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Atmosphere

This unit forms part of your second Module

F332 Chemistry of Natural Resources

This Module will be examined in June.

Name:

Group: .

Chemistry Tutor:

1


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including photochemical smog(r) recall and discuss aspects of the research leading to the discovery ofthe hole in the ozone layer and how the evidence was at firstoverlooked;(s) recall the following regions of the electromagnetic spectrum in order ofincreasing frequency: infrared, visible (redblue), ultraviolet; recall, interms of these, the

principal radiations of the Earth and the Sun

(t) recall that:(i) molecules change in vibrational energy states (bonds vibrate more)when they absorb IR radiation(ii) UV and visible radiation promote electrons to higher energy levels,sometimes causing bond breaking; vibrational and electronic energies ofmolecules arequantised

(u) calculate values for frequency and energy of electromagnetic radiationusing the equation E= h

(v) explain the greenhouse effect in terms of:(i) solar energy reaches Earth mainly as visible and UVs(ii) Earth absorbs some of this energy, heats up and radiates IR(iii) greenhouse gases (eg carbon dioxide and methane) in thetroposphere absorb some of this IR, in the IR window(iv) absorption of IR by greenhouse gas molecules increases thevibrational energy of their bonds, the energy is transferred to othermolecules by collisions, thus increasing their kinetic energy and raisingthe temperature(v) greenhouse gas molecules also re-emit some of the absorbed IR in alldirections, some of which heats up the Earth

(vi) increased concentrations of greenhouse gases lead to an enhancedgreenhouse effect(w) discuss the evidence for the relationship between the increasedconcentration of gases and global warming(x) recall and discuss different approaches to the control of carbon dioxideemissions: burning fewer fossil fuels (alternative fuels and economy ofuse), increasing photosynthesis, burying or reacting carbon dioxide.

The chemical ideas in this teaching module are:

giant covalent structures

rates of reaction

chemical equilibrium radical reactions

the electromagnetic spectrum and the interaction of radiation with matter

The following topics in this teaching module are also treated in other teaching modules: rates of reaction (EP); equilibrium (EP, SS, O); bond enthalpies (DF); radical reactions (PR); interaction of radiation with matter (EL, WM, SS, CD and MD).

The Storylines in this unit are:

A1 Whats in the air?

A2 Screening the sun

A3 Ozone: a vital sunscreen

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A4 The CFC Story

A5 What is the state of the ozone layer now?

A6 The greenhouse effect

A7 Trouble in the troposphere

A8 Keeping the window open the role of water vapour

A9 Focus on carbon dioxide

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Target SheetMy minimum target grade is A B C D E, which means I need to get at least

% in every homework.

I would like to achieve grade A B C D, which means I need to get at least

% in every homework.

CommonHomework Sheet

% Score

What do I need to do toimprove?

A 1

A 2

A 3

A 4

5


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The components of the Atmosphere

Chemical Storyline A1 Whats in the air?

The gases present in the atmosphere, including some major pollutants.

Use Tables 1 and 2 (Chemical Storyline A1) to complete these tables

Composition of the atmosphere

GasConcentration (by volume)

Nitrogen%

Oxygen %

Argon %

Carbon Dioxide ppm or %

Neon ppm

Helium ppm

Methane ppm

Krypton ppm

Hydrogen ppm

Dinitrogen oxide, N2O ppm

Carbon monoxide ppm

Xenon ppm

Nitrogen monoxide, NO and nitrogendioxide, NO2 (NOx)

ppm

Sources of Pollutant Gases

GasMain source as a result of human activities

Incomplete combustion of hydrocarbons (e.g. from car exhausts)

Combustion of hydrocarbon fuels (from the reaction of N2 and O2)

Fertilised soils; changes in land use (e.g. from the soil when land isploughed up)

Combustion of hydrocarbon fuels (e.g. in power stations, motor vehicles,deforestation)

Cattle farming; landfill sites; rice paddy fields; natural gas leakage

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Calculating composition by volume measured in percentage concentration and in partsper million (ppm)

In Storyline A1 Table 1, the composition of the dry air is given as % composition (by volume)and when the gas is only present in a very small concentration it is quoted in parts per million.

e.g. dry air is 1% Ar 1cm3 of Ar in every 100cm3 of dry air

0.1ppm CO0.1 molecules of CO in every million molecules of dry airor 0.1 x 100 = 0.00001% (very low)

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What happens when radiation interacts with matter?Chemical Ideas 6.1 Light and electrons

The structure of the atmosphere

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How many parts per million (by volume) of argon are there in a typical sample of troposphericair?

In 1 dm3 of tropospheric air, what is the volume of methane present?


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What happens when radiation interacts with matter?

Chemical Ideas 6.2 What happens when radiation interacts with matter?

When electromagnetic radiation interacts with matter energy is transferred to the chemicalsinvolved. This can increase the amount oftranslational, rotational, vibrationalandelectronic energyassociated with a molecule.

Briefly explain each of these types of energy in terms of molecular movement/electrontransition.

Translational energy

Rotational energy

Vibrational energy

Electronic energy

For these types of energy only certain fixed levels of energy can be occupied.Molecules cannot possess energies between these fixed levels. The diagram shows thevibrational energy changes in an HCl molecule.

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The relationship between frequency and energy of electromagnetic radiation

Remember E = h from Elements of Life?What do the letters represent?What are the units for each term?

Radio 1 transmits at a frequency of 98.9 MHz (1MHz = 106Hz). What is the energy of a photonof the transmission waves?

Different changes for different parts of the spectrum

The spacing between vibrational energy levels corresponds to the ______________________

part of the spectrum. Making molecules rotate requires less energy than making their bonds

vibrate, so changes in rotational energy correspond to the ____________________________

region of the spectrum. The spacings between _______________________ energy levels are

sol small, that we can treat __________________________ energy as being continuous.

Making ______________________ changes occur in a molecule requires higher energy than

for vibrational changes. Electronic changes involve electrons jumping to higher electronic

energy levels within the molecule and energy corresponding to _______________________

and ____________________________________ parts of the spectrum is required.

Use Chemical Ideas to help you complete the following table:

Change occurring Size of energy change / J Type of radiationabsorbed

1 x 10 -22 to 1 x 10 -20

1 x 10 -20 to 1 x 10 -19

1 x 10 -19 to 1 x 10 -16

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Homolytic and heterolytic bond fission

Chemical Ideas 6.3 Radiation and Radicals

Bond fission breaking a bond.Remember - a covalent bond involves a pair of electrons shared by two atoms.

Homolytic fission one electron goes to each atom, producing two radicals.

Heterolytic fission both electrons go to one of the atoms producing a positive ion and anegative ion.

The formation, nature and reactivity of radicals

A chlorine radical has an unpaired electron and can be represented as follows:

(1) showing all (2) showing unpaired (3) showing none of

outer shell electron only the electronselectrons (used in the SL)

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How are radicals formed?

What is the key feature of a radical?

Describe the reactivity of radicals


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Explaining their reactivityRadicals tend to be reactive because they try and fill their outer shell by grabbing an electronfrom another atom or molecule.The single headed curly arrow shows the movement of one electron (as compared with thefull-headed arrow, for a pair of electrons)

The mechanism of a radical chain-reaction involving initiation, propagation andtermination

The reaction between hydrogen and chlorine is a typicalphotochemicalradical chainreaction.

If a mixture of these two gases is exposed to ultraviolet light the Cl Cl bond breakshomolyticallyby the absorption of light energy (known asphotodissociation). This produceschlorine radicals which then react further in a radicalsubstitution reaction.Like all chain reaction mechanisms this reaction proceeds through three key stages: initiation,propagation and termination.

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Explain the term initiation

Explain the term propagation

Explain the term termination

What does the term photochemical mean?


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Use Chemical Ideas to help you complete the boxes below:

As few radicals are present at any one time termination isnt common.

The overall reaction for these 3 stages is: H2 + Cl2 2HCl

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Initiation - This stage produces radicals which go on to react with something else (hindicates the photon of ultraviolet radiation absorbed)

Propagation - This involves a reaction between a molecule and a radical to produce anew molecule and another radical. Show the two propagation steps below

Termination - In this stage radicals react together and so the reaction chain is terminated.Give three possible termination steps.


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The reaction of alkanes with halogens

Examples: CH4 + Cl2 CH3Cl + HCl equation (1)

C6H14 + Br2 C6H13Br + HBr

These are both radical substitution reactions where the halogen replaces hydrogen. Thistype of reaction leads to a mixture of halogen substituted products because we cannotcontrol the radicals that are produced.

The reaction of methane (CH4) with chlorine in the presence of sunlight proceeds via a radicalchain reaction. The overall equation for the reaction is equation (1) above.

Now answer the questions from the end of Chapter 6.3 of Chemical Ideas.

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b) Suggest why the reaction product also contains some CH2Cl2, CHCl3 and CCl4. Showadditional reactions in your propagation/termination parts of the mechanism to showthe formation of these products.

Write out a possible mechanism for the reaction showing clearly which reactions correspondto the three stages: initiation, propagation and termination.

Initiation:

Propagation:

Termination:


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Halogenoalkanes

Chemical Ideas 13.1 Halogenoalkanes

Nature: one or more halogen atom (F, Cl, Br, I) attached to a hydrocarbon chain

Naming: (1) identify parent alkane

(2) identify halogen present and correct prefixe.g. chlorine - chloro, bromine - bromo

(3) assign numbers to show position of halogens on the chain (make numbersas low as possible)

(4) list prefixes alphabetically.

Examples: CH2ClCH2CHBrCH3

H C C C C H

H H H H

Cl H Br H

3 - bromo - 1 - chlorobutane

Draw the full structural formula ofCH3CHClCBr2CH3and give its systematic name.

Now answer the questions 1, 2 and 4 from the end of Chapter 13.1 of Chemical Ideas.

Formation of radicals:One way this can take place is when radiation of the right frequency (visible or ultraviolet) isabsorbed by the halogenoalkane.

e.g. CH3 Cl + h CH3 + Clchloromethane

As the carbon - halogen bond becomes weaker radicals form more easily (and the compoundbecomes more reactive).

C F bond becomes weaker

C Cl radicals form more easily

C Br compounds more reactiveC I

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How the atmosphere acts as a sunscreen

Chemical Storylines A2

The damage that ultraviolet radiation can do to the skin is shown by the diagram below.

The structure and reactivity of ozone

The bonds are identical and intermediate between single anddouble oxygen oxygen bonds

Ozone is very reactive and a powerful oxidising agent.

The following reactions summarise the formation and destruction of ozone in the stratosphere

(1) O2 + h O + O H = + 498 kJmol-1

(2) O + O2 O3 H = - 106 kJmol-1

(3) O + O O2 H = - 498 kJmol-1

(4) O + O3 O2 + O2 H = - 392 kJmol-1

(5) O3 + h O2 + O H = + 302 kJmol-1

Oxygen atoms are formed in the stratosphere byphotodissociation of oxygen molecules(reaction 1). The oxygen atoms produced can then do a number of things depending on whatthey collide with (reactions 2, 3 or 4) but remember not every collision will necessarily result ina reaction.

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Different substances can absorb light in the visibleultraviolet region. Use Storylines tofind out the names of a variety of sunscreens

Why is ozone in the stratosphere (upper atmosphere) an important sunscreen?

O

O

O

Which reaction or reactions remove ozone from the atmosphere?

Which reaction or reactions absorb ultraviolet radiation?


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Destruction of Ozone

Chemical Storyline A3

Ozone is produced by the following reaction: O.

+ O2 O3

Ozone is very reactive and reacts with oxygen atoms in the atmosphere. Oxygen atoms arenot the only radicals to be found within the stratosphere. There are other radicals which can

remove ozone by reacting with it.

The two most important radicals are the chlorine atom (Cl.) and the bromine atom (Br.). Smallamounts of chloromethane and bromomethane reach the stratosphere as a result of naturalprocesses.

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What is the bond energy, in kJ per mol, of the oxygen oxygen bond in O2?

What is the bond energy, in joules per molecule, of the oxygen oxygen bond in O2?Avogadro constant, L= 6.02 x1023

Use the expression E= hto calculate the frequency of radiation that would break this bond.Planck constant, h= 6.63 x10-34 JHz-1

Chloromethane (CH3Cl) and bromomethane (CH3Br)are given off from the oceans and from burningvegetation. They are responsible for the small amountsof naturally produced chlorine and bromine in thestratosphere. CH3Br is also released into theatmosphere as a result of human activity as it is usedas a fumigant for soils and grain.


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Other chlorine containing compounds reach the stratosphere in greater concentrations as aresult of human activity. These also absorb high-energy solar radiation and break down toform chlorine radicals.

From the reactions above it can be seen that Cl is regenerated and can then go on to reactwith more yet more O3 (the catalytic cycle). These Cl atoms have a devastating effect becausethey also react 1500 times faster than O atoms do with O3.

A single Cl atom can destroy on average 1 million ozone molecules!

There are other radicals present in the stratosphere that can remove ozone in a catalytic cyclein this way.

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How do chlorine atoms react with ozone? Write a balanced symbol equation

How does the radical formed by this reaction react with oxygen radicals?Write down two reactions that remove ozone from the atmosphere

Add together these two reactions for an overall reaction

How is the chlorine acting?

Write an equation for the overall reaction using X. to represent the radical


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Rates of Reaction

Chemical Ideas 10 Factors affecting reaction rates

Use your knowledge from GCSE to recall what factors affect the rate of reaction

We can also add intensity of radiation to that list

What is meant by the term collision theory?

Any factor which increases the number of collisions or the kinetic energy of colliding particleswill increase the rate of the reaction

The meaning of the terms: enthalpy profile and activation enthalpy.

With reference to the enthalpy profile diagram for an exothermic reaction above explain themeaning of the terms enthalpy profile and activation enthalpy.

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Enthalpy profile

Activation enthalpy


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Draw and label an enthalpy profile diagram for an endothermic reaction

Considering the exothermic reaction:

CH4 + 2O2

CO2 + 2H2O

O O C OO

O

HH

O O

O

HH

Look closely at the molecules involved and calculate the activation enthalpy and H for thisreaction (use bond enthalpies from the Data Book)

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C

H

H

H

H


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The use of the concept of activation enthalpy to explain the qualitative effect oftemperature changes on rates of reaction

As the temperature increases, more molecules move at __________________________ and

have higher _____________________________. The above graph shows how increasing the

temperature by _____ K changes the spread of energies in a sample of gas. At 310 K it can beseen that more molecules have energies above 50 kJ mol-1 (given by the shaded area under

the curve) and so these molecules will have enough energy to ___________________,

therefore there will be more chance of ___________________collisions at higher temperature

and thus ____________________rate of reaction.

The role of catalysts in providing alternative routes of lower activation enthalpy

With reference to the enthalpy profile diagramand the collision theoryexplain howcatalysts speed up reactions.

Homogenous catalysis in terms of the formation of intermediates

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A homogenous catalyst is in the ____________ physical state as the reactants. This type ofcatalyst works by forming an intermediate compound with the reactants (that is why theenthalpy profile for the catalysed reaction has two humps one for each step). Theintermediate then breaks down to give the products.

An example of homogenous catalysis (in the gas state) takes place in the stratosphere whereozone is destroyed in a reaction with chlorine atoms through a catalytic cycle

The CFC Story


The nature and uses of chlorofluorocarbons (CFCs)

CFCs are compounds derived from alkanes by replacing H atoms with Cl and F atoms. One ofthe earliest useful CFCs was dichlorodifluoromethane, CCl2F2, produced by Thomas Midgley in1930. It was developed as a refrigerant for freezers and fridges until then only ammoniabased refrigerants had been used which were toxic and had an overpowering odour.

List some other uses of CFCs:

CFCs are generally unreactive, non-flammable, insoluble and volatile (toxicity varies) but the

drawbacks are the environmental effects ozone depletion and global warming.

Why are stratospheric CFCs no longer unreactive?

You need to be able to discuss the evidence that was gathered to support understanding ofhow ozone depletion in the stratosphere due to halogenoalkanes occurs and how the scientificcommunity validated the results of this experiment.

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What did Sherry Rowland and Mario Molina first investigate about CFCs


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Why did they carefully check for errors in their data?

Where did they publish their research?

Why did other scientists try to pick holes in their work?

When were the researchers awarded the Nobel Prize for their work on CFCs?

Why was the hole in the ozone layer not originally found by NASA satellites?

When were CFCs eventually banned?

Why will there continue to be problems with low ozone concentrations for many years tocome?

What is a chlorine reservoir molecule? Give examples

How reactions in polarstratospheric cloudscontribute to the dramaticloss of ozone in theAntarctic spring


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CFC replacements

Now answer Assignment 7 from the end of A4 in Chemical Storylines.

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What compounds were used in the short term to replace CFCs?

Why is the use of these compounds also being phased out?

What compounds are believed to be a better long-term replacement?

What other global problem do CFCs also contribute to?


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The Greenhouse Effect


The radiation form the Sun that reaches the outer limits of the atmosphere is mainly in thevisible and ultraviolet regions of the spectrum.

Part of the energy is absorbed by the Earth and its atmosphere and part is reflected back intospace. The part that gets absorbed is changed into lower frequency radiation that warms theEarth and its atmosphere and in turn radiates energy back into space. A point is reachedwhere the Earth is radiating energy as fast as it absorbs it.

Greenhouse gases (e.g. carbon dioxide and methane) in the troposphere absorb some of thisradiation, in the IR window,

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In what form does solar energy reach the Earth?

When the Earth absorbs some of this energy it radiates the energy as what type ofradiation?

What does the absorption of infrared radiation by greenhouse gas molecules do to themolecules themselves?

How is this energy transferred to other molecules in the atmosphere?

What 2 effects does this have on the other molecules in the atmosphere?

Greenhouse gas molecules also re-emit some of the absorbed IR in all directions, someof which heats up the Earth, so what happens if concentrations of greenhouse gases isincreased?


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Use the following diagram to help you answer the next four questions

You can get a rough idea of the relative contribution of a

gas to the total greenhouse effect in the atmosphere by multiplying the global warmingpotential by the abundance of the gas.

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Methane absorbs radiation in the frequencyranges 0.39 x 1014 0.46 x1014 and 0.85 x 1014

1.03 x1014 Hz.

Will methane absorb the Suns incomingradiation?

Will methane absorb the Earths outgoingradiation?

What will be the effect of methane on the balance between incoming and outgoing radiationon Earth?

What will be the effect of methane on the temperature of the Earth?

List the gases in order of how much they contribute to the total greenhouse effect

Which of the gases are produced in significant amounts by human activities?


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Which of these gases would it be most fruitful to try to control in order to try to control thegreenhouse effect?


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Chemical Equilibrium

Chemical Ideas 7.1 Chemical Equilibrium

CLOSED SYSTEM (bottle of soda water)

At equilibrium, provided the temperature stays thesame, if you measure the pressure of CO2(g) above the water and theconcentration of CO2(aq) dissolved in the wateryou will find that they both remain constant.

Nothing appears to change on a macroscopicscale.

On a microscopicscale explain why the picture is

rather different.

In soda water :

This means that this is a dynamic equilibrium. Nothing appears tochange but the forward and backward reactions are happening at thesame time and at the same rate.

Using the soda water example again:Water itself is also present in two phases and so there are two physical equilibria inside thebottle of soda water.

H2O

(g)

H2O

(l)

and

When CO2 dissolves a small proportion of it actually reacts with the water instead of justdissolving. It is a reversible reaction and hence a dynamic equilibrium will be established.

hydrogencarbonate the hydrogen ion ion responsible for the acidic

nature of carbon dioxidesolutions

Le Chateliers Principle

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CO2(aq)

CO2 (g)

CO2 (aq)CO2 (g)

CO2 (aq)+ H 2O (l)

HCO3_

(aq)+ H +

(aq)


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This is a simple rule of thumb to help predict the effect of changes on the position ofequilibrium.

If a system is at equilibrium, and a change is made in any of the conditions, then the systemresponds to counteract the change as much as possible.

Looking at this equilibrium, if the CO2(aq) concentrationis increased the position of equilibriumwill move to the right (to the product side) in order to reduce the CO2(aq) concentration andcounteract the change.

We can use the shorthand of square brackets to mean concentration e.g. [CO2(aq)] means theconcentration of CO2(aq). Concentration is usually measured in mol dm

-3.

We can apply Le Chateliers principle to exothermic and endothermic reactions

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CO2 (aq) + H2O (l) HCO3

_

(aq) + H +(aq)

Explain what happens to the [HCO3-]

in the equilibrium mixture if the [CO2] is increased

Which way will the equilibrium move if [CO2(aq)] is decreased?

What mathematical sign appears in front of the enthalpy value for an exothermic reaction?

Which way will the equilibrium move if [H+(aq)] is increased?

Explain the use of this symbol by drawing a labelled enthalpy profile diagram


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This is the equation for the Haber Process, which is used to make ammonia gas (NH3)

( H = -92 kJ mol_1)

The forward reaction is exothermic,therefore the backwards reaction is endothermic.

(By convention we use the term forward reaction to talk about the reaction going fromreactants on the left to products on the right)

We can also apply Le Chateliers principle to predict the effects on the position ofequilibrium of changes in pressure.

4 molecules 2 molecules

If the pressure is increased the position of equilibrium will move in the direction offewer

moles of gas in order to counteract the change. If pressure is decreased, the equilibrium willshift in the direction ofmore moles of gas in order to counteract the change.

In the case of the Haber process, if we increase the pressure the equilibrium will shift to theright in order to reduce the pressure as there are fewer gas molecules on that side of theequation. This will also increase the [NH3(g)] at equilibrium.

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N2 + 3H2(g) (g)

2NH3(g)

N2 + 3H2(g) (g)

2NH3(g)

What happens to the position of equilibrium in the Haber Process if the temperature isincreased?

What effect does this have on the [NH3(g)] at equilibrium?

What happens to the position of equilibrium in the Haber Process if the temperature isdecreased?


What happens to the position of equilibrium in the Haber Process if the pressure isdecreased?



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Structure and Bonding

Chemical Ideas 5.8 Bonding, structure and properties: a summary

Use your research skills to complete the missing boxes in the table

Giant covalent Simple molecular

Which substances havethis structure?

Some non-metal elementsand some non-metal/non-metal compounds.

Examples

What type of particle doesit contain?

How are the particlesbonded together?

Strong covalent bondsholding the atoms together ina giant network.

Strong covalentintramolecular bonds holdingthe atoms together in themolecule, weakintermolecular bondsbetween the molecules

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An example of an intramolecular bond is a covalent bond. Give some examples ofintermolecular bonds

Intermolecular bonds bonds between molecules

Intramolecular bonds bonds within molecules


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