KISS Notes.energy

keep it simple science TM

www.keepitsimplescience.com.au

Baulkham Hills High School SL#802445

1

but first, an introduction...

Preliminary Chemistry Topic 4

ENERGYWhat is this topic about?To keep it as simple as possible, (K.I.S.S.) this topic involves the study of:1. HIGH ENERGY COMPOUNDS & FUELS

2. CARBON & CARBON COMPOUNDS3. THE CHEMISTRY OF COMBUSTION

4. RATE OF CHEMICAL REACTIONS...all in the context of society’s use of fuels.

Energyincludes heat, light, sound, the kinetic energy of a

moving object, and Potential Energy which is stored invarious ways.

This topic is all about Chemical Potential Energywhich is stored in the chemical bonds within manycompounds and released during chemical reactions,especially

Combustion

From the humble match to arocket launch, Combustion isthe chemical reaction thatreleases the energy of the

Fuelsupon which our entirecivilization is dependent fortransport, industry, heating &cooking.

To understand the Chemistry of Combustion you need to learn about the element

and its many and varied compounds.

You will learn more about

Exothermic & Endothermicchemical changes,

and then extend your chemical knowledge to include

RateofReaction

Carbon

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Preliminary Chemistry Topic 4Copyright © 2005-2007 keep it simple science


www.keepitsimplescience.com.auPreliminary Chemistry Topic 4Copyright © 2005-2007 keep it simple science


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Slow, Fast&

ExplosiveCombustions.

Safety Issues

Originof

Fossil Fuels

Significance of

Photosynthesis

Natural Gas

Propertiesof

Alkanes & Alkenes

Alkanes & Alkenes

NamesFormulasStructures

Temperature&

Kinetic Energyof Particles

Models ofCatalystAction

Factorsthat Effect

Reaction Rate

• Temperature• Concentration• Size of solid

particles• Catalysts

Catalysts&

Activation Energy

ActivationEnergy

The Combustion

Reaction

Refiningof Petroleum

Allotropesof

Carbon

Safety Issues

Exothermic&

EndothermicPollution from

Fuel Combustion

IncompleteCombustion

The Variety of

Possible CarbonCompounds

ENERGY

High Energy Compounds&

Fuels

Carbon&

Carbon Compounds

Chemistryof

CombustionRate ofChemicalReactions

CONCEPT DIAGRAM (“Mind Map”) OF TOPICSome students find that memorizing the OUTLINE of a topic helps them learn and remember the concepts andimportant facts. As you proceed through the topic, come back to this page regularly to see how each bit fits thewhole. At the end of the notes you will find a blank version of this “Mind Map” to practise on.

Bond Breaking,Bond Making

Chemical Potential Energyis the energy stored within chemical compounds. It isstored in the bonds between the atoms, and every chemicalreaction results in the release, or absorption, of energy asbonds are broken and formed.

Some bonds store more energy than others. Many of thecomplex compounds of carbon are especially important ashigh-energy compounds.

Photosynthesisis the process by which plants make food. Photosynthesisis the most important Biological process on Earth, becauseit makes all the FOOD and OXYGEN for all living things.

This essential process captures the energy of the Sun, andstores it in the chemical bonds in the glucose molecule,C6H12O6.

Glucose is just one of many compounds known as“carbohydrates”... the sugars and starches. All are highenergy compounds made from or derived from glucose,which in turn is made from the low energy compoundscarbon dioxide and water.

Plants make their own carbohydrates; animals get theirs byeating their food via the food chains.

The Origin of the “Fossil Fuels”Coal, Petroleum and Natural Gas are the main fuels whichpower our industries, our homes and vehicles, and generateour electricity.

They are all mixtures of high-energy compounds becausethey are the fossil remnants of life-forms which livedmillions of years ago. They have been chemically altered bythe fossilization process, but are basically derived fromcarbohydrates originally made by photosynthesis.

Therefore, whenever coal, petroleum or natural gas areburned as fuels, the energy released is energy which cameoriginally from the Sun and was captured byphotosynthesis in some ancient plant cell.

Composition of Natural GasAs an example of just which high-energy compounds arepresent in a fossil fuel, the following table summarizes thecomposition of “Natural Gas” which is chemically thesimplest of the fossil fuels. (K.I.S.S. Principle!)

Compound Formula Typical % in Name Natural Gas

Methane CH4 90%Ethane C2H6 5%Propane C3H8 2%Butane C4H10 1%Other gases CO2, SO2, H2 2%

Location of Australian Deposits of Natural GasNatural Gas, or “Petroleum Gas” is usually associated with“Oil Fields” where petroleum (crude oil) is found. Themajor Australian deposits are marked on this map:

3


1. HIGH ENERGY COMPOUNDS & FUELS




WATER + CARBON GLUCOSE + OXYGENDIOXIDE

6H22O + 6CO22 C66H1122O66 + 6O22

chlorophyll

light energy

North WestShelfOil-ffield

Bass StraitOil-ffield

MMoooonniiee,, QQLLDD

MMoooommbbaa,, SSAA

MMeerreeeenniiee,, NNTT

GlucosemoleculeC6H12O6

green pigmentin chloroplastsof plant cells

fromsoil

fromair

high-energysugar (food)

toair

The Element CarbonCarbon is element number 6 in the Periodic Table.

Therefore, each atom has 6electrons.

ElectronConfiguration = 2.4

AllotropesCarbon can exist in several different physical forms called“allotropes”. Each allotrope is the same element,containing exactly the same atoms, but the atoms arearranged differently, giving each form quite differentphysical properties.

Some other elements that have allotropes include sulfur,phosphorus, oxygen and tin.

Allotropes of Carbon

Diamond is also a covalent lattice of carbon atoms, butthe atoms are arranged in a tetrahedral pattern, forming ahuge 3-D crystal lattice.

Diamond hasextremely high m.p.& b.p., and is thehardest naturalsubstance known.

The beauty of itssparkling crystalshas made diamondtreasured (literally)for jewellerythroughout history.

In modern times, its extreme hardness has resulted in thewidespread use of “industrial diamonds” for drill bits (e.g. for oil-drilling equipment to bore through solid rock)and high-speed saw blades.

“Bucky Balls”Discovered much more recently is a third allotrope ofcarbon... the Buckminster-Fullerenes. Named after thearchitect who invented the “geodesic dome” structure,Bucky Balls and Bucky Tubes come in a variety of shapesand sizes.

The best known has the formula C60, in which the carbonatoms are arranged to form a sphere resembling a soccer ball.

The Bucky Balls have not yet found a practical use, butthey have potential for use as high temperature lubricants,for making super-conducting polymers or even asspecialized “capsules” for administering medicines.

4


2. CARBON & CARBON COMPOUNDS




6

CCarbon

12.01

In Graphite, the atoms are arranged in hexagonal ringswhich connect to form flat sheets.

The atoms ineach sheet arestrongly bonded,(m.p. & b.p. arehigh) but thebonds betweenthe sheets arevery weak. Theycan easily slidepast each other,so graphite is“slippery”.

Because of its “slipperiness”, graphite is an excellentlubricant, used for example, in door locks. Its most familiaruse is the “lead” in a lead pencil.

Unusually for a non-metal and covalent lattice, graphite is agood conductor of electricity and is used in electric motorsin the rotating contacts called “brushes”; in this role it bothconducts electricity and helps lubricate the rotating axle.

C60

ALLOTROPES:Same element, same atoms.

Different atomic arrangements,different physical properties.



5

Carbon CompoundsAll life on Earth is based on carbon compounds, operatingwithin an aqueous (water) environment. We have goodreason to believe that, if there is life elsewhere in theUniverse, it will also be carbon and water based, althoughthe details of E.T.’s Chemistry, Biology and appearancecannot be predicted.

Water is the solvent without equal, and carbon is the onlyelement capable of forming the variety of intricate andcomplex molecules needed to make a living organism.

Carbon rarely forms ions. The immense scope of carbon’sChemistry is based on covalent bonding, and the key is thata carbon atom can readily form bonds with other carbonatoms, as well as (notably) hydrogen, oxygen and nitrogenatoms.

The HydrocarbonsWith so much variety of carbon compounds, where do youstart?

It turns out that many of our important fuels are composedof mixtures of the very simplest carbon compounds... theHydrocarbons. These are compounds containing onlycarbon and hydrogen.

Names & Formulas for the “Alkanes”

Methane

Molecular Formula CH4

Structural Formula

Ethane

Molecular Formula C2H6

Structural Formula

“CondensedStructural Formula” CH3CH3

Propane

Molecular FormulaC3H8

Structural Formula

“CondensedStructural Formula” CH3CH2CH3

See the pattern beginning to emerge?The Syllabus requires that you learn the first 8 compoundsin this series... the “Alkanes”



C C

C C

C C

Single BondA single C-C bond involves sharing one pair of electrons.Each carbon atom has 3 other bond positions available,allowing the formation of chains, rings and networks.

Double Bonds (sharing 2 pairs of electrons)andTriple Bonds (sharing 3 pairs) create even more possibilities.

The result is that carbon can form more possiblecompounds than all the other elements put together.

HHH

H

C

H HH

H

H

H

C C

Molecular Model

Molecular Model

H H HH

HH

H

H

C C C

Molecular Model

The ALKANESare HYDROCARBONS

containing only SINGLE C-C Bonds.

The Molecular Formulas form a pattern

CnH2n+2where n = number of carbon atoms




6

How to Name the AlkanesThe Alkanes are just one of many “Homologous Series” ofcarbon compounds. An Homologous Series means a groupof chemicals which form a logical series or pattern, interms of their molecular structure. As you’ll learn, theyhave similar properties which also follow patterns.

You must learn the following rules for naming:

Number of PrefixCarbon Atoms

1 Meth-2 Eth-3 Prop-4 But-5 Pent-6 Hex-7 Hept-8 Oct-

...there are many more,but this is as far as the Syllabus expects you to learn.

The first 3 were on the previous page. We now continue...

Butane


Structural Formula

“CondensedStructural Formula”

CH3(CH2)2CH3

Pentane


Structural Formula

“CondensedStructural Formula”

CH3(CH2)3CH3

Hexane C6H14 CH3(CH2)4CH3

Heptane C7H16 CH3(CH2)5CH3

Octane C8H18 CH3(CH2)6CH3

H H H

HH

H

H

C C CH

HC

HH

H

C

H H H

HH

H

H

C C CH

HC H

To name any ALKANE,just add “-ANE”

to the appropriate prefix above.

The Alkenesare another, similar Homologous Series.The Alkenes contain one C=C double bond.

Since there have to be at least 2 carbon atoms to have adouble bond, the series begins with

Ethene

Molecular Formula C2H4

Structural Formula

“CondensedStructural Formula” CH2CH2

Propene


Structural Formula

“CondensedStructural Formula” CH2CHCH3

Butene


Structural Formula

“CondensedStructural Formula”CH2CHCH2CH3

Pentene C5H10 CH2CH(CH2)2CH3

Hexene C6H12 CH2CH(CH2)3CH3

Heptene C7H14 CH2CH(CH2)4CH3

Octene C8H16 CH2CH(CH2)5CH3

The ALKENESare HYDROCARBONS

containing one DOUBLE C=C Bond.

The Molecular Formulas form a pattern

CnH2n

H H

HH

C C

H H H

H

H

H

C C C

H H H

HH

C C C

HH

H

C

Double C=C Bond

Double C=C Bond

Double C=C Bond

To name, just add “-ENE”to the appropriate prefix.



7

Properties of the Alkanes and AlkenesAs you know by now, the properties of any chemicalsubstance are determined by the bonding within.

Alkanes and alkenes contain only 3 types of bond withinthe molecules:

• Single C-C bonds• Double C=C bonds• C-H bonds

Therefore, you would expect these compounds to:

• have relatively low m.p. & b.p.’s and you’d• be insoluble in water be correct !• be non-conductors of electricity

To keep it simple, (K.I.S.S. principle) consider just theboiling points:

Alkane b.p. State Alkene b.p. State (oC) 25oC (oC) 25oC

Methane -162 gasEthane -89 gas Ethene -104 gasPropane -42 gas Propene -48 gasButane -1 gas Butene -6 gasPentane 36 liquid Pentene 30 liquidHexane 69 liquid Hexene 64 liquidHeptane 98 liquid Heptene 94 liquidOctane 126 liquid Octene 121 liquid

When these values are graphed the pattern becomesobvious, and also the great similarity between the alkanesand alkenes (at least for this property).

Inter-molecular Forces?Inside each molecule are strong, non-polar, covalent bonds.

However, the only forces between the molecules are thevery weak “Dispersion Forces”, so m.p. & b.p. aregenerally low.

Dispersion forces become stronger as the size and mass ofthe molecule increases, which explains the pattern of thegraph.

Another Important Physical Property:Volatility

A substance is said to be volatile if, at room temperature, itvaporizes readily. A simple indicator of volatility is theboiling point, but its not the only factor.

Obviously, the alkanes and alkenes with 4 or less carbonatoms are already gases at room temperature, but eventhose which are liquids are highly volatile.

Consider octane, a component of petrol. Its boiling pointis a little above that of water, but it is much more volatile.At room temperature, octane (petrol) in an open containervaporizes rapidly compared to water under the sameconditions.

The explanation is, again, all about bonding. Watermolecules tend to cling together because of the dipole-dipole attractions of hydrogen bonding. Octane moleculeshave only the weak “Dispersion Forces” holding them, andmany molecules have enough energy to escape into the gasstate, even at temperatures well below the boiling point.

The volatility of the alkanes and alkene has important...

Safety Consequences.We haven’t yet looked at the Chemical Properties ofalkanes and alkenes, but don’t forget that these are fuelcompounds... they contain a lot of energy, are highlyinflammable, and now we find out they are highly volatileas well! DANGER, DANGER!

Natural Gasis mostly methane.It must be stored inhigh pressure cylinders,outdoors and keptcool.

Propane & Butaneare used as bottled“BBQ gas”. They arestored as pressurizedliquids in pressurecylinders and usedoutdoors, or in well-ventilated areas only.

Small amounts of “smelly” chemicals are added to gas fuelsso that leaks are easily detected by smell.

Petrol, Kerosene & Dieselare highly volatile liquid fuels. They must be stored in sealeddrums or tanks, and all sparks or flames (even mobilephones) kept well away. All transfer of fuel from tank totank (e.g. filling the car) must be done outdoors.

SAFETY IS CHEMICAL COMMON SENSE



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Boili

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(oo C)-22

0000-11

000000

110000

11 22 33 44 55 66 77 88

No. of Carbon Atoms

AllkkaannessAllkkenness



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The Refining of PetroleumPetroleum deposits (including Natural Gas) are thefossilized chemical remnants of ancient sea creatures.Petroleum is a complex mixture containing alkanes, alkenesand many other carbon compounds, trapped insedimentary rocks.

To obtain the fuels we need, the first step is, of course, tolocate the petroleum and pump it to the surface.

The next step is to separate the petroleum mixture intomore useful “fractions”. This is achieved by...

Fractional Distillationwhich separates the mixture according to differences inboiling point.

Prac Work; Fractional DistillationYou may have done a laboratory exercise to separate amixture by fractional distillation. A common mixture usedis water and ethanol, a vastly simpler mixture thanpetroleum, but the principle is the same.

Ethanol’s b.p. = 78oC. Water’s b.p. = 100oC.

The mixture is heated gently until the vapour temperatureis about 80oC. At this temperature the “distillate” collectedfrom the condenser is relatively pure ethanol, perhapsabout 90%. Once all the ethanol has evaporated from themixture, the vapour temperature will rise to around 100oC,indicating that now water is being collected.

By watching the thermometer, and changing collectionbeakers at the right time, it is possible to collect 2 separate“fractions” from the original mixture.

In an Oil Refinery the same basic process separates crudepetroleum into many fractions... petrol, diesel, kerosene, etc.based upon different boiling points.

Each fraction is still a mixture. For example, petrol containsdozens of individual compounds, and the exactcomposition varies from place to place, and from time totime. It depends on which crude oil source is being refined,and on any adjustments made by the chemical engineers atthe refinery.



Oil Well Pump,Alberta, Canada

Oil RefineryFractionating Towers

Fractional Distillationthermometer monitors

temp. of vapours.

MixtureCondenser

HHeeaatteerr

Tem

pera

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dec

reas

es

up t

he t

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PPeettrrooll

HHeeaattiinngg &&FFuueell OOiill

LLuubbrriiccaattiioonnggOOiillss

RReessiidduuee::GGrreeaassee &&BBiittuummeenn

DDiieesseell &&KKeerroosseennee

Fractionating Tower in a Refinery

SimplifiedSchematicDiagram

Photo: Nick Fletcher

Photo by Diana

VaporizedCrude

Oilinjected

GGaass ffrraaccttiioonnLL..PP..GG..

Vapours rise through

the tower

VVaappoouurrss ccoonnddeennssee oonnccoolllleeccttiioonn ttrraayyss......

aatt ddiiffffeerreenntt lleevveellss......

aaccccoorrddiinngg ttoo bbooiilliinngg ppooiinntt

Worksheet 1

Fill in the blank spacesChemical a).................................. Energy is the energy storedin the b)........................................................... within acompound. Biologically, all high-energy compounds arecompounds of c)....................................

In all living things, chemical energy came originally fromthe d)....................... The plants capture the energy duringe)......................................................... and store it initially inthe compound f)......................................, a member of theg)................................................. group which includes all theh)................................... and .........................................

Photosynthesis is the most important Biological process onEarth because it produces all the i).....................................and j)..................................... for all living things.

Coal, k)................................... and Natural Gas are all“l)................................... fuels” because they are thefossilized remains of ancient living things. When thesefuels are burned, the energy released came originally fromthe m).................... via the process of n)...................................in plants.

Natural Gas is associated with o).......................................deposits. Its composition is about 90% p)..............................

Worksheet 2Part A Fill in the blank spaces.

Carbon is element No.6 and therefore has electronconfiguration a)...................... Carbon has 3 allotropes;b)......................................., ........................................... and theso-called “buckyballs”. Allotropes are different physicalforms of the same element. Allotropes are composed ofc).............................. atoms, but they are d)................................differently, giving each allotrope different e)...............................................................

Carbon’s 2 main allotropes have different properties whichdetermine their uses:• In Graphite, the atoms are arranged in f)..............................which can slide easily, so it is used as a g)...............................It is also a good electrical h)..........................................., it isused in the rotating contacts in i).............................................Its soft, slipperiness and black colour make it suitable forits major use, as the “j).........................” in a pencil.

• In k).................................. the atoms are arranged in a 3-Dl)......................................... (shape) pattern, producing a hardcrystal which has been prized for m).................................throughout history. Its extreme hardness makes it usefulfor n)..............................................................................................

The huge variety of carbon compounds is due to the abilityof carbon atoms to each form o)................. (number) of

p)................................ bonds. Carbon atoms readily bondwith each other forming q).........................., rings ornetworks. They can form single, r)......................... or............................ bonds.

The simplest carbon compounds, containing only carbonand hydrogen, are collectively called thes)............................................., of which the t)............................and the u)........................................ are 2 types. Hydrocarbonsare named by using a prefix for the number of v)............................................., and adding an ending to identify whichprecise type it is.

The Alkanes have names ending in “w).......................”. Theyall contain x)............................... C-C bonds and have ageneral formula y).................................... Alkanes arez)............................................. in water and have relativelyaa)....................... melting and boiling points. Theirproperties relate to their molecules being non-ab)......................... The only inter-molecular forces are thevery weak ac)............................................ forces. Melting andboiling points show a very regular pattern of rising steadilyas molecular size ad).....................................

The Alkenes have very ae)................................ properties tothe alkanes. Alkenes contain one af)........................................bond, and have general formula ag)....................................

Each of these “types” form an ah)........................................Series, with the same general formula and similarproperties.

Another property of alkanes and alkenes is that they arehighly volatile. This means that they tend to ai).......................readily. This has important safety consequences for thesehighly inflammable, aj)......................-energy compounds.

Crude petroleum is a complex ak)............................ of manycompounds. At a refinery, it is separated into useful“al).............................” by the process of am)............................................................................... The crude mixture isvaporized and as the vapours rise and the temperaturean)............................., each fraction ao)...................................at a different level in the tower.

Part B Practice Questions1. There are other Homologous Series which have differentname endings, but the prefixes follow the same rules.How many carbon atoms in:a) pentanone? b) butanal?c) propanal? d) octanone?e) methanoic acid? f) hexanol?

2. What is the molecular formula, structural formula andcondensed structural formula for “nonene” and “decane”,given that the prefixes “non” = 9 and “dec” = 10.

9





Indications of a Chemical ChangeBy now you should be aware that when any chemicalreaction occurs, one or more of the following will beobserved:

• reactant substance(s) will “disappear” or be eaten away.• new substance(s) will appear.• colour changes may occur.• precipitates may occur in a solution.• a gas may be evolved (bubbles, if in a liquid).• there will be significant energy changes, observed aschanges of temperature, or in the case of combustionreactions, as visible flames.

Combustion Reactionsoccur when something burns. Combustion always involvesa “fuel” compound combining with oxygen to form oxidecompound(s) of whatever elements are present in the fuel.

An example is a reaction you studied in an earlier topic:

magnesium + oxygen magnesium oxide2Mg(s) + O2(g) 2MgO(s)

Combustion reactions are always exothermic, releasingchemical energy. In many cases, there is a visible flamewhich is a region of gas (in which the reaction is occurring)heated up by the energy release so that it glows.

Our most common and important fuels are mixtures ofhydrocarbons.

Natural Gas and L.P.G. (Liquified Petroleum Gas) containMethane:

methane + oxygen carbon dioxide + waterCH4(g) + 2O2(g) CO2(g) + 2H2O(g)

Petrol is a mixture of many hydrocarbons, but we can useOctane as an example:

octane + oxygen carbon dioxide + waterC8H18(g) + 25 O2(g) 8CO2(g) + 9H2O(g)

2

Note: When necessary, it is usual to balance the equationwith a fractional mole quantity of oxygen so that theequation shows 1 mole of fuel. (Simply add total oxygenatoms on the right side. If an odd number, write thisnumber over 2, as the coefficient for O2(g))

Changes of State During CombustionNotice that in both the previous examples all the reactantsand products are shown as being in the gas state. We areused to water being a liquid, but due to the heat producedby combustion, the water always forms as water vapour,H2O(g).

Also, notice that octane (a liquid at room temperature) isshown as a gas because the liquid vapourizes before thecombustion reaction begins.

Combustion generally involves reactants and products in the gas state.

Change of Mass in a CombustionIf combustion occurs in theopen, the gas products (oftencarbon dioxide and watervapour) escape and disperseinto the surroundings.

You may have done a simplepractical experiment involving weighing a match before,and after burning. You would have found that it lost mass,due to the escape of the reaction products.

If carried out in a sealed container, there is no masschange, since the same atoms are still present, but simplyrearranged to form the reaction products.

10


3. THE CHEMISTRY OF COMBUSTION




“Fuel”

“Fuel”

Oxygen

Oxygen

Oxide compound

Oxide compounds ofthe elements

in the fuel

+

+

A Burning CandleYou may have done a simple practical exercise toexamine carefully a burning candle to study the changesof state occurring.

The wax melts, soaks upinto the absorbent wick,and vaporizes before itactually burns.

Some fuels, just like wax,will not burn easily unlessa wick is provided to allow easy vaporization so the fuelvapour can ignite. Kerosene and heating oils are likethis. Visible flame is the

zone wherecombustion is

occurring

zone of vaporization

Wick absorbsliquid

Molten, liquidwax

Solid wax



11

Break Bonds, Make BondsAll chemical reactions involve rearranging the atoms of thereactant molecules. This must involve 2 stages:

• Breaking the bonds in the reactant molecules, so theatoms can be rearranged, and• Making new bonds to create the product molecules.

For example, in the combustion of methane:

CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

the reactants are methane and oxygen molecules:

Energy must be supplied to break these bonds.

Once the atoms are freed from the reactant molecules, theycan begin combining into new molecules to form theproducts.

The making of these new bonds always releases energy.

Activation EnergyThe reaction cannot begin until some energy is supplied tobreak some bonds in reactant molecules. This energyrequirement is called the “Activation Energy”.

Once a combustion reaction gets started, the energyreleased by the exothermic reaction provides the activationenergy for other molecules, so the reaction continues.

It is this activation energy requirement that explains whyfuels, like petrol, do not just spontaneously burst into flamewhen exposed to oxygen in the air. There has to be a sparkor flame to ignite them.

The temperature at which a fuel-air mixture will ignite iscalled the Ignition Temperature. Its value is related to theActivation Energy required to get the combustion started.

Fuel Ignition Temperature (oC)Petrol 500 approxDiesel 250Methane (Natural Gas) 540

Endothermic & Exothermic ReactionsThese concepts have been introduced in previous topics.

Now you must learn more detail about the shape of theenergy profile diagrams introduced previously.

Exothermic Reactions release energy because theproducts contain less Chemical Potential Energy than thereactants. Because the chemicals lose energy, the energychange (ΔH) is negative.

Here is the Energy Profile Diagram as you’ve seen itbefore:

Now we must add the idea of Activation Energy:

The Activation Energy is like a “hill” or energy barrierwhich must be overcome before the chemicals can turninto products. The Ea is the energy needed to break somebonds in reactant molecules so the atoms can begin torearrange themselves. Remember, that this is connected tothe value of the “Ignition Temperature” of a fuel.

These 2 fuels release the same amount of energy (ΔH is thesame). The one on the left has a lower Ea and therefore alower Ignition Temperature than that on the right.



Bondsmust bebroken...Energyneeded

New bondsare made.Energyreleased

Reactants EnergyLevel

Ener

gy C

onte

nt

Products haveLESS energy

Energyreleased bychemicalsduringreaction

= ΔΔHnegative

Reactants

Ener

gy C

onte

nt

Products

ΔΔH

Ea Eaa = Activation Energy

Ener

gy C

onte

nt

ΔΔH

Eaa

Ener

gy C

onte

nt

ΔΔH

Eaa



12

Endothermic Reactionsare those that require energy to proceed. The productshave more Chemical Potential Energy than the reactants.Since the chemicals have gained energy, ΔH is consideredpositive.

These reactions also have an Activation Energyrequirement.

Practical WorkYou may have carried out experiments to observe anddescribe various exothermic and endothermic reactions.

In this topic, you have seen combustions, while back intopic 2 you saw active metals react withwater or acids. In topic 3 you measured

the heat releasedduring dissolving

These were all Exothermic.

Way back in topic 1, you studiedand observed electrolysis causing water to decompose

when electrical energy was added. Intopic 2 you decomposed carbonates byheat and studied the Endothermicextraction of metals from ores.

Incomplete CombustionEarlier, you learned about the combustion of typical fuelcompounds like octane, a component of petrol:

octane + oxygen carbon dioxide + waterC8H18(g) + 25 O2(g) 8CO2(g) + 9H2O(g)

2

This reaction describes complete combustion.

In many real situations however, there is not enoughoxygen available, or the fuel and oxygen do not get mixedthoroughly enough for all the molecules to “find” eachother in the time available. In these cases, the combustionmay be incomplete:

lessoctane + oxygen carbon monoxide + waterC8H18(g) + 17 O2(g) 8CO(g) + 9H2O(g)

2

If there is even less oxygen, or really poor mixing ofmolecules, the reaction can be even more incomplete:

lessoctane + oxygen carbon + waterC8H18(g) + 9 O2(g) 8C(s) + 9H2O(g)

2

In this case, the solid carbon formed is “soot”, oftenobserved from diesel exhausts, burning candles, andbunsen burners set to a yellow flame.

In real situations, the total result of burning a fuel can besome combination of all the 3 situations above... somemolecules burn completely forming CO2, and some burnincompletely to CO or even soot.

Try the Worksheet at the end of this section



Reactants

Ener

gy C

onte

nt

Products

ΔΔHpositive

Ea

Eaa = Activation Energy

EndothermicReactions

Incomplete Combustionof carbon-based fuels

results in the formation ofCarbon Monoxide and/or “soot”



13

Pollution from Combustion of FuelsWhen pure hydrocarbon fuel compounds burn completelythe products are carbon dioxide and water vapour. Neitherof these can be considered as “pollution” since they arepart of the natural chemical cycles on Earth.

Carbon DioxideHowever, the release of billions of tonnes of CO2 fromfossil fuel combustion is believed to be leading to a“Greenhouse Effect”, and causing global warming. Thismay be leading to (is leading to?) major environmentalchanges, such as alterations to climates and weatherpatterns.

The Greenhouse Effect can be minimized by reducing theusage of fossil fuels, and switching to fuels which do notincrease CO2 levels, such as ethanol from plant sources.

Toxic pollution results from either incomplete combustion,or from impurities in the fuels.

Sulfur DioxideFossil fuels, especially coal, contain small amounts of sulfurcompounds. When these are burnt with the fuel:

sulfur + oxygen sulfur dioxideS + O2 SO2(g)

This gas is a serious pollutant because it reacts with waterin the environment to form a strong acid. This can lead to“Acid Rain” which can destroy natural environments suchas forest and lake ecosystems.

Emission of SO2 is minimized by• removing sulfur impurities during the refining of liquid

and gas fuels.• “scrubbing” the exhaust emissions from coal-burning

power stations and sulfide-ore metal smelters. The SO2can be collected and used to make sulfuric acid forindustry.

Carbon Monoxideis produced in all petrol engines due to incompletecombustion of the fuel. It does not cause any large-scaleenvironmental damage, but is very toxic to people.

Oxides of NitrogenOxygen and nitrogen in the air do not normally react witheach other. However, at the high temperatures inside anengine, they react:

nitrogen + oxygen nitric oxide N2(g) + O2(g) 2 NO(g)

Nitric oxide (or nitrogen monoxide) reacts further to formtoxic nitrogen dioxide, NO2(g), and is involved in theformation of “petrochemical smog” which causes a toxicand serious pollution problem in cities.

To avoid the problems caused by both carbon monoxideand the oxides of nitrogen, modern cars are fitted with“catalytic converters” on their exhaust systems. Thesepromote the reaction:

2NO(g) + 2CO(g) N2(g) + CO2(g)

Modern car engines also have advanced fuel-air controlsystems to ensure the most efficient, complete combustionpossible. These measures have been very successful inreducing air pollution problems in some cities.

The final solution to the problems associated with burningfossil fuels may be solved for us when petroleum suppliesrun out. Petroleum is a non-renewable resource, and manyestimates suggest that world supplies will only last about 30years, at current usage rates.

In 2005, the world economies were shocked by a sudden“spike” in the price of petroleum. It is highly probable thatsuch events will continue to happen (for both political andeconomic reasons). This is certain to encourage researchand development of “alternative”, sustainable fuels. It’salready happening...

Now there’s a career idea for today’s Chemistry students!



Photo: Daniel West

Photo: Ken Kiser

Worksheet 3Part A Fill in the Blank Spaces

The indications that a chemical change hasoccurred are:• reactants a)..........................................• b)..................................... appear.• c)................................. changes may occur.• in a solution, a d)................................ may appear.• a e)............................ may be evolved.• there may be significant f).............................changes.

Combustion reactions always involve 2 reactants:the “g)...............................” and ...............................The product(s) of the reaction are always theh)........................... compound(s) of the elementsin the fuel. For all the common hydrocarbonfuels, the products of complete combustion arei)............................................. and ..............................In general, hydrocarbon fuels need to be in thej)............................ state before they will ignite.Many fuels, such as k).................................... need awick to help them vaporize.

For a reaction to get started, energy must besupplied to l).............................. some of thebonds in m)........................... molecules. Thisenergy requirement is called the“n)............................................... Energy”. Theatoms can then form new bonds to form theo)...................................... of the reaction. Makingnew bonds always p)......................... energy whichcan provide the activation energy for furtherbond breaking so the reaction becomesq).............................. ...................................

On an “Energy Profile Diagram”, activationenergy is like an energy “r).....................................”which must be overcome before the reaction canproceed. For fuels, the activation energy is relatedto the value for “s)..............................................................”, which is the temperatureat which a fuel-air mixture will ignite.

Incomplete Combustion occurs when there isinsufficient t)............................................ available,or when the fuel and air are notu)............................................. thoroughly. Inhydrocarbon fuels, incomplete combustion leadsto the production of v).............................................gas, or even particles of solidw).....................................

Large scale burning of fossil fuels has added somuch extra x)................................................. to theatmosphere that this has resulted iny)................................. warming due to thez)........................................... Effect. Fuelscontaining sulfur will produceaa)........................................... when burned, andthis can cause “ab)........................ Rain”. Petrolengines produce ac)..................................... gasfrom incomplete combustion, andad)................................................ gas from the hightemperature reaction of ae).................................and ........................................ NO(g) is involved inthe production of “af)..............................” in cities.

Part B Practice Questions

1. Equations for CombustionWrite a balanced equation for the completecombustion of

a) calcium metalb) hydrogen gasc) ethaned) ethenee) pentanef) penteneg) propane

2. Equations for Incomplete CombustionWrite a balanced equation for the incompleteburning of

a) hexane, if all the carbon atoms form carbon monoxide.

b) hexane, if the carbon atoms form CO2 & CO in equal proportions.

c) hexane, if CO and “soot” are formed in equalproportions.

d) butane, if CO2 & CO are formed equally.

3. Energy Profile DiagramsLabel each diagram to show ΔH and Ea.Sketch diagrams to represent:

a) an exothermic reaction with large ΔH andsmall Ea.b) an exothermic reaction with small ΔH andlarge Ea.c) an endothermic reaction.d) Which diagram (a, b or c) could describe a fuelwith a high ignition temperature? Explain youranswer.

14





Slow, Fast and Explosive CombustionsCombustion reactions can occur at different rates,depending on conditions.

In a camp fire, or slow combustion stove, large pieces ofsolid fuel (e.g. wood) burn slowly and steadily. There’splenty of fuel, so why is the reaction slow?

Although there is a large piece of fuel available, it can onlyburn at the surface where oxygen can get to it. The reactionrate is limited by the surface area available. If the wood hadbeen chopped into small “chips”, there would be a greatersurface area and it would burn faster, assuming plenty ofair could get to it. (Solid fuel heating stoves are oftenslowed right down by limiting the amount of air getting in.)

Fast CombustionsIn a gas stove or bunsen burner,the fuel is mixed with air beforeignition. The fuel and oxygenmolecules are intimately mixedtogether, then ignited as theyreach the burner nozzle.

The reaction is fast, and limitedonly by the flow of fuel-airmixture to the burner.

The point is that the rate of a combustion reactiondepends on the fuel and oxygen molecules colliding witheach other

Explosive Reactionsoccur when fuel molecules are well mixed with air, andthere is no limit on the flow of fuel to the combustion area.For example, if a gas leak fills a room with a fuel gas mixedwith air, any spark or flame can start a reaction thataccelerates so fast that the heat release and gas pressurecreates a shock wave... it explodes!

Its not just gas molecules that can mix with air to formexplosive mixtures. Fine dust particles suspended in air canexplode too, if the particles are at all capable ofcombustion.

Explosions have occurred with wheat dust, coal dust,cotton fibres and even custard powder, in factories wheredust or fibre was allowed to mix with air.

The safety consequences are obvious. Any workenvironments where dust can be produced need to ensuredust supression, good ventilation, air filters, or whatever isnecessary to prevent explosive mixtures forming in the air.

15


4. RATE OF CHEMICAL REACTIONS




O2

CO2

CH4

H2O

collision

energyreleased

The RATE of a Chemical Reactiondepends on the reactant molecules

COLLIDING

Any situation which increases the Rate of Collisionswill increase the

REACTION RATE

Reaction




16

Temperature & Kinetic Energy of ParticlesAccording to the “Kinetic Theory of Matter”, all matter iscomposed of particles (molecules, atoms or ions) which areconstantly moving.

• In solids, the particles can only vibrate in one place.• In liquids, they are close together, but move around.• In gases, they are far apart and flying in all directions.

As temperature increases, the movement increases.Increased heat energy causes an increase in the kineticenergy of the particles.

Therefore, at higher temperatures the particles move fasterand are more likely to collide with enough “activationenergy” for the reaction to proceed. The result is that:

The Effect of ConcentrationIf a chemical reaction is occurring in solution, theconcentration of the reactant(s) can affect the chances of acollision, and control the reaction rate.

At higher concentration, the chances of reactants collidingis increased, so reaction rate is higher.

In reactions involving gases, higher gas pressure increasesthe concentration of the particles, by forcing them closertogether. So gas pressure also increases reaction rates.

TEMPERATUREis a measure of the average

KINETIC ENERGYof the particles

The RATE of Chemical ReactionsIncreases with TEMPERATURE

Practical Work:Temperature & Rate of Reaction

You may have done experiments in the laboratory toobserve and measure this relationship.

Many reactions are suitable for study; a simple one is:

Mg(s) + 2HCl(aq) H2(g) + MgCl2(aq)

Three identical test tubes ofthe same acid solution can beset up at differenttemperatures.

Different temperatures areachieved by immersing eachtube in a beaker containing

• ice and water• tap water• hot water

Identical pieces of magnesiumare dropped into each tube.The rate of the reaction canbe observed by the rate of bubbling (of H2 gas). Tomeasure the rate, the time taken for the magnesium to“disappear” could be measured. (For graphing purposes,the reciprocal (1/time taken) can be used as a value for “rateof reaction”.)

You would find that higher temperature faster rate of reaction

Concentration Low,collisions less likely,

Rate of ReactionSlow

PPrroodduucctt

PPrroodduucctt

PPrroodduucctt

The RATE of Chemical ReactionsIncreases with CONCENTRATION

or with GAS PRESSURE

More Practical Work:Effect of Concentration

Using the same reaction as on the left, you could set upthree test tubes of acid at different concentrations by usingthe same acid, but diluting the solution in each tube:

Tube 1 Tube 2 Tube 3Vol. of acid (mL) 10 10 10Vol. of water (mL) 20 10 0

30 20 10Note that the amount of acid is the same in each tube, butthe concentrations are different.

Identical pieces of magnesium could then be droppedinto each tube, and therate of reactionobserved or measuredas before.

You would find thatTube 3 would reactthe fastest.



17

CatalystsA “Catalyst” is a chemical substance which increases therate of a reaction, without being consumed or permanentlychanged by the reaction.

Catalysts are widely used in Industrial Chemistry (examplesat right) but perhaps the most notable example of catalystsis in Biology. All living cells carry out thousands ofchemical reactions which would occur far too slowly for lifeto function if not for catalysts. In every cell, every reactionis made possible by protein catalysts called “Enzymes”.

Catalysts Lower Activation EnergyThe effect of catalysts is to reduce the Activation Energyrequirement for the reaction.

If the Activation Energy requirement is lower, then at anygiven temperature there will be more reactant moleculeshaving the energy to react... so the reaction goes faster.

How Catalysts WorkThere is no single, simple model to explain how a catalystspeeds up reactions by reducing the Activation Energy.There may be dozens of ways that various catalysts work;here are just two.

Model 1: Surface AdsorptionSome catalysts work by holding the reactant(s) in such away that collision and reaction become more likely. In somecases too, the bonds within a reactant molecule are placed“under strain” so that less energy is required to breakbonds to start the reaction process.

Many Industrial processes rely on catalysts of this type:

• In the manufacture of Ammonia

the reaction rate is speeded up using an iron catalyst, finelydivided to provide greater surface area.

• In the “Catalytic Cracking” of petroleum compounds atan oil refinery (to be studied in a later topic) largerhydrocarbon molecules are broken up to increase the yieldof the valuable petrol fraction. The reaction is speeded upby a catalyst of Zeolite; a natural clay mineral with a largesurface area, and an affinity for hydrocarbon molecules.

Model 2: Alternative Chemical PathwaysSome catalysts work by providing an alternative chemicalpathway which has a lower Activation Energy. In thesecases, the catalyst might be a reactant in a series ofreactions which then “regenerate” the catalyst at the end.

For example, the reaction

2SO2(g) + O2(g) 2SO3(g)

is very slow, but if a small amount of NO2(g) is added to themixture, 2 faster reactions occur in sequence:

Reaction 1: SO2(g) + NO2(g) SO3(g) + NO(g)

Reaction 2: 2NO(g) + O2(g) 2NO2(g)

The NO2 is regenerated at the end, and has not beenconsumed... it is a catalyst for the reaction, and each NO2molecule can be recycled over and over.



Reactants

Ener

gy C

onte

nt

Products

Without Catalyst

With Catalyst.Activation Energyreduced

NNoottee tthhaatt ΔΔHH iiss NNOOTT aaffffeecctteedd

ΔΔH

Ea

PPrroodduuccttmmoolleeccuullee

RReeaaccttaannttmmoolleeccuulleess

BBoonndd ppllaacceedd uunnddeerrssttrraaiinn bbyy aaddssoorrppttiioonn

ttoo ccaattaallyysstt

RReeaaccttaannttmmoolleeccuullee((ss))aaddssoorrbb ttoo ssuurrffaaccee

CCoolllliissiioonn&&

RReeaaccttiioonn

CATALYST

N2(g) + 3H2(g) Fe catalyst 2NH3(g)

Effect of Surface Area on Reaction RateYou may have done a simple experiment as suggested bythe diagram:

The reaction occurring is

CaCO3(s) + 2HCl(aq) CO2(g) + H2O(l) + CaCl2(aq)

You would have found that the reaction occurs faster withthe powdered solid than with larger lumps. This is becausethe smaller solid pieces in the powder add up to a largertotal surface area for the acid to react with. Collisionsbetween reactants occur more often, so the reactionproceeds faster.

Same quantity of same strength acid

Same quantity of solid calcium carbonateon each spoon

LumpsPowder

Both lumpsand powderreact with

acid inexactly thesame way,

but youwould

observe thatthe ppoowwddeerr

rreeaaccttss ffaasstteerr..



18

Practical Work: CatalystsYou may have carried out a practical experiment to see theeffect of a catalyst on a reaction rate.

A popular experiment is to use the familar reaction

Mg(s) + 2HCl(aq) H2(g) + MgCl2(aq)

with, and without a catalyst.

The catalyst used is a smallpiece of copper metal, which astrip of magnesium ribbon iswrapped around. (The metalsmust be in contact.)

In contact with the copper, itwill be found that themagnesium reacts much fasterthan an identical experimentwithout the copper.

At the end of the reaction, the copper is totallyunchanged... it speeds the reaction up, but is notconsumed or changed itself; it is a catalyst.

The method by which the copper catalyses the reactiondoes not fit either of the “models” described on theprevious page.

Another Exampleof an experiment on catalysts uses the following reaction:

hydrogen + iodide + hydrogen iodine + waterperoxide ions ionsH2O2(aq) + 2I-

(aq) + 2H+(aq) I2(aq) + 2H2O(l)

The reaction is quite slow, but can be followed visually overseveral minutes because while the reactants are colourless,the iodine product is a yellow-brown colour.

If a few drops of a dilute solution of sodium molybdate(NaMoO4) is added to another identical mixture, thereaction proceeds to completion much faster... a fewseconds compared to many minutes.

The molybdate ion (MoO4-, a polyatomic ion) acts as a

catalyst and remains in the solution unchanged at the end.

It works by providing an alternative sequence of lower-activation energy reactions, which regenerate the molybdateion at the end.



ColourlessReactants

Solutiondarkens asProduct I22forms

Worksheet 4Fill in the Blank Spaces

The rate of a combustion reaction depends onthe reactant molecules a)................................. witheach other. In a slow combustion, such as theburning of b)............................... ..............................,the reaction only occurs at the surface wherec)..................................... can reach the fuel. In agas stove or bunsen burner the fuel and air aremixed before d)......................................... The rateof reaction is limited only by the flow ofe).................... & ................................ to the burnernozzle.

If a fuel-air mixture is allowed to build up, a sparkor flame can result in anf).......................................... Any combustible gasor even g)............................ can become anexplosive mixture if allowed to mix with air. Thishas important h)...........................................implications for workplaces which produce dustor fibres, such as i).................................,.................................., etc. (examples)

According to the “j)................................. Theory ofMatter”, all substances contain particles whichare constantly k)................................... Adding heatenergy causes the particles to l)......................................................... Temperature is a measureof the average m).................................................. ofthe particles.

An increase in temperature causes all chemicalreactions to n).............................................................This is because the particles will moveo)......................................, and the chances ofp)........................................... increases. Also, thereactant molecules are more likely to have thenecessary q).................................. energy at highertemperatures.

Higher r)......................................... also increasesreaction rates, by making it more likely thats)...................................... will occur. In gases, thet)............................................. of the gas isequivalent to the concentration.

If solid substances are involved, then reactionrate can be increased by increasing theu).................................................... of the solid, suchas by v).......................................................... .............

A “catalyst” is a substance whichw)..................................... the rate of a reaction,without x)............................................... or............................................ by the reaction. Inliving things, all chemical reactions are controlledby catalytic proteins called y).....................................Catalysts effect the energy profile of a reaction;they have no effect on the value ofz)......................, but reduce theaa).............................. .................................. Thismeans that, at any given temperature, there aremore reactant molecules with enoughab)............................................ to commence thereaction, so the ac).................................................increases.

Some catalysts work by ad).......................................reactant molecules to their surface in such a waythat collisions become moreae)..................................... Chemical bonds mayalso be placed under af)......................................which makes it more likely that they can beag)................................... to begin the reaction.This type of catalyst is common in industry, suchas the use of ah).................................. catalyst inthe production of ammonia, and the use of theclay mineral ai)......................................... in the“aj).......................... Cracking” in petroleumrefining.

Other catalysts work by providing anak)................................ ......................................pathway from reactants to products. In thesecases, the catalyst actually takes part in thereactions, but is al).......................................unchanged at the end.

19





WHEN COMPLETED, WORKSHEETSBECOME SECTION SUMMARIES




20

ENERGY

CONCEPT DIAGRAM (“Mind Map”) OF TOPICSome students find that memorizing the OUTLINE of a topic

helps them learn and remember the concepts and important facts.Practise on this blank version.

Practice QuestionsThese are not intended to be "HSC style" questions, but tochallenge your basic knowledge and understanding of thetopic, and remind you of what you NEED to know at theK.I.S.S. Principle level.

When you have confidently mastered this level, it isstrongly recommended you work on questions from pastexam papers.

Part A Multiple Choice

1.The chemical raw materials for photosynthesis areA. water & oxygenB. glucose & oxygenC. carbon dioxide & glucoseD. water & carbon dioxide

2.When a fuel such as coal is burned, large amounts ofenergy are released. This energy was previously in the formofA. low energy compounds such as CO2 and water.B. chemical energy stored in carbohydrates.C. geothermal energy from mineral formation.D. chlorophyll used in photosynthesis in ancient plants.

3.Allotropes of an element:A. contain identical atoms in different arrangements.B. are different atoms arranged in the same way.C. have the same physical properties as each other.D. have the same appearance and uses.

4.The molecular modelshown isA. penteneB. buteneC. propaneD. butane

5.The compound with molecula formula C15H32 is likely to:A. be a member of the alkene homologous series.B. contain a triple carbon-carbon bond.C. have a name ending in “-ANE”.D. have polar molecules.

6.Which of the following is NOT a property of the alkanes?A. soluble in waterB. relatively low melting & boiling pointsC. non-conductors of electricityD. hydrophobic

7.In petroleum refining, Fractional Distillation separates themixture according to differences in:A. melting pointB. densityC. solubilityD. boiling point

8.If the compound hydrogen sulfide (H2S) underwentcombustion, you would expect the products to be:A. carbon dioxide and waterB. water and oxygenC. sulfur dioxide and waterD. hydrogen and sulfur dioxide

9.The correctly balanced combustion equation is:A. CH4(g) + 3O2(g) CO2(g) + H2O(g)

2B. CH4(g) + 3O2(g) CO2(g) + 2H2O(g)

C. CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

D. CH4(g) + 3O2(g) 2CO2(g) + 2H2O(g)

10.During a chemical reaction, some chemical bonds arebroken and others are formed. The correct statement is:A. energy is released by both making & breaking of bonds.B. energy is released when bonds are made, and absorbed

when bonds are broken.C. energy is released when bonds are broken, and absorbed

when bonds are made.D. energy is absorbed by both making & breaking bonds.

11.The diagram shows an energyprofile for a chemicalreaction. It would be true tosay that the reaction is:A. exothermic,

and Ea = +20kJ/mol.B. endothermic,

and Ea = +40kJ/mol.C. exothermic,

and Ea = +40kJ/mol.D. endothermic,

and ΔH = -10kJ/mol.

12.A major problem associated with large scale use of fossilfuels is:A. the toxic effects of CO2 gas.B. SO2 gas release which causes petrochemical “smog”.C. carbon monoxide destruction of the ozone layer.D. global climate change due to CO2 gas release.

13.The risk of an explosive combustion is increased by:A. increasing the particle size of solid fuels.B. decreasing the collision rate of reactant molecules.C. storing liquid fuel in larger tanks or drums.D. allowing build-up of dust or fibres in the air.

21





Ener

gy C

onte

nt (

kJ/m

ol)

0

10

20

30

40

14.The rate of a chemical reaction could NOT be increased byA. increasing the temperature.B. reducing the pressure of a reactant gas.C. adding a catalyst.D. increasing the concentration of a reactant solution.

15.Catalysts can:A. decrease ΔH of a reaction by providing alternative

chemical pathways.B. speed reactions up by increasing Ea.C. lower Ea by increasing reactant collisions.D. cause different products to form, via different pathways.

Longer Response QuestionsMark values shown are suggestions only, and are to giveyou an idea of how detailed an answer is appropriate.

16. (6 marks)Using a named fossil fuel as an example:a) give the name, and formula for a significant compoundwithin the fuel.b) justify the use of the term “fossil fuel”.c) explain how the fuel comes to have energy stored withinit, including mention of the origin of that energy.

17. (6 marks)For each of the 2 main allotropes of carbon:a) list the significant physical properties of the substance.b) explain how a named use of the allotrope is linked to itsphysical properties.

18. (6 marks)a) Write the molecular formula for hexane.b) Draw the structural formula for pentene.c) Write the condensed structural formula for heptane.d) Name the following compounds:

i) CH2CH(CH2)5CH3ii)

iii) C2H6

19. (5 marks) Use this grid to help answer the questionfollowing.

19. (continued)The boiling points (oC)for some alkenes:

ethene -104propene -48hexene 64heptene 94

a) Plot the data as points on the grid provided.b) Use the graph to estimate b.p.’s for butene and pentene.c) Account for the fact that the alkenes have low b.p.’srelative to some other similar sized molecules.

20. (5 marks)a) What is meant by the property of “volatility”?b) Octane, a compound found in petrol, has a boiling pointcomparable to that of water, but is much more volatile.Explain why.c) Outline any safety concerns and precautions arising fromthe volatility of octane and associated compounds inpetrol.

21. (5 marks)Give an outline of the key process in the refining ofpetroleum, including a list of some of the main products ofthe process.

22. (4 marks)In an experiment, a student burnt 2 identical pieces of coal;one in an open container, the other in a sealed containerwhich had an adequate supply of air within it.

The containers (including pieces of coal) were weighedbefore and after burning. One was found to lose mass, theother did not change.

Identify which container lost mass and account for thedifferent results between the 2 containers.

23. (9 marks)Write a balanced equation to describe:a) the complete combustion of butane gas.b) the complete combustion of ethene gas.c) the incomplete combustion of hexane, assuming that 2different oxides of carbon are formed in equal molarquantities.

24. (4 marks)During the combustion of petrol and diesel fuels bothrelease approximately the same amount of energy (per unitof mass). However, petrol has a significantly higher ignitiontemperature.

Sketch an energy profile diagram to compare these twofuels. No values are required, but the diagrams should belabelled to emphasize any important comparisons.

22





H H H

H

H

H

C C C

Boili

ng P

oint

(oo C)-22

0000-11

000000

110000

11 22 33 44 55 66 77 88

No. of Carbon Atoms

25. (5 marks)a) Name a toxic pollutant associated with the use of anamed fossil fuel.b) Write a balanced symbol equation to describe theproduction of this pollutant during the use of the fuel.c) Describe the steps that can be, or have been, taken tominimize the release of this pollutant into the environment.

26. (5 marks)During your study of this topic you have carried out a“hands-on” practical exercise to observe the effect ofcertain factors on the rate of a chemical reaction.

Describe the experiment you did to investigate the effect ofeither temperature or concentration on reaction rate.

Include a) a brief outline of the method used.b) description of exactly what observation/measurementwas used to get a result.c) what general conclusion was reached.

27. (5 marks)In the upper atmosphere is a region called the “ozonelayer” where the gas ozone (O3) protects the Earth byabsorbing dangerous U.V. radiation from the Sun. Certainhuman-made chemicals can release single chlorine atomswhich react with ozone as follows:

Cl(g) + O3(g) ClO(g) + O2(g)

Then the ClO(g) reacts with the occasional single oxygenatoms that occur at this altitude:

ClO(g) + O(g) Cl(g) + O2(g)

Overall, the result is the destruction of the ozone allowingmore U.V. radiation to penetrate to the surface of theEarth.

a) In what sense can the chlorine atom (Cl(g)) be consideredas a “catalyst” in the overall reaction?

b) The diagram showsthe energy profile forthe overall reaction which can occur withoutthe presence of chlorine.

i) Describe this reactionas “endo-” or “exo-”thermic, and state the signof the value for “ΔH”.

ii) Alter the diagram to show the expected change whenchlorine atoms are present.

23





ReactantsEn

ergy

Con

tent

Products



24

Answer Section

Worksheet 1

a) Potential b) chemical bondsc) carbon d) Sune) photosynthesis f) glucoseg) carbohydrate h) sugars & starchesi) food j) oxygenk) petroleum l) fossilm) Sun n) photosynthesiso) petroleum p) methane

Worksheet 2

Part Aa) 2.4 b) graphite, diamondc) identical d) arrangede) physical properties f) flat sheetsg) lubricant h) conductori) electric motors j) leadk) diamond l) tetrahedralm) jewellery n) drill bits/saw bladeso) 4 p) covalentq) chains r) double or triples) hydrocarbons t) alkanesu) alkenes v) carbon atomsw) -ANE x) singley) CnH2n+2 z) insolubleaa) low ab) polarac) dispersion ad) increasesae) similar af) doubleag) CnH2n ah) Homologousai) vaporize aj) highak) mixture al) fractionsam) Fractional Distillation an) dropsao) condenses

Part B1.a) 5 b) 4 c) 3 d) 8 e) 1 f) 6

2.Decane: C10H22 CH3(CH2)8CH3

Worksheet 3

Part A

a) disappear b) new substancesc) colour d) precipitatee) gas f) energyg) fuel & oxygen h) oxidei) carbon dioxide & water j) gask) kerosene/ wax l) breakm) reactant n) Activationo) products p) releases

Worksheet 3 (continued)

q) self-sustaining r) hill/barriers) ignition temperature t) oxygenu) mixed v) carbon monoxidew) soot/carbon x) carbon dioxidey) global z) Greenhouseaa) sulfur dioxide ab) Acidac) carbon monoxidead) nitrogen monoxide /nitric oxideae) nitrogen & oxygen af) (petrochemical) smog

Part B1.a) 2Ca(s) + O2(g) 2CaO(s)

b) 2H2(g) + O2(g) 2H2O(g)

c) C2H6(g) + 7 O2(g) 2CO2(g) + 3H2O(g)2d) C2H4(g) + 3O2(g) 2CO2(g) + 2H2O(g)

e) C5H12(g) + 8O2(g) 5CO2(g) + 6H2O(g)

f) C5H10(g) + 15 O2(g) 5CO2(g) + 5H2O(g)2

g) C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

2.a) C6H14(g) + 13 O2(g) 6CO(g) + 7H2O(g)2b) C6H14(g) + 8O2(g) 3CO2(g) + 3CO(g) + 7H2O(g)

c) b) C6H14(g) + 5O2(g) 3CO(g) + 3 C(s) + 7H2O(g)

d) C4H10(g) + 11 O2(g) 2CO2(g) + 2CO(g) + 5H2O(g)2

3.

d) Diagram (b). To be a fuel, it must be exothermic. To havea high ignition temperature Ea must be large.



H H H

HHH

C C CH

HC

HH

H

C

H H H

HH

H

H

C C CH

HC

H

H

C

Ener

gy C

onte

nt large

ΔΔH

ssmmaallllEaa

a)

c)

Ener

gy C

onte

nt

small

ΔΔH

llaarrggeeEaa

b)

Reactants

Ener

gy C

onte

nt

Products

ΔΔHpositive

Ea



25

Worksheet 4

a) colliding b) a large piece of woodc) oxygen d) ignitione) fuel & oxygen f) explosiong) dust h) safetyi) wheat, cotton, coal j) Kinetick) moving l) move fasterm) kinetic energy n) proceed fastero) faster p) collisionq) activation r) concentrations) collisions t) pressureu) surface areav) breaking it into smaller/finer particlesw) increases x) being consumed or changedy) enzymes z) ΔHaa) activation energy ab) energyac) reaction rate ad) adsorbingae) likely/frequent af) strainag) broken ah) ironai) zeolite aj) Catalyticak) alternative chemical al) regenerated

Practice Questions

Part A Multiple Choice1. D 5. C 9. C 13. D2. C 6. A 10. B 14. B3. A 7. D 11. A 15. C4. B 8. C 12. D

Part B Longer ResponseIn some cases there may be more than one correct answer possible. The following “model”answers are correct, but not necessarily perfect.

16.Natural Gas (or L.P.G.)a) is largely composed of methane, CH4b) It is a “fossil” fuel because it is a chemical derivativefrom ancient living things. The life forms were fossilized insediments and their cell chemicals altered by heat &pressure in the rocks, forming petroleum.c) The energy in it came originally from the Sun. It wascaptured by ancient plants during photosynthesis, andstored in the carbohydrates in living cells. Fossilization hasmodified the chemicals, but the energy is still there.

17.Graphite:a) high m.p., electrical conductor, “slippery”.b) its slipperiness makes it useful as a dry lubricant in (forexample) door locks.Diamond:a) high m.p., clear sparkling crystals, extremely hard.b) its hardness makes it useful for high performance drillbits and saw blades.

18.a) C6H14 b)

c) CH3(CH2)5CH3

d) i) octene ii) propene iii) ethane

19.a) & b) on graph

c) All the bonds within the molecules are non-polar sothere are no dipole-dipole attractions between molecules.The only forces are very weak “dispersion forces”. Themolecules are easily separated from each other by heatenergy, so m.p. & b.p. are relatively low.

20.a) A substance is “volatile” if it readily evaporates at roomtemperature, or at temperatures well below its boiling point.b) Octane, like all alkanes, has only very weak dispersionforces between its molecules. (because its intra-molecularbonding is non-polar) So, even at temps well below b.p.,many molecules have enough energy to escape into the gasstate.c) Since it is volatile and combustible, and the vapour canform explosive mixtures with air, safety is a major concern.Precautions include:• storage must be in sealed containers.• fuel transfer must occur outdoors, with no sparks, flamesor electrical devices nearby.

21.Key process is “Fractional Distillation” in which themixture is separated according to boiling points. Heatedcrude oil is injected into the “fractionating tower” and thevapour mixture rises through the tower. Temperatures fallwith height and each “fraction” condenses at differentlevels according to b.p.Important fractions include: lubricating oils, fuel oil,kerosene, diesel, petrol and petroleum gas.

22.The coal in the open container lost mass because the gasproducts of combustion (e.g. CO2 & H2O) escaped intothe surroundings.

In the sealed container there is no mass change becausenothing escapes.

23.a) C4H10(g) + 13 O2(g) 4CO2(g) + 5H2O(g)2b) C2H4(g) + 3O2(g) 2CO2(g) + 2H2O(g)

c) C6H14(g) + 8O2(g) 3CO2(g) + 3CO(g) + 7H2O(g)



H H H

H

H

H H

HH

H

C C C C C

Boili

ng P

oint

(oo C)-22

0000-11

000000

110000

11 22 33 44 55 66 77 88

No. of Carbon Atoms

butene ≅ 0oC

pentene ≅ 30-40oC



26

24.

25.a) Sulfur dioxideb) Fossil fuels contain small amounts of sulfur compounds.When burnt:

S + O2(g) SO2(g)c) Petroleum fuels can be refined to remove sulfurcompounds before combustion.Coal burning is the main source. The exhaust gases are“scrubbed” to collect all the SO2. This is used to makesulfuric acid for industrial use... a useful by-product.

26.Effect of temperature.a) Method outline:• set up 3 identical test tubes of acid; one in an ice bath, oneat room temp., one in hot water bath.• Identical pieces of magnesium were dropped into each, inturn.b) The time taken for the Mg to disappear was measured.The faster the reaction, the shorter this time was.c) Reaction rate increases at higher temperatures.

27.a) It is a catalyst in that it is involved in a series of reactions,but it is “regenerated” at the end, and can be used over andover. It is not consumed or permanently changed by thereactions.b) i) Endothermic, ΔH = +ve

ii)



Ener

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onte

nt

ΔΔH

Eaa

ΔΔH

Eaa

Diesel PetrolPetrol’s Ea is larger;= higher Ig.Temp.

ΔH’ssame

Reactants

Ener

gy C

onte

nt

Products

With Cl atoms;Ea reduced

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