee trade is increasingly

marketing its decaffeinated products while tea companies have sponsored research on the effects of the various polyphenolics. It turns out that dietary iron (except that present in meat) is complexed by polyphenolics and is then not absorbed by the body: vegetarians are therefore recommended not to drink tea, or even coffee, together with their main meal. However, these tea polyphenols can play a beneficial role as antioxidants. Like vitamin C, they can mop up oxygen radicals which have been implicated in causing cancers and heart disease.

(935 words)

Adapted from Coffee and Tea Chemistry, Michael Spiro, Chemistry Review, Volume 6, Number 5, May 1997, Philip Allan Publishers.

(a) Why should athletes avoid drinking tea or coffee? (1)

(b) What is a ‘volatile’? (1)

(c) How can the different compounds responsible for the aroma of coffee be identified?

(1)

(d) Describe how you would use a graph of (c∞ – c) against time to show that coffee brewing is a first order process.

(2)

(e) Explain why tea is NOT a suitable drink for vegetarians to have with meals. (1)

(f) Write a summary in continuous prose, in no more than 100 words, describing the process and changes involved in roasting coffee beans.

(9)





3. Read straight through the passage below on “Consolidating chemistry”.

Then read it more carefully, in order to answer the following questions.

Consolidating chemistry

Conservation is often thought to be concerned mainly with the natural environment, but the conservation of man-made objects is also important. Such objects may be made from wood, glass or metal and also include documents, paintings, buildings and monuments. Important materials used in conservation work are cleaning materials, adhesives, coatings and consolidants.

Consolidants are materials that restore structural strength to objects in danger of disintegration. Stone buildings and monuments deteriorate for many reasons, such as frost damage, the effects of acid rain and the internal crystallisation of salts. Wooden objects may have lain in water for prolonged periods or been attacked by woodworm.

A consolidant must be capable of penetrating the object and then binding it together. Normally it needs to be applied as a liquid or solution but then solidify after application. The ease and extent of penetration will depend on the liquid’s viscosity. The viscosity of a liquid is a measure of its mobility or runniness; water has a low viscosity compared to treacle. Ideally a liquid used as a consolidant should have a low viscosity as this will assist penetration. It is sometimes possible to change the viscosity by using a different solvent or increasing the temperature, but this is often undesirable.

Another factor affecting penetration is the nature of the material. Both wood and stone have an essentially porous structure, but only a fraction of the pores will be connected to the surface and therefore accessible to the consolidant. This fraction is called the effective porosity of the material.

One successful consolidant is polyethylene glycol (PEG). This is a long-chain polymer with the following structural formula:

being the repeat unit in the formula.

Different varieties of PEG are identified by their molar masses; for example, PEG 1500 has a molar mass of 1500 g mol–1 and an average of around 34 repeat units per molecule. PEG is a good consolidant for wood, which consists chiefly of cellulose, a polymer having a structure made up of the following repeat unit:


PEG readily forms hydrogen bonds to cellulose.

Another method of consolidation uses liquids that react together to form a solid product within the damaged or corroded object. A problem here is that the liquids may be too viscous to penetrate effectively and need to be made into solutions. A more successful approach has been to apply the consolidant as a monomer which polymerises within the object. The monomer is likely to have a much lower viscosity than the polymer.

A good example is the organosilane group of stone consolidants based on trimethoxymethylsilane. The polymerisation reaction takes place in several stages, but its effect is to convert Si O C linkages into Si O Si linkages and build up a polymer resembling silica (silicon dioxide).

Silica is found naturally as quartz and sand. It is very stable and has a giant covalent structure in which each silicon atom is joined tetrahedrally to four oxygen atoms, each oxygen being joined in turn to another silicon atom.

After the reaction has occurred the solid polymer shrinks back on to the pore walls and binds the object together.

(516 words)

Adapted from Science for Conservators, published by The Conservation Unit, Museums and Galleries Commission.

(a) (i) Why is the viscosity of a consolidant liquid or solution important? (1)

(ii) Suggest a reason why some liquids have much higher viscosities than others. (1)

(b) Explain what is meant by the term effective porosity of a material. (1)

(c) Draw a diagram to show one way in which a hydrogen bond can be formed between a repeat unit in a molecule of polyethylene glycol (PEG) and part of a molecule of cellulose.

(1)

(d) Carry out a calculation to confirm that there are approximately 34 repeat units in one molecule of PEG 1500. Use the Periodic Table as a source of data.

(2)

(e) Draw a diagram to illustrate the structure of silica (silicon dioxide). (1)

(f) Describe in no more than 120 words:

• why consolidants are used on stone objects;

• what properties consolidants must have;

• how organosilane consolidants act on stone. (8)


You are not asked to summarise the whole passage.

Credit will be given for answers written in good English, using complete sentences and with the correct use of technical words. Avoid copying long sections from the original text. Numbers count as one word, as do standard abbreviations, units and hyphenated words. Any title you give your passage does not count in your word total.


4. Read the passage below on EMISSION CONTROL straight through and then more carefully, in order to answer the following questions.

EMISSION CONTROL

Photochemical smog, a brown haze just above the horizon, was first observed in Los Angeles in the 1940s. Research showed that the smog was produced by the action of sunlight on air containing a variety of pollutants, many of them produced by burning fuels.

In a conventional petrol engine, a mixture of fuel and air is ignited by a spark. Petrol is essentially a hydrocarbon mixture so if combustion is complete the reaction products are carbon dioxide and water. The proportions of air and fuel are vitally important; the most efficient is the stoichiometric ratio. This is the ratio of air to fuel which, in theory, leads to complete combustion of the fuel. In practice, incomplete combustion results in the formation of toxic carbon monoxide and also leaves unburnt hydrocarbons which are the main cause of photochemical smogs. There is also reaction between nitrogen and oxygen gases in the engine, producing oxides of nitrogen, usually referred to as NOx.

‘Lean-burn’ engines use a higher air:fuel ratio. They work at lower temperatures, producing less NOx but more unburnt hydrocarbons. Diesel engines also work at lower temperatures, producing less of all the gaseous pollutants but significantly more solid particles such as carbon.

Catalytic converters are fitted to vehicle exhaust systems to deal with gaseous pollutants. The earliest catalysts tried were metals such as nickel, copper and cobalt. These were relatively cheap but were adversely affected by high temperatures and were ‘poisoned’ by sulphur and lead compounds in the fuel. Platinum was found to be very much more satisfactory. Very small particles of the metal were supported on a ceramic honeycomb to increase its surface area.

Getting the right conditions for combustion is not easy. Carbon monoxide and unburnt hydrocarbons need to be oxidised but the nitrogen oxides have to be reduced to nitrogen gas. An ingenious solution is to use the nitrogen oxides to oxidise the other pollutants, but this still requires some oxygen for complete removal of the hydrocarbons and carbon monoxide. Modern three-way catalysts (so-called because they deal with all three types of pollutants) only work effectively if the composition of the original air:petrol mixture is carefully controlled; too much fuel will mean insufficient oxygen in the exhaust fumes. The exhaust system must therefore be fitted with oxygen sensors linked back to an electronically controlled fuel injection system. Efficiency is also improved by incorporating other ‘platinum group’ metals such as rhodium and sometimes palladium.


Catalysts for diesel-engined vehicles have to include an efficient filter to remove carbon

particles. This leads to a further problem because the solid carbon (soot) needs to be burned to allow the filter to continue to work effectively. Soot combustion needs a temperature of 500–600°C and the exothermic reaction with oxygen can raise it to as much as 1000 °C,which can damage the filter. This problem has been tackled by first converting all nitrogen oxides to nitrogen dioxide (NO2) using a platinum catalyst, and then using the nitrogen dioxide to oxidise the carbon, a reaction which takes place at 200 °C.

The catalyst will only work if it is hot. Platinum needs a temperature of 240°C but rhodium helps by reducing this to 150°C. This means that the catalyst is ineffective when the engine is first started, a major problem under investigation. Unleaded fuel is essential and the introduction of ‘very low sulphur’ fuel has helped to eliminate the bad-egg smell noticed if hydrogen sulphide is produced.

(573 words)

Adapted from ‘Emission control’ by B. Harrison, Education in Chemistry, September 2000.

(a) Why is the brown smog first seen in Los Angeles said to be photochemical? (1)

(b) Write a balanced equation for the reaction between carbon monoxide and nitrogen monoxide (NO), producing carbon dioxide and nitrogen gas.

(1)

(c) Explain what is meant by the incomplete combustion of a fuel. (1)

(d) Why do conventional petrol engines give higher NOx emissions than ‘lean-burn’ or diesel engines?

(1)

(e) Three ‘platinum group’ metals used in catalyst systems are mentioned in the passage. Suggest another metal of this type which might be tried in the search for improved catalysts.

(1)

(f) Why has it been found necessary to change the composition of motor fuel for engines fitted with converters?

(1)

(g) Why are catalysts least effective when the engine is first started? (1)

(h) Describe, in no more than 120 words, how major exhaust pollutants are dealt with by modern exhaust systems.

(8)


You are not asked to summarise the whole passage, nor to include equations in your summary.

At the end of your summary state the number of words you have used.

Credit will be given for answers written in good English, using complete sentences and using technical words correctly and chemical names rather than formulae. Avoid copying long sections from the original text. Numbers count as one word, as do standard abbreviations, units and hyphenated words. Any title you give your passage does not count in your word total.


5. Read the passage below on CYANIDE – A TOXIC PARTNERSHIP straight through, and then more carefully in order to answer the following questions.

CYANIDE – A TOXIC PARTNERSHIP

Hydrogen cyanide was discovered by the Swedish apothecary and chemist Carl Wilhelm Scheele in 1782. He isolated hydrogen cyanide while working on the destructive distillation of the pigment Prussian Blue. By distilling the fumes formed and heating the product, he obtained a small quantity of aqueous acid which he named prussic acid. It had, he reported ‘. . . a taste which almost borders slightly on sweet and is somewhat heating in the mouth’. Surprisingly, having tasted his new compound, Scheele was to survive for another three years. Although his death has been attributed to him inhaling HCN vapour when a flask broke, a more satisfactory historical account suggests that he actually died from a combination of disorders, including rheumatism and gout.

In the early 19th century the study of prussic acid was taken up by Gay-Lussac, who in the course of his work prepared pure HCN, and also the gas cyanogen, (CN)2.

Hydrogen cyanide is a colourless, low viscosity liquid with the smell of bitter almonds, though in fact not everyone can smell it. It is a weak acid and is found in nature, for example, during photosynthesis and is produced when a number of synthetic polymers, such as polyurethane furniture foams, burn.

Today, hydrogen cyanide is prepared by reacting ammonia with hydrocarbons by either the Andrussov process or the Degussa process. In the first process, the mixture is oxygenated, and passed over a precious metal catalyst at slightly elevated pressure and high temperatures.

CH4(g) + NH3(g) + 1½O2(g) HCN(g) + 3H2O(g)

In the alternative process, developed by the company Degussa, a simpler catalyst system is used and it operates in the absence of oxygen.

CH4 + NH3 HCN + 3H2

Both processes employ rapid flow rates and each gives yields approaching 90%.


Hydrogen cyanide polymerises to give a black solid. To prevent this happening, small quantities

of stabilisers, typically strong acids such as sulphuric or phosphoric acid, are added. The major use of hydrogen cyanide is in the synthesis of other compounds, such as the monomer methyl methacrylate (used to make Perspex), as well as pharmaceuticals and speciality chemicals.

From 1887, sodium cyanide was used for extracting gold and silver from their ores. Gold occurs in its ores in a chemically free state as finely divided metallic particles. Extraction by the cyanide process involved crushing the gold-bearing rock, followed by treatment with a dilute solution of sodium cyanide plus lime to control the pH. The mixture was aerated and reacted according to the following equation

4Au + 8NaCN + O2 + 2H2O → 4Na[Au(CN)2] + 4NaOH

The resulting gold complex was then treated with zinc dust to precipitate the product, after which the gold was liberated by electrolytic refining. Potassium cyanide is used in electroplating and also in the manufacture of dyestuffs. Typically, plating is carried out by depositing silver onto the object by making it the cathode in a bath consisting of an aqueous solution of both silver and potassium cyanides.

A surprising application for a cyanide complex is sodium hexacyanoferrate(II), Na4[Fe(CN)6], in table salt to prevent caking. Manufacturers of table salt seem coy about admitting to this use, at least to the consumer, and bland terms appear in the list of ingredients, such as ‘anti-caking agent’. Whatever it is called, this particular cyanide complex acts by absorbing water and forming hydrates. This counteracts the natural tendency of sodium chloride to take up water and thus to form solid lumps. Like all cyanides, this substance is toxic, but the levels added to table salt are such that the daily dose is well below the recommended maximum of 0.025 mg kg–1 body weight.

(612 words)

Adapted from ‘Cyanide – a toxic partnership’, by John W. Nicholson, Education in Chemistry, March 1996.

(a) (i) Draw a ‘dot-and-cross’ diagram for hydrogen cyanide, HCN. (1)

(ii) Predict the bond angle in HCN. (1)

(b) Give ONE reason for and ONE reason against the use of a slightly elevated pressure in the Andrussov process for the preparation of hydrogen cyanide.

(2)


(c) Draw the formula for the repeat unit of Perspex.

(1)

(d) The reaction between gold and sodium cyanide to form a gold complex is a redox reaction.

(i) Give the name of the element which is oxidised and show its change in oxidation number.

(1)

(ii) Give the name of the element which is reduced and show its change in oxidation number.

(1)

(e) Describe in no more than 100 words

• the two methods currently used to make hydrogen cyanide

• a use of hydrogen cyanide

• a use of another cyanide.

You are not asked to summarise the whole passage nor to use chemical formulae or equations in your answer.

At the end of your summary, state the number of words you have used. (8)

Credit will be given for answers written in good English, using complete sentences and with the correct use of technical words. Avoid copying long sections from the original text. Numbers count as one word, as do standard abbreviations and hyphenated words. Any title you give to your passage does not count in your word total.

There are penalties for the use of words in excess of 100.

6. Read the passage below on THE DANGERS OF EXPOSURE TO THE SUN AND HOW TO AVOID THEM straight through, and then read it again more carefully, in order to answer the following questions.

THE DANGERS OF EXPOSURE TO THE SUN AND HOW TO AVOID THEM

During the latter part of the twentieth century a suntan became associated with fitness, youth and health. Increasing leisure time and cheap travel have helped certain populations to increase their lifetime ultraviolet radiation (UVR) exposure way beyond previous generations. Unfortunately this level of UVR exposure has increased the risk of damage to the skin caused by sunlight, called cutaneous photodamage. This photodamage has two potentially undesirable effects – premature ageing of the skin, called photoaging, and skin cancer.


When skin is exposed to ultraviolet radiation a proportion of the energy of the radiation is

absorbed. Initially electrons in molecules in the skin are promoted to higher energy levels, producing electronically excited molecules. These electronically excited molecules may react in several ways. They might simply break up heterolytically to produce fragment ions which react to produce toxic photoproducts. They may form highly reactive free radicals, or powerful oxidising agents by reaction with oxygen, which damage cells or other biologically important molecules. Finally, they may directly interact with cells in a damaging way.

There are three ways of reducing the risk from UVR exposure. These are directly limiting sun exposure by avoiding going out in the sun, the use of appropriate clothing and the careful use of sunscreen products.

Directly limiting sun exposure is effective. However, it is socially unacceptable and, with a desirable increase in outdoor activities like jogging, it is impractical.

Clothing which is made of opaque fabric is effective. It is important that the fabric is not flimsy or ‘see-through’. It should also be noted that many fabrics lose as much as 70% of their sun protection factor when wet. Broad-brimmed hats can reduce head and neck exposure significantly, but will still allow about 30% of damaging radiation exposure from reflected light, particularly when near sand or water.

There are two main types of sunscreen preparations, those that reflect the sun’s rays, and those that filter out the harmful UVR.

Historically, the first reflective preparation was issued to soldiers during the Second World War. Known as ‘Red Vet Pet’, it was a suspension of iron(III) oxide in petroleum jelly. It was extremely effective but was thick, greasy and gave a bizarre appearance. Nowadays similar effective preparations, which reflect harmful UVR, use zinc oxide or titanium dioxide. The only problem with these is one of cosmetic acceptability. As well as reflecting UVR, they tend to reflect visible light giving the user the appearance of a circus clown. Recently this problem has been overcome by using a very fine particle size of titanium dioxide which limits the reflection of visible wavelengths. This type of sunscreen has the major advantage of reflecting all wavelengths of ultraviolet radiation.

A wide variety of chemicals are used in filter preparations. These include esters of 4-aminobenzoic acid, esters of cinnamic acid, benzophenones, salicylates, and anthranilates. These usually act by absorbing harmful ultraviolet radiations. The disadvantage is that these chemicals only absorb over a small range of ultraviolet frequencies, so some harmful radiations will still reach the skin. The key is to check on the package of sunscreen that it is effective against both UVA and UVB, the two ranges of UVR frequencies which are known to be harmful.

(533 words)

Adapted from ‘The benefits of lifetime photoprotection’ by J Gray and J L M Hawk published by the Royal Society of Medicine Press, and ‘Sun damaged skin’ by Ronald Marks published by Dunitz.

(a) What are the TWO reasons for increased exposure to the sun nowadays? (1)

(b) What are the TWO undesirable effects of exposure to ultraviolet radiation? (2)


(c) What is meant by the following terms:

(i) heterolytically

(ii) free radical

(iii) oxidising agent? (3)

(d) Write a summary, in no more than 100 words, describing the methods for avoiding exposure to ultraviolet radiation and how effective they are.

(9)

You are NOT asked to summarise the whole passage, or to include equations in your summary.




7. Read the passage below on CHEMISTRY THAT WON’T LET YOU DOWN straight through, and then more carefully, in order to answer the following questions.

CHEMISTRY THAT WON’T LET YOU DOWN

The current deodorant, antiperspirant and body spray market in the UK is estimated to be worth £447 million. This represents approximately 30% of the toiletries sector which makes up almost a third of the entire cosmetics market. So what is the story behind antiperspirants and deodorants?

Sweating plays a vital role in controlling the body’s temperature. The evaporation of a watery fluid from sweat glands on the surface of skin has a cooling effect. During sweating, pheromones, chemicals that identify us through odour, are secreted. Sweat glands, of which there are two types, occur over most of the body surface. They are most abundant in the armpits, the groin, on the soles of the feet, the palms of the hands and the forehead. Eccrine glands, of which there are about two to four million over 99% of the body, are triggered by emotional, thermal and sensory stimuli. The sweat which they produce is virtually odourless and is essentially a dilute aqueous solution containing mainly sodium chloride and urea, and also other metabolic waste products, such as the lactates produced in muscles. Emotional stimuli also trigger the aprocrine glands, which are found only in the armpit and groin. Aprocrine glands produce small amounts of secretions rich in proteins and lipids, together with cholesterol and steroids. These fatty compounds are broken down by bacteria on the skin surface, mostly to low carbon chain (C4–C10) fatty acids, RCOOH, such as 3-methylhexanoic acid, which we recognise as body odour (BO).


Antiperspirants and deodorants share many of the same ingredients. However, each has its own

distinct role and contains different active compounds.

Deodorants act solely to reduce BO by killing the odour-causing bacteria. The first deodorant, marketed in the US in 1888, was “Mum”, a zinc oxide based cream. Chemists soon discovered that other zinc-based compounds such as zinc peroxide were anti-bacterials and therefore also had deodorant potential. In the early 1960s Gillette introduced an aerosol deodorant, “Right Guard”, that contained essentially two antibacterial agents, zinc phenolsulphonate and hexachlorophene. While zinc phenolsulphonate remains one of the few zinc salts clinically accepted in deodorants, hexachlorophene was banned in the mid-1970s because of its toxicity. In today’s deodorants, ethanol is the principal antibacterial agent. Further antibacterial activity is derived from some of the added perfume oils, for example, the essential oils of sage and lemon, and additional antibacterial agents such as triclosan.

In contrast to deodorants, antiperspirants have a dual action, reducing both odour and wetness. Antiperspirants usually contain aluminium salts, which physically block the eccrine sweat glands. Aluminium salts are also antibacterial agents and therefore have a deodorising effect.

“Everyday”, launched in 1902, was the first branded antiperspirant. It was an aqueous solution of aluminium chloride which was dabbed onto the armpits with cotton wool. Unfortunately with a pH of about 2 the “Everyday” solution was so acidic that it irritated the skin and rotted clothes. Early attempts in 1921 to counteract the corrosive nature of this solution used borax and alum, but eventually urea became the agent of choice. These products were cold and wet to apply and slow drying. It was not until John H. Wallace, a Princeton chemist working for Carter Medicine, developed “Arrid Cream” in 1934, which used the less corrosive aluminium sulphate, that antiperspirant products began to have as much appeal as deodorants like “Mum”.

In 1947, two chemists, T. Gorett and M. G. deNavarre, used the more basic aluminium salts, known as aluminium chlorohydrates (ACH) with the general formula Al2(OH)mCln, where m + n = 6, in antiperspirant formulations, with more success. These are still used extensively in antiperspirants today.

Deodorants and antiperspirants are sold in three different formulations: solutions, suspensions and emulsions. These are applied in several different ways: pump sprays developed from World War II insecticide sprays, roll-ons, an idea inspired by Ladislas and Georg Biro’s ballpoint pen, as well as sticks, aerosols and gels.

In the personal hygiene market cosmetic companies are continually trying to win over customers. Formulation chemists will continue to develop products that provide optimum performance, effective application and pleasant use.

[635 words]

Adapted from “Chemistry That Won’t Let You Down” by James Berressem, Education in Chemistry, July 2002.

(a) What does sweat, produced by the eccrine glands, contain? (1)

(b) State THREE factors which stimulate the creation of sweat. (1)

(c) Suggest the structural formula for 3-methylhexanoic acid. (2)


(d) What was the main problem associated with the use of the earliest antiperspirants?

(1)

(e) Write the formula of the “ACH” antiperspirant in which m = 2. (1)

(f) Suggest ONE advantage of applying an antiperspirant as a “roll-on”, as opposed to using it in an aerosol.

(1)

(g) Write a summary in no more than 100 words, in which you • explain the distinction between deodorants and antiperspirants • describe their composition nowadays and how they work • give the different formulations and ways in which they can be applied to the skin.

(8)

You are NOT asked to summarise the whole passage, nor to include equations in your summary. At the end of your summary state the number of words you have used.

Credit will be given for answers written in good English, using complete sentences and with the correct use of technical words. Avoid copying long sections from the original text. Numbers count as one word, as do standard abbreviations, units and hyphenated words. Any title you give your passage does not count in your word total.


8. Read the passage on petroleum refining straight through, and then more carefully, in order to answer the following questions.

(a) Petrol produced by simple crude oil distillation has a low octane rating and is not suitable for modern car engines.

(i) What technological change led to the need for higher octane rating fuels? (1)

(ii) What is the effect of using a low octane fuel in a modern petrol engine? (1)


(b) CH3CH2CH2CH2CH2 CH2 CH2 CH3

CH

Methylbenzene

3

Benzene

CH3C(CH3)2C(CH3)2CH3

2,2,3,3–Tetramethylbutane

(i) Name the two chemicals from the four hydrocarbons above which are isomers of each other.

(1)

(ii) Name the hydrocarbon with the lowest spontaneous ignition temperature. (1)

(c) (i) How has the cost of producing petrol been affected by the reduction in permitted lead content?

(1)

(ii) Why have limits been placed on the maximum concentration of benzene in petrol? (1)

(d) Write a summary in continuous prose, in no more than 100 words, of the chemical processes involved in the manufacture of petrol today.


At the end of your summary state the number of words you have used. (9)

Credit will be given for answer written in good English, using complete sentences and with correct use of technical words. Avoid copying long sections from the original text. Numbers count as one word, as do standard abbreviations and hyphenated words. Any title you give to your passage does not count in your word total.



Petroleum refining in the post–lead era

Commercial petroleum refining can be traced back to 1863 when Samuel Andrews, a candlemaker and lard oil refiner, extracted lamp oil from newly discovered reserves of crude petroleum. Lamp oil was the only petroleum product of interest in 1863, but demand for a much wider product range grew over the next 100 years from developments in transport, in power generation and the organic chemical industry. And nowadays we are aware of the toxic nature of some of the compounds in crude petroleum and petroleum products.

Oil refining

Today oil refineries have to provide products in the quantities and qualities required to satisfy market requirements – no easy matter, considering the complex chemistry of crude oil, and a global consumption of its products exceeding 3000 million tonnes per annum.

Alkanes and cycloalkanes predominate in crude oils, and alkenes exist only in trace amounts. A single crude oil may contain upwards of 10,000 individual compounds, though this is of little significance to the refiner, whose main concern is to break down the starting materials into suitable boiling ranges. Separation by fractional distillation is the most convenient way of doing this, producing a range of products which usually require further conversion and treatment processes.

In the early days of oil refining, products from fractional distillation were used with little or no further processing. This is no longer the case. Today, motor car engine compression ratios approach 10:1 compared with 4:1 in 1914, and fuel of high spontaneous ignition temperature must be used to avoid the damaging and wasteful effects of preignition (“knock”) and autoignition (“pinking”). The octane scale is used as an indication of resistance to engine knock. A high octane rating indicates good resistance – modern petrols (known as gasolines in the refining industry) need ratings of between 95 and 98. Arenes and highly branched alkanes, in comparison with straight chain alkanes, have high spontaneous ignition temperatures and excellent resistance to knock. Petrol that is produced simply by crude oil distillation has an octane rating of less than 50 and requires considerable conversion to make it acceptable.

Conversion processes

Here, molecular sizes and shapes are modified to provide refinery products. Cracking, both thermal and catalytic, reduces molecular size and converts heavy residues into lighter and more marketable products.

Catalytic cracking is particularly attractive for petrol production. A low demand, high boiling point product of distillation is converted to a high octane fuel.

The first catalytic cracking unit contained acid–treated clays, in the form of pellets, as the catalyst. Nowadays crystalline aluminosilicates known as zeolites, are used.


Thermal cracking, the method of increasing petrol output in about 1914, fell out of

favour with the advent of catalytic processes, but is re–emerging as an important method for converting very heavy residues into petrol blendstocks. In one variant, the residues from vacuum distillation are heated to 500°C for several seconds and the cracked products are distilled. About 10 percent of the feed is converted to petrol, and some gas oil suitable for catalytic cracker feedstock is produced.

Catalytic reforming is used to raise the octane rating of distillates in the petrol boiling range by converting straight chain alkanes and cycloalkanes to arenes. The octane rating is increased from 50 to greater than 95. The dehydrogenation reactions taking place during reforming provide a valuable source of hydrogen for use in sulphur removal.

Greater amounts of unsaturated hydrocarbons have become available in the refinery as a result of increased cracking activity. These have potential as polymer feedstocks.

Treatment processes

The final stage in the manufacture of petrol involves blending components from distillation, cracking and the various reforming processes to produce a fuel with the required volatility and knock resistance – at minimum cost. In principle, any octane rating fuel likely to be required by the car industry can be produced in this way. However, compensation for the reduction in allowable lead alkyls tends to raise costs dramatically. Furthermore, limits have been placed on the maximum concentration of benzene in petrol.

(640 words)

9. Read the passage on pages 13 and 14 on ESCA – a Practical Method for Determining Charges on Atoms in Molecules – straight through, and then more carefully, in order to answer the following questions.

(a) (i) In ammonia, NH3, the ionic character of the bond between nitrogen and hydrogen (N–H) is 18%. State the charge on each hydrogen atom and hence calculate the charge on the nitrogen atom.

(2)

(ii) Explain why liquid ammonia is likely to be a good solvent for ionic compounds. (1)

(b) Use the results given for the sulphate ion to calculate the charge on the sulphate ion. (1)

(c) Describe the shape of the thiosulphate ion with an appropriate diagram. (1)

(d) (i) Using the normal rules, calculate the oxidation number of sulphur in the thiosulphate ion, -2

32OS . (1)

(ii) Suggest a reason why oxidation numbers are not always the same as the true electric charge on atoms in molecules or ions consisting of several atoms.

(1)


(e) Describe the process of electron spectroscopy for chemical analysis (ESCA) and explain

how it can be used to give important information about atoms in molecules, in not more than 100 words.


(8) Credit will be given for answers written in good English, using complete sentences and with correct use of technical words. Avoid copying long sections from the original text. Numbers count as one word, as do standard abbreviations and hyphenated words. Any title you give to your passage does not count in your word total


(Total 15 marks)

ESCA – a Practical Method for Determining Charges on Atoms in Molecules

It is often useful to know actual charges on atoms in polar molecules. Such knowledge can help in the predictions of physical properties of compounds like the melting point, boiling point and solubility. It is also an aid to predicting and understanding chemical reactions.

In molecules like HCI, H2O or NH3, determining the distribution of electric charge on the atoms is simple. The degree of ionic character of the bonds can be found by calculating the difference in electronegativity between the elements and then consulting an appropriate table of data.

Consider the example of water, H2O. The difference in electronegativity gives 39% ionic character for the bond between oxygen and hydrogen. This means that the charge on each hydrogen atom is +0.39, and the charge on the oxygen atom is–0.78.

+0.39 +0.39

–0.78

H H

O

This calculation shows that the oxygen atom in water has a very significant negative charge. This helps to explain the solubility of ionic compounds in water. Positive ions will be powerfully attracted to negative oxygen atoms in water molecules, and negative ions to the positive hydrogen atoms.

The calculation of charge distribution becomes much more difficult when the molecules contain multiple or delocalized bonds, or the molecule is non–symmetrical. It is more convenient to use experimental data instead of relying on theoretical calculations.

The main source of experimental data about charge distribution in molecules is the permanent dipole moment, which is based on measurements of the effect of a substance on an applied electric field. Unfortunately the interpretation of such data is still difficult for complex molecules.

However, in the last twenty years the method of X–ray spectroscopy has opened up new horizons in determining the distribution of charge in molecules and ions. There are three branches of spectroscopy which involve the interaction of X–rays with molecules: ESCA (electron spectroscopy for chemical analysis), Auger spectroscopy and X–ray fluorescence spectroscopy.

When X–rays of known energy are absorbed by an atom an electron from an inner shell is expelled and the atom becomes a positive ion in a high energy state. ESCA is concerned with measuring the kinetic energy of the expelled electrons using an electron spectrometer.

The kinetic energy of emitted electrons depends upon their binding energy within the atom. When electrons


are held tightly the energy needed to release them is greater and the kinetic energy of electrons released is less. The kinetic energy of emitted electrons, Ek, is related to the energy of the X–rays, hvk, and the binding energy of the electrons, Eb, by the equation:

Eb = hvk – Ek

The binding energy of an electron depends on the attractive force exerted by the positive nucleus on the negative electron. This is often referred to as the Coulombic force of attraction. The Coulombic attractive force is dependent on the effective charge on the nucleus; the higher the effective nuclear charge, the stronger the force of attraction. The effective charge on the nucleus is related to the partial charge on the atom. So the binding energy of an electron depends on the partial charge on an atom.

A more detailed analysis shows that there is a linear relationship between electron binding energies and partial charges on atoms.

Thus by measuring kinetic energies of electrons, it is possible through calculating electron binding energies to find partial charges on atoms relatively easily.

The reason ESCA is so important is that it enables the charge on the individual atoms within molecules or ions to be calculated.

Examples of ions where ESCA helps us to understand their electronic structure are the sulphate ion and the thiosulphate ion.

In the sulphate ion ESCA shows that the ion consists of a sulphur atom with a charge of +1.12, and four oxygen atoms with equal charges of –0.78.

–0.78

OO

O

S +1.12–0.78

–0.78

–0.78

O

In the thiosulphate ion, -2

32OS , the oxygen atoms carry the same charge of –0.83, but one sulphur atom has a charge of –0.50, and the other has a charge of +0.99.

These results are interesting and are quite different from the ‘charges’ which would be assigned to sulphur and oxygen using the concept of oxidation number in these ions. They suggest that oxidation number, while useful as a concept, is not a good indicator of the true pattern of electric charge in an ion consisting of several atoms.

(698 words) Adapted from ‘ESCA and molecular charge distribution’, Jacques Furnemont, Education in Chemistry, Volume 31, Number 5, September 1994, The Royal Society of Chemistry.


10. Read the passage on BENZENE straight through, and then more carefully, in order to answer the

following questions.

(a) (i) Draw the displayed formula for benzene (1)

(ii) Write a balanced equation for the dehydrogenation of cyclohexane, C6H12, to benzene, C6H6.

(1)

(iii) Suggest ONE reason why platforming might be preferred to using aluminium oxide (1)

(b) Classify the following reactions as substitution, elimination or addition reactions.

(i) The reaction between benzene and chlorine to form 1,2,3,4,5,6–hexachlorocyclohexane

(1)

(ii) The reaction between benzene and chlorethene to form ethylbenzene (1)

(c) Name the chemical used to produce dodecylbenzenesulphonic acid from dodecylbenzene. (1)

(d) Write a summary in continuous prose, in no more than 100 words, describing the production of benzene from petroleum.

You are NOT asked to summarise the whole passage, nor to include equations in your summary.



There are penalties for use of words in excess of 100. (Total 15 marks)

BENZENE

Benzene was first discovered by Michael Faraday in London in 1825. By this time gas lighting was becoming common in London. The Portable Gas Company was producing gas by heating whale oil in a furnace. When they compressed the gas to put it in cylinders an oily liquid separated out. They told Faraday about this and he set about identifying the liquid. He distilled it and collected a fraction which boiled at 80°C and condensed to a clear liquid. We now know this as benzene.

In 1834 Mitscherlich discovered that the same colourless liquid could be obtained by heating benzoic acid with lime.

In 1845 Hofmann demonstrated the presence of benzene in coal naphtha, the lowest boiling fraction obtained by the distillation of coal tar, and in 1848 Mansfield succeeded in isolating benzene from coal naphtha.


Now most benzene is produced from petroleum. Petroleum is fractionally distilled to separate the crude oil into fractions on the basis of their boiling points. A typical crude oil composition is 2% butane, 11% petrol, 14% naphtha, 17% furnace oil, 39% gas oil and 17% residue. The naphtha fraction which contains hexane is used to produce benzene.

The naphtha fraction is purified to remove sulphur compounds which would poison the catalyst used in the process. Sulphur compounds are removed by reduction to form hydrogen sulphide. This is known as hydrodesulphurisation (HDS) and leaves a very low concentration of sulphur in naphtha. The hydrogen sulphide produced is used to manufacture sulphur and sulphuric acid. This treatment was originally only used in the production of sulphur–free fuels but it is now used to reduce the sulphur content of most crude oil fractions.

Purified naphtha is now heated to about 770 K. It then passes into a reactor where one of two processes may be used, depending on the catalyst.

An aluminium oxide catalyst may be used with the reactants at a pressure of 40 atmospheres. Hexane first reacts to form cyclohexane and hydrogen. Then cyclohexane is dehydrogenated to form benzene. Other aromatics like methylbenzene and dimethylbenzenes are also produced. The mixture of products is dissolved in a suitable solvent. The aromatic products are separated from the solvent by further distillation in fractionating towers. Residual impurities are removed, for example, by passing through an active clay catalyst. A final fractionation is then used to separate and purify benzene.

The second process is called platforming because it uses the metal platinum. In platforming the same chemical reactions are involved. The difference is that a platinum catalyst is used, in spite of the extra expense, and a lower pressure of 15 atmospheres is sufficient.

Benzene is the starting material for a large number of useful chemicals and materials.

The insecticide BHC, benzenehexachloride (systematic name, 1,2,3,4,5,6–hexachlorocyclohexane) is made by passing chlorine through benzene irradiated by ultra–violet light. BHC is particularly valuable in the fight against the locust.

Benzene is the starting material for the manufacture of many plastics.

By reacting benzene with chloroethane in the presence of an aluminium chloride catalyst at 80°C, ethylbenzene is formed. This is dehydrogenated by heating to 600 °C with a zinc oxide catalyst to produce phenylethene (styrene) which polymerizes to form polystyrene.

Benzene is also the starting material for making many soapless detergents.

Benzene reacts with dodec–l–ene in the presence of a suitable catalyst to produce dodecylbenzene.

(CH ) CH

12 24

112 3

+ C H


The dodecylbenzene is then sulphonated to produce dodecylbenzenesulphonic acid which can then be neutralized to produce sodium dodecylbenzenesulphonate, a biodegradable soapless detergent.

So it can be seen that from Faraday's discovery a vast and vital range of chemicals has developed.

(590 words)

11. Read the passage below on ARGON – IN THE SPOTLIGHT straight through, and then more carefully, in order to answer the following questions.

(a) Give a chemical reason for the derivation of the name “argon”. (1)

(b) Ramsay made two experimental measurements which led him to the discovery of argon.

(i) What were these measurements? (1)

(ii) How did they lead him to this discovery’? (1)

(c) Why is nitrogen not suitable for a gas in electric light bulbs? (1)

(d) Suggest why oxygen is used to adjust the carbon content in steel making. (1)

(e) (i) Explain why argon is so unreactive in terms of its electronic structure. (1)

(ii) Use your answer to (i) to explain how the discovery of argon helped chemists to understand chemical bonding.

(1)

(f) Describe how argon is manufactured from air, in not more than 100 words. (8)






ARGON – IN THE SPOTLIGHT

It may seem surprising that uses exist for a colourless, odourless, totally unreactive gas like argon. Being denser than air, it cannot be used to lift balloons as its fellow Group member helium can, yet more argon is used than all the other members of the Noble gas group put together. How was it discovered? Why are unreactive substances of interest to chemists? Of what possible use can it be?

1994 was the centenary of the discovery of argon by Sir William Ramsay. Ramsay discovered argon whilst investigating apparent differences in the density of nitrogen made in different ways. Ramsay found the density of nitrogen made by chemical reactions to be 1.2505 g dm–3, whereas he found a value of 1.2575 g dm–3 for nitrogen obtained from dry air by removal of carbon dioxide and oxygen. He repeated the measurements enough times to be certain that the difference of about 0.5% was not due to experimental error. He concluded that either ‘air nitrogen’ contained traces of a heavier gas or that ‘chemical nitrogen’ contained some lighter gas. One idea was that the heavier gas was N3, akin to ozone O3. He removed the nitrogen from ‘air nitrogen’ by passing it repeatedly over heated magnesium:

3Mg + N Mg N2 23 There remained about 1% of residual gas which turned out to be mainly a new element, argon, a name chosen because of its chemical unreactivity. The word is derived from the Greek argos meaning lazy or easy.

Like the noble metals they were named after, the noble gases exist uncombined in nature. But thinly spread out. Physical rather than chemical processes are needed to separate them from their surroundings. The noble gases are obtained by fractional distillation of liquefied air. This is basically a scaling up of the method pioneered by Ramsay in discovering this family of elements.

Filtered air is compressed in five or six stages. The effect of each compression is to heat up the gas mixture. To minimise this, the air is compressed slowly. The air is then allowed to expand rapidly to give a maximum cooling effect and the process of compression followed by expansion is carried out repeatedly until the air is liquefied.

Between the second and third stages, carbon dioxide is removed by passing the air through aqueous sodium hydroxide. Residual moisture is removed by passage through silica gel or activated alumina.

After the compression stages, dry carbon dioxide–free air is fractionally distilled. This separates liquid oxygen from nitrogen. As nitrogen has a lower boiling point (77 K) than in oxygen (90 K), nitrogen boils first and leaves the top of the distillation column as a gas. Oxygen collects at the bottom of the column as a liquid. Because the boiling point of argon is nearer the boiling point of oxygen, argon is mainly found in the liquid oxygen fraction. This fraction is passed to another distillation column where 80-90% pure argon is produced. Residual oxygen is removed by reaction with hydrogen. This reaction produces water, so the resulting gas is then dried to leave pure argon.

Most of the uses of argon arise because it is the most abundant, hence the cheapest, noble gas. Its very unreactivity is an advantage. Many electric light bulbs are filled with argon, as this enables them to run at higher temperatures, where there is a more efficient transformation of electrical energy to light energy. Although nitrogen is much cheaper and unreactive at normal temperatures, it combines with the hot metal filaments to form nitrides, which would not conduct electricity.


The steel industry is the largest single user of argon. The gas is used as an inert stir gas to ensure homogeneity as oxygen is blown through the molten metal to adjust the carbon content.

So lazy, unreactive argon really can be useful in different ways, and it has also helped chemists to understand chemical bonding.

(645 words) Adapted from ‘Argon – in the spotlight’ by Gordon Woods. Chemistry Review, May 1995.

12. Read the passage below on GROWING DIAMONDS straight through, and then more carefully, in order to answer the following questions.

(a) Give TWO reasons why alchemists tried to make gold from copper, rather than diamonds from graphite.

(2)

(b) (i) State the physical property of diamond which suggested that high pressure would be needed in its synthesis from graphite.

(1)

(ii) Explain why high pressure is needed in the synthesis of diamond from graphite (1)

(iii) Suggest a reason why high temperatures are needed to make diamond from graphite.

(1)

(iv) Where do many scientists believe that the conditions of pressure and temperature for making diamonds occur naturally?

(1)

(c) Explain why large natural diamonds rather than synthetic ones are still used in jewellery. (1)

(d) Describe in no more than 100 words how diamonds can be made from graphite. (8)




(Total 15 marks)


GROWING DIAMONDS

From the fabled Golconda mines of ancient India to Paul Simon’s “Diamonds on the Soles of Her Shoes”, diamonds have been a source of fascination to men and to women. Had medieval alchemists known that graphite and diamond are merely different forms of the same element, carbon, they might have spent as much time attempting to synthesize diamonds as they did attempting to transmute base metals into gold. But copper at least looks something like gold, whereas graphite and diamond could scarcely be more dissimilar; one is dull, black, and soft enough to rub off on paper, the other is brilliant, transparent, and hard enough to drill granite. It was not until 1954 that modern alchemists learned how to synthesize diamonds from graphite.

The first clues toward the successful synthesis of diamond came from theoretical arguments and from considerations of the circumstances under which diamonds are found in nature. Diamond has a density of 3.51 g cm–3 whereas the density of graphite is only 2.26 g cm–3. Thus very high pressures are required to squeeze the graphite carbons into the tighter packing arrangement of the diamond crystal. Many scientists believe that natural diamonds are formed under such conditions, 100 or 200 miles below the surface of the Earth, and are later brought by volcanic action closer to the surface where they can be mined.

Laboratory duplication of these harsh conditions was finally accomplished in 1954 by four General Electric scientists, Francis Budy, Tracy Hall, Herbert Strong, and Robert Wentorf. The team of General Electric scientists developed a small chamber that could maintain temperatures up to 2500°C and pressures of 1.0 × 105 atm.

Their first attempts not only failed to produce diamond but also failed even to melt the graphite. The melting temperature of graphite is above 4000 °C, far beyond the chamber’s maximum of 2500°C.

The researchers finally hit upon the idea of adding small amounts of a metal that does melt below 2500°C, in the hope that some of the graphite might dissolve in the molten metal then crystallize out as diamond. The strategy worked beautifully with many metals: chromium, manganese, iron, cobalt, nickel, tantalum, ruthenium, rhodium, palladium, iridium, and platinum. When the mixture reached 1200–2400°C under a pressure of 1 .0 × 105 atm small diamond crystals formed at rates as high as 0.1 per minute. At last, a practical synthesis of diamond had been achieved.


The crystals – usually yellow or green and less than 1 mm across – were not gem quality, but they were as hard as natural diamond and were useful as an industrial abrasive.

Cross-section of apparatus used in diamond making. Synthetic diamonds appear within the graphite in the cylindrical cell, which is squeezed to a pressure of 1.0 10 atm by hydraulically driven pistons. A strong belt of concentric metal rings keeps the material from squeezing out the sides.

× 5

Today, the same method is used. It is called the high–pressure, high–temperature method (HPHT). Annual world–wide production of synthetic diamonds is measured by the ton, far outstripping natural diamond production from mines. Synthetic HPHT diamonds as large as 8 mm across have been made, but the long heating and pressing time they require – seven days – makes them more costly than natural diamonds of the same size. Therefore, the large diamonds used in jewellery are still natural diamonds.

(550 words) Adapted from Growing Diamonds by Harold Zaugg and Burning Diamonds and Squeezing Peanuts by Dr. K.A. Davenport published in Chem Matters, April 1990.

13. Read the passage below on CHEMICAL BONDING straight through, and then more carefully, in order to answer the following questions.

CHEMICAL BONDING

Atoms come together to form compounds. By so doing they can pair off electrons and the resulting product is energetically more stable. Not all combinations of atoms, however, are known and some elements such as the rare gases are particularly loath to enter into compound formation. A fruitful avenue to understanding chemical compounds is via chemical bonding.


The basis of chemical bonding is electron interaction, and in particular electron (spin) pairing.

Not all the electrons of an atom are involved in the formation of bonds to other atoms. The electrons in filled inner shells around the nucleus play little or no part. The electrons in the outer, partly filled, shells are the important ones. These are known as the valence or bonding electrons and the shell is called the valence shell. If the atom has a full complement of electrons in its outermost shell, then it shows little inclination to compound formation and for this reason the rare gases, helium, neon, and argon, form no true chemical bonds. Krypton and xenon can form compounds if enough energy is supplied to break into the filled shell and promote some of its electrons to a higher energy level, whereupon bond formation becomes feasible. Even so, only fluorine and oxygen can provide bonds with sufficient energy to do this.

There are three kinds of strong chemical bond – ionic, covalent, and metallic. The part played by the bonding electrons in each of these is quite different. In ionic (or electrovalent) bonding the electrons in the valence shell of a metal atom are transferred to the valence shell of the non–metal atom. In other words, the bonding electrons become associated with one of the atoms and charge separation occurs. The coulombic attractions between the ions so formed are the real basis of the binding energy of ionic compounds.

–22

1 ClNa Cl Na +→+ +

The structure of ionic compounds is determined mainly by the physical parameters of ion size and charge. The result is a lattice of alternate metal cations and non–metal anions in which there is no particular cation associated with a particular anion. This means that there is no particular direction to the bonding – the ions bond equally to neighbouring ions in all directions.

In covalent bonding the bond is said to be directed. The bonding electrons associated with two atoms occupy the region of space between them. The result of this is that the internuclear repulsion is lowered and the nuclei can approach one another more closely. This is represented in terms of energy in Figure 1. Most covalent bonds are of this type, that is, the bonding electrons are associated with two atoms. There are, in addition, some examples of three–atom covalent bonds in which a pair of bonding electrons serves to hold three nuclei together. These bonds are called three–centre bonds. This ability of electrons to hold several nuclei together reaches its ultimate form in metallic bonding.

atoms A + B

molecule ABinternuclear distance

bond length

bond energy

potential energy


In metals the bonding electrons are associated with no particular atom or group of atoms but are

free to move throughout the body of the metallic compound. This freedom is reflected in the properties of metals such as electrical conductance, which is invariably high.

metals

Na

metalsnon metals

NaClionic compounds covalent compounds

non-c

ovale

nt

cova

lent

non-ionic

ionic

Cl2

This method of classifying chemical compounds through their bonding can be illustrated by reference to Figure 2. In this the three corners of the triangle represent ideal forms of each kind of compound, but in reality these are not attained. No bond is purely ionic, covalent, or metallic. Because electrons are not fixed they can spend some of their time in a covalent bond with one of the atoms, so that in effect we have an equilibrium between covalent and ionic forms.

A B A B A B+ + – –

ionic formscovalentform

Similarly, deformation of the electron clouds of ions can produce a covalent contribution to the bonding in ionic compounds.

Adapted from ‘Chemical Compounds’ an article by John Emsley in the Physical Science Sourcebook published for the Nuffield Foundation by Penguin Books 1974.

(a) (i) Why do atoms come together to form compounds? (1)

(ii) Suggest a reason why electrons in filled inner shells around the nucleus play little or no part in bonding.

(1)

(iii) Explain what is meant by the term valence shell. (1)

(iv) Explain why krypton and xenon form compounds only with fluorine and oxygen. (2)

(b) Why do metals have a high electrical conductance? (1)

(c) Why is Figure 2 helpful in understanding real compounds? (1)


(d) Summarise the information given in the passage about the three types of chemical bond.

Use continuous prose and no more than 100 words.





14. Read the passage below on SIR HUMPHRY DAVY AND THE SAFETY LAMP straight through, and then more carefully, in order to answer the following questions.

SIR HUMPHRY DAVY AND THE SAFETY LAMP

Today it is not uncommon to find buildings decorated with brightly polished brass miners' lamps. Indeed, display seems to be the major use of these devices. But soon after the safety lamp was developed in late 1815, it was hailed as "the pride of science, the triumph of humanity, and the glory of the age". Over the next 150 years, thousands of the lamps were carried into mines the world over, and an unknown number of miners were saved from the ravages of poisonous and explosive gases.

Reports of coal miners being overcome by toxic gases and burned in mine explosions appear as early as the sixteenth century. In time, the gas responsible for both catastrophes came to be called fire–damp, but its chemical nature and origins remained unknown. The problem posed by fire–damp was two–fold. To avoid disaster, it was necessary both to devise a method of detecting the gas and to find a way of illuminating the mine without igniting the gas. Canaries and other birds served as early detectors. When they stopped singing and fell off their perches, it was a clear sign for the miners to head for the exit shaft. Illumination posed a greater problem. The open flames of candles and oil lamps could easily ignite explosive mixtures of fire–damp and air. One book reports that attempts were made to light some mines by the dull phosphorescent glow from rotting fish—a method that no doubt had a stronger effect on the sense of smell than the sense of sight. One solution to the detection problem was the flint and steel mill in which a stream of sparks was generated by pressing a piece of flint against a rotating steel wheel. Although fire–damp caused changes in the size and luminosity of the sparks and could thus be detected, the mill could and did ignite explosions.

The event that ultimately led to the invention of Davy's safety lamp was an explosion at the Felling Colliery near Sunderland on 26 May 1812. A total of 92 miners were killed, and a great public outcry resulted. A group of public–spirited citizens contacted Davy in 1815.


The development of the Davy lamp, as it soon came to be called, was a blend

of scientific research and practical inventiveness. Sir Humphry began by demonstrating that fire–damp is "pure light carburetted hydrogen" consisting of "four portions of hydrogen in weight 4, and one portion of charcoal in weight 11.5". John Dalton's applications of atomic theory to chemistry had been published only 7 years earlier, but Davy must have used atomic ideas to come up with the first correct statement of the formula of the gas we know as methane, CH4. Davy was also able to show that when methane is mixed with 6–14 times its volume of air, an explosive mixture results. Mixtures that are richer in methane extinguish a candle flame; those that are poorer cause the flame to burn more brightly, but do not explode. Fortunately, even explosive mixtures of methane and air proved to require considerably higher temperatures to ignite than do mixtures of hydrogen and oxygen.

In late October 1815 Davy described three promising preliminary lamp designs, but his real breakthrough was the introduction of a "chemical fire sieve". In a letter dated 1 January 1816 (Davy was working on New Year's Day!) he reported that a candle or oil lamp surrounded with a wire mesh cylinder was absolutely safe in the presence of even potentially explosive mixtures of fire–damp and air. The flame would not propagate through the mesh, and the mesh would diffuse the heat of the lamp so that the methane would not be explosively ignited. In the presence of low concentrations of methane, the size of the flame would increase as the methane burned, much as it does in a Bunsen burner. When methane was present in high concentrations, the flame would be extinguished. Thus, the Davy lamp is both an explosion–proof source of light and a detector of methane.

The same principle of design has been incorporated in all subsequent lamps, though various refinements have been made. One of the first was the introduction of a metal shield to prevent draughts from blowing the flame horizontally into the mesh, which might thus become hot enough to ignite the gas. A second layer of iron or bronze mesh was soon added to further enhance safety. Modern lamps also have cylindrical glass windows around the flame, and the gauze cylinder, which is situated above the glass, is fully protected by a metal wind shield. Thus, the screen, the key to the safety of the Davy lamp, is not obvious from a casual inspection of the typical decorative lamp.

Davy appears to have taken great satisfaction from his invention. His contribution was recognised by a valuable silver dinner service, a gift from a group of wealthy mine owners. Davy was made a baronet in 1819, but perhaps he prized even more highly a letter from those who benefited the most from his invention. It reads thus:

We, the undersigned, miners at the Whitehaven Collieries, belonging to the Earl of Lonsdale, return our sincere thanks to Sir Humphry Davy, for his invaluable discovery of the safe lamps, which are to us life preservers; and being the only return in our power to make, we most humbly offer this, our tribute of gratitude.


The 82 `signatures' include 47 marks made by miners unable to write their

names.

(913 words)

Adapted from `Sir Humphry Davy and the Safety Lamp' by A. Truman Schwartz, Chemistry Review, March 1996.

(a) State the two types of catastrophe caused by fire–damp. (1)

(b) Why did fire–damp pose a two–fold problem? (2)

(c) Show how the formula of methane, CH4, can be deduced from the information "four proportions of hydrogen in weight 4, and one proportion of charcoal in weight 11.5".

(1)

(d) (i) Suggest a property of iron which makes it a suitable metal for the wire mesh in a safety lamp.

(1)

(ii) Suggest another metal that could be used for the mesh. (1)

(e) Why was the safety lamp hailed as "the pride of science, the triumph of humanity and the glory of the age"?

(1)

(f) Write a summary in continuous prose, in no more than 120 words, describing the development of the safety lamp and how it works.




Credit will be given for answers written in good English, using complete sentences and

with the correct use of technical words. Avoid copying long sections from the original text. Numbers count as one word, as do standard abbreviations and hyphenated words. Any title you give to your passage does not count in your word total.


15. Read the passage below straight through and then more carefully.

A RADICAL ALTERNATIVE TO IONS

Reactions involving free radicals were once believed to be unselective and low-yielding and therefore of little use to chemists. There is no doubt that free radicals get a pretty bad press. Radicals generated from chlorofluorocarbons are known to destroy the Earth’s ozone layer, while radicals made in the body are believed to contribute to many diseases, including heart disease and stroke. Even the rancidity of margarine is blamed on undesirable radical reactions. In these instances, it is easy to see why the formation of radicals should be avoided, though this is not always the case. There are many useful and efficient radical reactions currently being developed by organic chemists.

The most important method for making polymers involves radicals. In industry, radical chain reactions are used to make a number of useful polymers, including poly(chloroethene), poly(phenylethene) and poly(ethene). Such polymers have numerous uses, for example in electrical insulation, grocery bags, raincoats, swimming pool liners, tubing and foam packaging. Radical polymerisation is used to prepare poly(tetrafluoroethene), (PTFE or Teflon), which is used to coat non-stick pans. Polymerisation reactions involving radicals can produce polymers of varying chain length and degrees of chain branching.

Uses of radicals in organic synthesis include producing valuable halogenoalkanes from alkanes. Chlorination of alkanes, for example, can give a mixture of mono-, di-, tri- and tetra-chlorinated products. Radicals provide a route to useful products, but mixtures can be formed which may be difficult to separate. Reactions may be uncontrollable and of little use in preparing valuable organic molecules such as insecticides and pharmaceuticals.

Why use radicals?

There are many efficient organic reactions involving ions such as R3C+ or R3C– as intermediates (R represents an alkyl group). These ions are generally prepared under acidic or basic conditions. This can be a problem for acid-sensitive or base-sensitive reagents because, rather than forming the desired ion, the reagent can decompose. For radical reactions this is much less of a problem because radicals are formed under mild and neutral conditions.

Radical reactions are not very sensitive to changes in solvent. The formation of intermediate ions often requires a polar solvent which can solvate and stabilise the ions. Because radicals are neutral they are generally not solvated and changing the solvent will be less important.

Finally, most radicals are highly reactive and can be used for changes that are difficult or impossible to achieve using ions. This includes halogenation and oxidation of alkanes and preparation of highly strained molecules. So radical reactions do have some important advantages over ionic reactions, but what about controlling them so that only one, rather than many, organic products are formed?


Getting in control

Most radical reactions involve chain processes. Efficient chain reactions require a selective initiation step in which only one bond is broken in an initiator molecule. To do this effectively, relatively low temperatures (below 150 °C) need to be used and the organic starting material should contain one particularly weak bond.

At present, the most important reagent used in radical chain reactions is tributyltin hydride, (C4H9)3SnH. A typical reaction involves heating this compound with an initiator to make the

tributyltin radical, (C4H9)3Sn•, which can then react with a variety of organic compounds. Though tributyltin hydride has played a crucial part in the development of radical chemistry, it is relatively expensive and neurotoxic and the tributyltin halide by-products, such as (C4H9)3SnCl, can be difficult to separate from the organic products on a large scale.

So synthetic chemists faced with the most perplexing problems are now finding that radical reactions can provide the perfect solution.

(589 words)

Adapted from “A radical alternative to ions” by Andrew F. Parsons, Education in Chemistry, March 2003.

Answer the following questions.

(a) Explain what is meant by a free radical.

.......................................................................................................................................

....................................................................................................................................... (1)

(b) Name the type of bond breaking which occurs when a chlorine molecule forms free radicals.

....................................................................................................................................... (1)

(c) Suggest the condition which causes chlorine free radicals to be produced from chlorofluorocarbons in the Earth’s atmosphere.

....................................................................................................................................... (1)

(d) Write an equation for the reaction between ethane and chlorine to form a dichloroalkane.

(2)


(e) Tetrafluoroethene is made from an ethene molecule in which each hydrogen atom is

replaced by a fluorine atom. Draw a section of the polymer poly(tetrafluoroethene) showing at least two monomer units.

(1)

(f) Propanone, CH3COCH3 is an example of a polar solvent. Draw a diagram of a molecule of propanone indicating where it is polar.

(1)

(g) Summarise, in no more than 110 words, the problems and advantages of using radicals in organic synthesis.

You are NOT asked to summarise the whole passage, nor to include equations in your summary. At the end of your summary state the number of words you have used.


(8)


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