3. kinetics

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AQA AS Chemistry (CHEM 2) Mo Idriss, Quick Revision Notes

22 Unit 2: Chemistry In action 2.2 Kinetics

Kinetics

LearningObjec

tive(s)

After studying this section, you will be able to:

understand that reactions can only occur when collisions take place betweenparticles having sufficient energy

define the term activation energy and understand its significance understand that most collisions do not lead to reaction have a qualitative understanding of the MaxwellBoltzmann distribution

of molecular energies in gases

draw and interpret distribution curves for different temperatures understand the qualitative effect of temperature changes on the rate of

reaction

understand how small temperature increases can lead to a large increasein rate

understand the qualitative effect of changes in concentration on rate ofreaction

know the meaning of the term catalyst understand that catalysts work by providing an alternative reaction route

of lower activation energy

For this topic, you should have reviewed the following definitions:

1. Activation Energy2. Collision Energy3. Collision Frequency

4. Heterogeneous catalyst5. Homogeneous catalyst6. Maxwell-Boltzmann distribution

1. Collision TheoryThe collision theory: Reactions only happen when:

I. Particles collideII. Particles have the correct orientation

III. And they possess the minimum amount of kinetic energy particles need to react(activation energy)

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Therefore, to increase the rate of a reaction, particles must:

Collide more frequently (collision frequency) either;o By increasing particle speed OR Having more particles in the same volume

Have more successful collisions either;o By having more energy (collision energy) OR By lowering the activation energy

Specific factors that affect the rate of a reaction are:

Increase the temperaturegenerally, raising the temperature by 10oC doubles the rate.How does energy increase rate? ____________________________________________

_____________________________________________________________________

Increase the concentration(for solutions) Increases the frequency of collisions of thereacting molecules since there are more of them in the same volume

Increase the pressure(for gaseous reactions)Explain Increase the surfacearea(for solids reacting with gases or liquids)Explain Use a catalystA catalyst works by providing an alternative route with lower activation

energy. Thus at a given temperature, a greater proportion of the colliding molecules will

possess the lower activation energy of the catalysed route and so the reaction will be

faster.

Use a light source (for photochemical reactions only)Simple Energy profile diagrams are given below:

_______thermic reaction _______thermic reaction

I. Label the two graphs above as either exothermicor endothermicII. Add labels for the diagram on the right to indicate

a. activated complex (transition state)b. activation energy(Ea)c. labels for the two axesd. Enthalpy change.

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2. The Maxwell-Boltzmann distributionIn any system, the particles present will have a wide range of speeds and therefore of energies

(energy of a particle is proportional to the speed - KE = mv2). If a distribution graph is drawn to

show the molecular speed of gases on the x axis and the number of molecules on the y axis, we

get a Maxwell-Boltzmann distribution curve as shown below.

Note: these curves are not symmetrical & the peak of the

curve represents the most probable energy (Emp). The

average/mean energy (E bar) is further to the right.

A few particles will have very low energy (none will have zero energy) and a few will have very

high energy but the majority will have energies within a narrow range in the middle.

Although drawn as a curve, the distribution is actually a histogram and the area under the

curve isa measure of the total number of particles present.

Using M-B distribution to explain Effect of Temperature Change

Since temperature affects the energy of molecules, the temperature of the system has an effect

on the size and shape of the Maxwell-Boltzmann distribution.

The effect of an increase in temperature is to make the molecules move faster and have a

higher average kinetic energy. The proportion of colliding molecules with energies equal to or

higher than the activation energy increases, and this causes an increase in successful collisions.

This is seen in the M-B distribution curve below. The curve for the higher temperature is shifted

to the right but slightly lowered. However, the area under the curve (proportional to the

number of molecules) stays the same.

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For a gaseous mixture, the effect of an increase in pressure is the same as that of an increase in

concentration of a liquid reactant. The greater the pressure in a gaseous mixture, the greater

the number of species per unit volume and the greater the frequency with which they will

collide. Hence an increase in pressure causes the rate of reaction to increase by increasing the

collision frequency. The pressure of a system is generally increased by reducing its volume.

The collision energy, activation energy and hence the fraction of successful collisions are

unaffected.

3. CatalystsWe have already seen that a catalyst works by providing an alternative route with lower

activation energy which enables more particles to have the necessary energy for reaction.

This can be seen in the M-Boltzmann distribution at the bottom right. The green shaded area

represents the proportion of molecules initially with energy higher than the activation energy.

In the presence of the catalysed, an alternative route with lower activation energy is provided.

More particles (grey-shaded + green-shaded areas) now have the required energy to react.

Energy profile diagram Maxwell-Boltzmann distribution

A catalyst does not change the distribution curve but more molecules now have an energy

exceeding the new, lower activation energy.

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Heterogeneous catalyst

A heterogeneous catalyst is in a different phase (physical state) from the reactants. Examples

include gaseous reactants using a solid catalyst.

Examples of Heterogeneous catalystsProduction of sulphur trioxide and eventually sulphuric acid by

the contact process

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

Solid vanadium (V) oxide

Production of ammonia (NH3) by the Haber Process

N2(g) + 3H2(g)2NH3(g) Solid Ironcatalyst

Catalytic converter Reactions (to remove pollutants like unburnt

hydrocarbons, CO & NO from car exhausts)

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

hydrocarbons(g) + 2NO(g) 2CO2(g) +N2(g) + H2O(g)

Platinum and Rhodium

catalyst (supported on aceramic honeycomb)

How Heterogeneous catalysts work

In general reactant molecules diffuse towards the catalyst

surface, are adsorbed onto the catalyst and this weakens their

bonds allowing reaction to take place. Following reaction the

products are desorbed from the catalyst

Homogeneous catalyst

A homogeneous catalyst is in the same phase as the reactants.

Examples include gaseous reactions in the atmosphere which utilize gaseous catalysts or

reactions in aqueous solution where the catalyst is a liquid.

Examples of Homogeneous catalysts

Breakdown of ozone in upper atmosphere (stratosphere)

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

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

Cl. (g) (Chlorine radicals) formed by action of UV on

CFCs

Production of esters from alcohols & carboxylic acids

CH3COOH (l) + CH3OH (l)CH3COOCH2(l)

Acid catalyst (H+) usually

concentrated H2SO4

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Practice Questions

1. (a) State what is meant by the terms rate of reactionand activation energy.

Rate of reaction...........................................................................................................

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

Activation energy.........................................................................................................

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

(4 marks)

(b) The diagram below shows the Maxwell-Boltzmann energy distribution curve for

a sample of gas at a fixed temperature. Ea is the activation energy for the

decomposition of this gas.

Number ofmoleculeswith a givenenergy

EaEnergy

(i) On this diagram sketch the distribution curve for the same sample of gas at a

higher temperature.

(ii) What is the effect of an increase in temperature on the rate of a chemical

reaction? Explain your answer with reference to the Maxwell-Boltzmann distribution.

Effect.........................................................................................................................

Explanation.............................................................................................................

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

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

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(c) (i) Explain why, even in a fast reaction, a very small percentage of collisions leadsto a reaction.

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

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

(ii) Other than by changing the temperature, state how the proportion of successfulcollisions between molecules can be increased. Explain why this method causesan increase in the proportion of successful collisions.

Method for increasing the proportion of successful collisions........................

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

Explanation......................................................................................................

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

(4)

(Total 9 marks)

3. Explain why the rate of the reaction between magnesium and dilute hydrochloric acid is

increased much more by changing the magnesium from ribbon to powder than by doubling the

concentration of the acid.

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

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

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

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

(2)

(Total 2 marks)

4. The curve below shows how the volume of oxygen evolved varies with time when 50

cm3of a 2.0 mol dm3solution of hydrogen peroxide, H2O2, decomposes at 298 K.

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A

Time / s

Volume

of oxygen/ cm3

(a) State how you could use the curve to find the rate of reaction at point A.

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

(1)

(b) Sketch curves, on the above axes, to illustrate how the volume of oxygen

evolved would change with time if the experiment was repeated at 298 K using the

following.

(i) 100 cm

3

of a 1.0 mol dm

3

solution of H2O2. Label this curve X.

(ii) 25 cm3 of a 2.0 mol dm3 solution of H2O2 in the presence of a catalyst.

Label this curve Y.

(4)

(Total 5 marks)

5. The equilibrium yield of product in a gas-phase reaction varies with changes in

temperature and pressure as shown below.

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0 50 100 150 200 250 300 350 400

70

60

50

40

30

20

10

0

350C

450C

550C

% Yieldof product

Pressure/kPa

(a) Use the information given above to deduce whether the forward reaction

involves an increase, a decrease, or no change in the number of moles present.

Explain your deduction.

Change in number of moles................................................................................................

Explanation.........................................................................................................................

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

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

(4)

(b) Use the information given above to deduce whether the forward reaction is

exothermic or endothermic. Explain your answer.

The forward reaction is..............................................................................................

Explanation................................................................................................................

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

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

(3)

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(c) (i) Estimate the percentage yield of product which would be obtained at

350C use only and a pressure of 250 kPa.

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

(ii) The reaction is an example of a dynamic equilibrium.

State what is meant by the term dynamic equilibrium.

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

(iii) State what effect, if any, a catalyst has on the position of the equilibrium.

Explain your answer.

Effect on position ..............................................................................................................

Explanation .........................................................................................................................

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

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

(6)

(d) A 70% equilibrium yield of product is obtained at a temperature of 350 C and a

pressure of 400 kPa. Explain why an industrialist may choose to operate the plant at

(i) a temperature higher than 350 C;

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

(ii) a pressure lower than 400 kPa.

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

(2)

(Total 15 marks)

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6. Nitrogen, hydrogen and oxygen undergo the reactions shown below.

Reaction 1 N2(g) + O2(g) NO(g) H = +91 kJ mol1

Reaction 2 N2(g) + 1 H2(g) NH3(g) H =46 kJ mol1

Reaction 3 H2(g) + O2(g) H2O(g) H =242 kJ mol1

Use this information in answering the questions that follow.

(a) In which, if any, of the reactions above would the percentage yield of products atequilibrium increaseif the temperature were to be raised? Explain your reasoning.

Yield increased with increased temperature. ..................................................

Explanation. ....................................................................................................

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

..........................................................................................................................(3)

(b) In which, if any, of the reactions above would the percentage yield of products atequilibrium notincreaseif the pressure were to be raised? Explain your reasoning.

Yield not increased with increased pressure. .................................................

Explanation. ....................................................................................................

..........................................................................................................................(2)

(c) (i) In which direction, if any, would any equilibrium reaction move if H= 0 andthe temperature were to be decreased?

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

(ii) Predict for which of the reactions above the percentage yield of products atequilibrium would be most affected by a change in temperature. Explain youranswer.

Reaction most affected by a temperature change. ...........................................

Explanation. .....................................................................................................

.......................................................................................................................... (3)

(Total 8 marks)

7. Methanol can be produced by direct combination of carbon monoxide and hydrogenaccording to the equation below

21

21

21

21

21

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CO(g) + 2H2(g) CH3OH(g) H =91 kJ mol1

Side reactions can also produce unwanted by-products.

(a) Explain why a low temperature and a high pressure favour a high yield of methanol in

this reaction.

Low temperature ........................................................................................................

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

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

High pressure .............................................................................................................

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

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

(4)

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(b) The industrial manufacture of methanol using this reaction is carried out at acompromise temperature of 400 C under a pressure of 20 MPa in the presence of a

Cr2O3/ZnO catalyst.

(i) Justify the use of a compromise temperature.

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

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

(ii) What effect, other than on the yield, does the use of high pressure have on thereaction?

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

(iii) Suggest two reasons for using a catalyst in this method for producing methanol.

Reason 1 ...........................................................................................................

Reason 2 ...........................................................................................................(5)

(Total 9 marks)

8. (a) State and explain, using the collision theory, the effect of concentration,

temperature and a catalyst on the rate of a reaction.

(10)

(b) Ammonia is manufactured by the Haber process:

N2(g) + 3H2(g) 2NH3(g) H=92 kJ mol1

Explain how the equilibrium yield of ammonia in the Haber process is affected by

changing the pressure, changing the temperature and by using a catalyst.

(11)

(Total 21 marks)

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9. The gas-phase reaction between hydrogen and chlorine is very slow at room temperature.

H2(g) + Cl2(g) 2HCl(g)

(a) Define the term activation energy.

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

.....................................................................................................................................(2)

(b) Give one reason why the reaction between hydrogen and chlorine is very slow at room

temperature.

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

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

(c) Explain why an increase in pressure, at constant temperature, increases the rate ofreaction between hydrogen and chlorine.

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

.....................................................................................................................................(2)

(d) Explain why a small increase in temperature can lead to a large increase in the rate ofreaction between hydrogen and chlorine.

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

.....................................................................................................................................(2)

(e) Give the meaning of the term catalyst.

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

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

(f) Suggest one reason why a solid catalyst for a gas-phase reaction is often in the form ofa powder.

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

(Total 9 marks)

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10. The curve below represents the distribution of molecular energies at a temperature T1for a

mixture of gases which react with each other. Ais the activation energy for the reaction.

T1

Aenergy

(a) (i) Label the vertical axis.

(ii) Explain the meaning of the term activation energy.

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

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

(iii) Draw a second curve on the same axes, and label it T2, for the same mixture at a

lower temperature.

(iv) By reference to the curves, state and explain in molecular terms the effect of

reducing the temperature on the rate of reaction.

Effect .................................................................................................................

Explanation .......................................................................................................

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

...........................................................................................................................(8)

(b) The reaction is repeated in the presence of a catalyst.

Mark on the energy axis a possible activation energy Bfor the catalysed reaction.(1)

(c) Give an example of a catalyst and a reaction which it catalyses.

Catalyst .......................................................................................................................

Reaction using this catalyst ........................................................................................

..................................................................................................................................... (2)

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(d) Several small pieces of calcium carbonate (an excess) were allowed to react with

100 cm3

of 0.1 M hydrochloric acid. The volume of carbon dioxide formed during the

experiment is shown on the graph below.

140

120

100

80

60

40

20

0Time

VolumeofCO

/cm

3

2

Using the same axes, sketch curves labeled A, Band Cfor the following experiments

which were all carried out at the same temperature as the original experiment:

A 50 cm

3

0.2 M hydrochloric acid with an excess of small pieces of calciumcarbonate;

B 50 cm3

0.1 M hydrochloric acid with an excess of small pieces of calcium

carbonate;

C 50 cm3

0.1 M hydrochloric acid with an excess of powdered calcium carbonate.(6)

(Total 17 marks)

11. In the Haber process for the manufacture of ammonia, nitrogen and hydrogen react as shownin the equation.

N2(g) + 3H2(g) 2NH3(g) H=92 kJ mol

1

The table shows the percentage yield of ammonia, under different conditions of pressure and

temperature, when the reaction has reached dynamic equilibrium.

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Temperature / K 600 800 1000

% yield of ammonia at 10 MPa 50 10 2



(a) Explain the meaning of the term dynamic equilibrium.

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

.....................................................................................................................................(2)

(b) Use Le Chateliers principle to explain why, at a given temperature, the percentageyield of ammonia increases with an increase in overall pressure.

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

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

.....................................................................................................................................(3)

(c) Give a reason why a high pressure of 50 MPa is not normally used in the Haber

process.

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

(d) Many industrial ammonia plants operate at a compromise temperature of about 800 K.

(i) State and explain, by using Le Chateliers principle, one advantage, other thancost, of using a temperature lower than 800 K.

Advantage .........................................................................................................

Explanation ......................................................................................................

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

(ii) State the major advantage of using a temperature higher than 800 K.

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

(iii) Hence explain why 800 K is referred to as a compromise temperature.

...........................................................................................................................(5)

(Total 11 marks)

3. kinetics

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