Chemical Equilibrium (OCR A Level Chemistry)

Exam Questions

3 hours39 questions
1a2 marks

This question is about dynamic equilibria.

State what is meant by the term dynamic equilibrium.

1b1 mark

State the meaning of a closed system

1c1 mark

A general reaction exists in the following dynamic equilibrium. 

3A+ 2B ⇌ C2 + D
 

Which part of the equation shows that this reaction is a dynamic equilibrium?

1d1 mark

The gases nitrogen dioxide, NO2 (g), and nitrogen tetroxide, N2O4 (g), exist in the following dynamic equilibrium. 

2NO2 (g) ⇌ N2O4 (g)
 

An increase in pressure shifts the equilibrium to the product's side and increases the yield of nitrogen tetroxide. 

State why the increase in pressure shifts the equilibrium to the product's side.

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2a1 mark

Steam-methane reforming is where hydrogen gas, H2 (g) is produced by passing methane, CH4 (g) and steam, H2O (g), over a heated catalyst.

The equation for this reaction is shown below: 

CH4 (g) + H2O (g) ⇌ CO (g) + 3H2 (g)   ΔH = +206 kJ mol-1 
 

Why is this reaction a homogeneous equilibrium.

2b1 mark

State whether the forward reaction is endothermic or exothermic.

2c2 marks

State why increasing the temperature increases the yield of hydrogen.

2d3 marks

State, with reasons, how the pressure used for this reaction can be changed to increase the yield of carbon monoxide.

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3a1 mark

During the manufacture of sulfuric acid in the Contact process, sulfur dioxide, SO2, is oxidised into sulfur trioxide, SO3.

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)   H = −197 kJ mol−1 

State the effect on the rate of reaction when the temperature increases.

3b2 marks

State, with a reason, the effect on the yield of sulfur trioxide when the temperature increases.

3c3 marks

Describe and explain the effect of decreasing the pressure on the reaction rate.

3d3 marks

Describe and explain the effect of decreasing the pressure on the yield of sulfur trioxide.

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4a1 mark

Phosphorus pentachloride, PCl5 (s), is used in the manufacture of antibiotics such as penicillin and cephalosporin.

A reversible reaction can be used to prepare phosphorous pentachloride, PCl5

The Kc expression for this reaction is K subscript straight C equals fraction numerator 1 over denominator left square bracket PCl subscript 3 right square bracket space left square bracket Cl subscript 2 right square bracket end fraction

Suggest why phosphorous pentachloride does not feature in the Kc expression. 

4b1 mark

Use the Kc expression in part (a), to complete the equation for the formation of phosphorous pentachloride. State symbols are not required.

 
_____ + _____ ⇌ PCl5
4c3 marks

Explain how pressure can be used to increase the yield of phosphorous pentachloride.

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5a1 mark

Ammonia is manufactured from nitrogen and hydrogen using the Haber process.

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

State the effect that the addition of an iron catalyst would have on the position of the equilibrium.

5b1 mark

Write the expression for Kc for the production of ammonia.

5c1 mark

State the effect that the addition of an iron catalyst would have on the value of Kc.

5d
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2 marks

At 450 oC, the equilibrium concentrations are N2 (g): 17.4 mol dm-3, H2 (g): 0.85 mol dm-3, NH3 (g): 1.25 mol dm-3.  

Calculate the value of Kc to three significant figures.

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1a3 marks

Ethanol has a great number of uses. For industrial purposes, it can be manufactured via the following reversible reaction.

   C2H(g)   +   H2O (g)     ⇋     C2H5OH (g) ΔH = -46 kJ mol-1

The optimum pressure for this reaction is between 60 and 70 atm.  

Using Le Chatelier’s principle, state and explain the effect, if any, that increasing the overall pressure would have on the equilibrium yield of ethanol.

1b4 marks

Although Le Chatelier’s principle is used to suggest the best conditions for a reaction, often a compromise has to be made.

i)
Use Le Chatelier’s principle to suggest whether a high or low temperature should be used to produce the maximum yield of ethanol in the reaction from part (a).

ii)
State a problem that may occur from using this temperature.
1c1 mark

Ethanol can be used as a reactant in another equilibrium reaction; the manufacture of ethyl ethanoate. 

   CH3CH2OH(l)   +   CH3COOH(l)    ⇋    CH3COOCH2CH3 (l)   +   H2O(l) 

Give the expression for the equilibrium constant, Kc, for this equilibrium.

1d
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4 marks

A student set up the esterification reaction seen in part (c), adding ethanol and ethanoic acid to a reaction vessel. They set the reaction up in a closed system, at a constant temperature and allowed equilibrium to be reached. 

The reaction was done in a container with a volume of 250 cm3

Table below shows the amount of each substance present in the equilibrium mixture. 

Substance

Amount (mol)

CH3CH2OH

0.0375

CH3COOH

0.0615

CH3COOCH2CH3

0.0776

H2O

0.0834

i)
Calculate Kc for this reaction to 2 decimal places.

ii)
Deduce the units for Kc. 

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2a2 marks

Methanol is the first member of the alcohol homologous series.
It is an essential industrial chemical, and can be manufactured according to the following reaction: 

   CO2(g)   +   3H2(g)   ⇋   CH3OH(g)   +   H2O(g) ΔH = -91 kJ mol-1


The conditions used are called compromise conditions, used to find a balance between producing a high yield of product and having manageable and safe conditions.
 
Give the Kc expression for this equilibrium reaction and deduce the units.

2b2 marks

The reaction in part (a) often uses a copper-based catalyst, meaning that the usual temperature and pressure of the reaction can be reduced.  

State and explain the effect, if any, that using a catalyst for this reaction will have on the yield of methanol which is produced.

2c5 marks

State and explain why lowering the temperature of the reaction will contribute to a higher yield of methanol being produced, but why lowering the pressure would contribute to a lower yield of product.

2d1 mark

Excluding any reasons regarding the yield of the product, give a reason why scientists in industry much prefer to use a lower pressure and temperature where possible.

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3a1 mark

Some industrial processes involve reversible reactions.
If done under the correct conditions, then these reactions will reach a dynamic equilibrium, producing equilibrium mixtures of reactants and products. The conditions of the reaction can be altered and controlled to maximise the product yield. 

Le Chatelier’s principle is a rule which is used to determine how to move the equilibrium position to the left or right, by altering specific conditions of the reaction.  

State Le Chatelier’s principle. 

3b3 marks

An example of an equilibrium reaction can be seen between oxygen and nitrogen.

When heated to a high enough temperature, nitrogen will react with oxygen to form nitrogen monoxide, as shown below:

   N2(g)  +  O2(g)   ⇋   2NO(g)

When the temperature of the system is increased, the yield of NO increases. 

State whether the forward reaction in this system is exothermic or endothermic.
Explain your answer. 

3c3 marks

State and explain the effect, if any, on the yield of NO produced if the pressure was increased, but the temperature was kept the same.

3d
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6 marks

A chemist set up a different reaction, shown below:

W(aq)  +  2X(aq)  ⇋   3Y(aq)  +  Z(aq)

They started with a flask containing 1.75 x 10-2 mol of an aqueous solution W. They then added 0.050 mol of solution X to the flask, ensured a closed system, and allowed the

reaction to reach equilibrium. 

Once equilibrium was reached, the reaction mixture contained 0.012 mol of Z. 

The overall volume of the reaction mixture was 105 cm3

i)
Give the Kc expression for this reaction.

ii)
Calculate a value for Kc.

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4a2 marks

A reaction mixture was set up in a syringe between dinitrogen tetraoxide gas and nitrogen dioxide gas as shown in the equation below:

   N2O4(g)  ⇋  2NO2(g) ΔH = +58 kJ mol-1

The appearance of the gases is very different; dinitrogen tetraoxide is a colourless gas, whereas nitrogen dioxide is dark brown in colour. 

Give the Kc expression for this reaction and deduce the units.

4b3 marks

Explain why the reaction mixture turns darker in colour when it is heated.

4c3 marks

The reaction which takes place in part (a) has a Kc value of 3.21. A student claims that increasing the temperature of this reaction will increase the value of Kc

Is the student correct? Justify your answer. 

4d3 marks

Using Le Chatelier’s principle, explain what would be seen initially if the plunger of the syringe was pressed and the gases within the syringe were compressed.

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5a3 marks

During an esterification reaction, methanol and ethanoic acid react together to form the ester, methyl ethanoate, and water as shown below:

   CH3OH(l)  +  CH3COOH(l)  ⇋  CH3COOCH3(l)  +  H2O(l)

A chemist sets up the reaction and allows it to reach dynamic equilibrium at a constant temperature. 

i)
State the meaning of the term dynamic equilibrium. 

ii)
Give one key condition which must be satisfied for a reversible reaction to reach dynamic equilibrium. 
5b2 marks

Once the reaction in part (a) is set up, the students leave it for 24 hours to make sure that it has reached equilibrium.

State how the students could check to make sure that the reaction mixture had reached equilibrium.

5c
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4 marks

When the equilibrium was reached, the equilibrium moles of each component in the mixture were as follows:

    - Ethanoic acid = 0.378

   - Methanol = must be calculated 

   - Methyl ethanoate = 0.716

    - Water = 1.08

   Kc for the reaction was 7.21.

i)
Calculate the number of moles of methanol at equilibrium.

ii)
State why there are no units for Kc in this reaction.
5d3 marks

Adding more ethanoic acid to the reaction mixture will increase the yield of the ester produced. 

Use Le Chatelier’s principle to explain the above statement.

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1a3 marks

This question is about a dynamic equilibrium.

The following dynamic equilibrium was reached at temperature, T, in a closed container.

3A (g) + 2B (g) ⇌ 2C (g)    ΔH = - 65 kJ mol-1

The value of Kc for the reaction was 255 mol-1 dm3 when the equilibrium mixture contained 3.34 mol of A and 4.28 mol of C.

i)
Give the definition of dynamic equilibrium.
ii)
Write an expression for Kc for the reaction.
1b
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4 marks

Calculate the concentration of B in the equilibrium mixture if the volume of the container is 8.00 dm3. Give your answer to 3 significant figures. 

1c2 marks

Changing the conditions of a closed container can have an effect on the concentrations of the reactants, products and Kc

State the effect, if any, on the concentration of B at equilibrium if temperature, T, is increased and give a reason for your answer.

1d
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1 mark

Calculate the equilibrium constant for the following reaction at temperature, T

2C (g) ⇌ 3A (g) + 2B (g)

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2a
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6 marks

This question is about the decomposition of sulfur trioxide.

A 0.740 mol sample of SO3 is introduced into a 2.86 dm3 reaction container and allowed to reach equilibrium at temperature T. 48% of the SO3 had decomposed. 

Calculate the value for Kc in this reaction, giving your answer to 3 significant figures. 

2SO3 (g) ⇌ 2SO2 (g) + O2 (g)     ΔH = + 196 kJ mol-1

2b4 marks

The size of the container for the reaction in part (a) is decreased. State the effect if any on the equilibrium constant, Kc, and the position of equilibrium. Justify your answer.

2c3 marks

The temperature of the reaction in part (a) is decreased. State the effect, if any, on the equilibrium constant, Kc, and the position of equilibrium. Justify your answer.

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3a
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5 marks

This question is about equilibrium reactions.

A mixture of 1.46 moles of V, 1.29 moles of W and 0.68 moles of Z were placed into a 4.0 dm3 container at temperature, T, and allowed to reach equilibrium. At equilibrium, the number of moles of V was 1.76.

Calculate the value of the equilibrium constant, Kc, to 3 significant figures. 

2V (g) ⇌ W (g) + 3Z (g)     ΔH = -126 kJ mol-1

3b2 marks

The value of Kc for the reaction in part (a) at a different temperature, T1, is 1.07 mol dm-3.

Comment on the relationship between the concentration of the reactant V and products W and Z with regards to Kc.

3c
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5 marks

Reactants C and D react together to form products G and F according to the equation

C + 3D ⇌ G + 4F

A beaker contained 45 cm3 of 0.16 mol dm-3 of an aqueous solution of D

7.36 x 10-3 moles of C and 3.7 x 10-3 moles of F were also added to the beaker. The equilibrium mixture contained 4.4 x 10-3 moles of D. 

Calculate the number of moles of C, G and F at equilibrium.

3d
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1 mark

For the reaction in part (c) calculate the equilibrium constant, Kc

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4a1 mark

This question is about the formation of diesters.

Diethyl butanoate, as shown below in figure 1, is used as a building block in organic synthesis reactions to produce more complex molecules. 

Write an equation for the formation of Diethyl butanoate.

Figure 1

Diethylsuccinat.svg

4b
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3 marks

At equilibrium, the reaction mixture contained 2.95 moles of CH3CH2OH, 1.25 moles of HOOCCH2CH2COOH, and 1.14 moles of C8H14O4.

The value for Kc at temperature, T, is 1.21.

Calculate the concentration of water in the reaction mixture in part (a) at equilibrium. Give your answer to 3 significant figures. 

4c2 marks

A student deduced that in order to calculate the value of Kc for the reaction in part (a) you must work out the concentrations using the overall volume. 

Is the student correct? Justify your answer.

4d3 marks

The forward reaction in part (a) is slightly exothermic. At a different temperature, T1, the value for Kc increases to 21.32. 

State whether the new temperature, T1, is higher or lower than the original temperature.
Justify your answer.

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5a3 marks

This question is about the effect of pressure and temperature on the equilibrium yield of gaseous molecules.

The graph in Figure 1 shows the effect of pressure and temperature on the equilibrium yield of gaseous molecules.

Figure 1

3-6_q5a-ocr-a-as--a-level-hard-sq

Use Figure 1 to fully explain whether the forward reaction is exothermic or endothermic.

5b3 marks

Use Figure 1 to explain whether the forward reaction will involve either an increase or decrease in the number of moles of a gas.

5c3 marks

The graph shows the relationship between temperature and Kc for a different dynamic equilibrium to produce a gaseous product is shown in Figure 2.

Figure 2

3-6_q5c-ocr-a-as--a-level-hard-sq

Use the information in Figure 2 to establish whether the forward reaction is exothermic or endothermic. Justify your answer.

5d2 marks

Gaseous products can be manufactured by the direct combination of reactants.

Explain why it is important for a chemist to know the value of Kc, at a given temperature, for such a reaction.

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