A LevelChemistryCIETopic Questions8. Reaction Kinetics8.2 Effect of Temperature on Reaction Rates & the Concept of Activation Energy

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First teaching 2023

First exams 2025

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Effect of Temperature on Reaction Rates & the Concept of Activation Energy (CIE A Level Chemistry)

Topic Questions

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1a2 marks

This question is about the following general reaction

A + B ⇌ C + D

The reaction pathway diagram for the reaction is shown in Fig. 1.1.

1-8-2a-e-reaction-profile-a-and-b
 
 Fig. 1.1
 

Explain whether the forward reaction is exothermic or endothermic. 

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1b1 mark

Define the term activation energy.

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1c
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Use Fig. 1.1 to calculate the activation energy for the forward and backward reactions.

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1d3 marks

Explain, using Fig. 1.2, how the addition of a catalyst affects the rate of reaction.

 
3-5_q2d-ocr-a-as--a-level-easy-sq
Fig. 1.2

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2a2 marks

This question is about Boltzmann distribution curves.

The Boltzmann distribution curve of molecular energies for a general reaction at a given temperature is shown in Fig. 2.1.

  
3-5_5a-ocr-a-as--a-level-easy-sq
Fig. 2.1
 

State what will happen to the curve when the temperature of the reaction is decreased.  

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2b1 mark

Comment on how, if at all, the total area under a Boltzmann distribution curve changes with temperature.

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2c2 marks

State what happens to the Boltzmann distribution curve in Fig. 2.1 when a catalyst is added to the reaction.

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2d1 mark

The Boltzmann distribution of energies for a gas is shown in Fig. 2.2.

 
3-5_5d-ocr-a-as--a-level-easy-sq
Fig. 2.2
 

State what the shaded area of Fig. 2.2 represents. 

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1a4 marks

A Maxwell-Boltzmann distribution curve is shown below in Fig 1.1 

1-8-3a-h-q3a-botlzman-distribution-curve

Fig 1.1

For the changes detailed in part (i) and (ii) state and explain the effect the change would have on:

  • The area under the curve 
  • The value of the most probable energy of the molecules, Emp
  • The proportion of molecules with greater than or equal to Ea 
i)
The temperature of the original reaction is increased, but no other changes are made.
[2]
ii)
A catalyst is added to the original reaction mixture, but no other changes are made.
[2]

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1b4 marks

A chemist performed a reaction at three different temperatures, 100 K, 300 K and 700 K as shown by the Maxwell-Boltzmann distribution graph in Fig 1.2.

maxwell-boltzman-three-diff-temps

Fig 1.2

i)
Label each curve in Fig 1.2 with the correct temperature values, 100 K, 300 K and 700 K.
[1]

 

ii)
Consider the following statement, ‘All reacting molecules have higher kinetic energy at 700 K than they do at 300 K’.

State whether you agree this statement is correct and justify your reasons.
[3]

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1c5 marks

Hydrogen will react with chlorine to form the hydrogen halide, hydrogen chloride, a colourless gas.

H2 (g) + Cl2 (g) → 2HCl (g)

i)
Give one reason why most collisions between hydrogen and chlorine molecules do not lead to the formation of hydrogen chloride. 

[1]

ii)
Apart from changing the temperature, state and explain two ways of speeding up the formation of hydrogen chloride.
[4]

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1a3 marks

Fig. 1.1 below shows, for a given temperature T, a Boltzmann distribution of the kinetic energy of the molecules of a mixture of two gases that will react together.


The activation energy for the reaction, Ea, is marked.

1-8m-q1-boltzmann

Fig. 1.1

On Fig. 1.1 above,

i)
draw a new distribution curve, clearly labelled T2, for the same mixture of gases at a lower temperature, T2.

[2]
ii)
mark clearly, as H, the position of the activation energy of the reaction at the lower temperature, T2.

[1]

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1b2 marks

Explain the meaning of the term activation energy.

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1c2 marks
i)
On the energy axis in Fig. 1.1, mark the position, clearly labelled C, of the activation energy of the reaction when a catalyst is used.

[1]

ii)
Use your answer to (i) to explain how the use of a catalyst results in reactions occurring at a faster rate.

 

[1]

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1d4 marks

Two reactions involving aqueous NaOH are given below.

CH3CHBrCH3 + NaOH → CH3CH(OH)CH3 + NaBr reaction 1
H2SO4 + 2NaOH → Na2SO4 + 2H2O reaction 2

The reagents in reaction 1 must be heated together for some time for the reaction to occur.
Whereas, reaction 2 is almost instantaneous at room temperature.

Suggest brief explanations why the rates of these two reactions are very different.

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2a4 marks

Ammonia can be produced by the reaction of nitrogen and hydrogen.

N2 (g) + 3H2 (g) rightwards harpoon over leftwards harpoon 2NH3 (g)          ΔH = -92 kJ mol-1

The reaction can be catalysed and the activation energy for this catalysed reaction is +109 kJ mol-1

Complete the reaction pathway diagram in Fig. 2.1 for the uncatalysed and the catalysed reaction between nitrogen and hydrogen.

You should label the following:

  • products
  • the enthalpy change of reaction, ΔH
  • the activation energy of the forward, uncatalysed reaction, Ea
  • the activation energy of the forward, catalysed reaction, Ec

1-8-reaction-kinetics-5a-a

Fig. 2.1

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2b
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Calculate the value of the activation energy of the catalysed decomposition of ammonia into nitrogen and hydrogen.

Show your working.

activation energy .............................................. kJ mol-1

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2c1 mark

Catalysts, such as iron used in the production of ammonia, increase the rate of reaction.

Explain why. Use a labelled Boltzmann distribution to explain your answer.

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2d3 marks

Platinum is used as a catalyst in catalytic converters which are fitted to vehicle exhaust systems to remove nitrogen oxide from the exhaust gases.

2NO (g) + 2CO (g) → N2 (g) + 2CO2 (g)

i)
State the type of catalyst that platinum is in a catalytic converter

[1]

ii)
Explain, using oxidation numbers, if nitrogen is being oxidised or reduced in this reaction.

[2]

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3a4 marks

In any chemical reaction, the particles will all be moving in different directions, at different speeds and with different amounts of energy. Maxwell-Boltzmann distributions show the distribution of energy amongst particles within a chemical reaction. 

Fig. 3.1 below shows the Maxwell-Boltzmann distribution in a sample of a gas at a fixed temperature, T1. 

1-3

Fig. 3.1

i)
Label the x and y axes of the graph. 
 
[2]
 
ii)
Sketch a distribution for a sample of the same gas at a higher temperature, T2.
 
[2]

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3b2 marks

State why a Maxwell-Boltzmann distribution curve always starts at the origin and what the area under the curve represents. 

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3c6 marks

Some changes were made individually to the experiment completed in part (a).

Consider your Maxwell-Boltzmann distribution curve from part (a). For each of the changes in parts (i), (ii) and (iii) below, state and explain the effect that the change would have on:

  • The area under the curve
  • The value of the most probably energy of the molecules (Emp)
  • The proportion of molecules with energy greater than or equal to Ea
i)
The temperature of the original reaction is increased, but no other changes are made.
 
[2]
 
ii)
The number of molecules in the original reaction mixture is increased, but no other changes are made.
 
[2]
 
iii)
A catalyst is added to the original reaction mixture, but no other changes are made.
 
[2]

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