Equilibria (OCR A Level Chemistry A): Exam Questions

A chemist carries out an investigation on the equilibrium system shown below.

2CO (g) + 2NO (g) ⇌ 2CO₂ (g) + N₂ (g)   ΔH = –788 kJ mol^–1

The chemist mixes 0.46 mol of CO with 0.45 mol of NO. The mixture is left to reach equilibrium at constant temperature.

The student analyses the equilibrium mixture and finds that 0.25 mol NO remains. The total volume of the equilibrium mixture is 1.0 dm³.

i) Write the K_c expression for this equilibrium.

ii) What are the units of this equilibrium constant?

Determine the value of K_c for the equilibrium mixture in part (a). Show all your working.

What does your value of K_c suggest about the position of equilibrium in this experiment?

The chemist increases both the temperature and the pressure of the equilibrium mixture. The mixture is left to reach equilibrium again.

i) What is the effect, if any, on the value of K_c? Explain your answer.

ii) Explain why it is difficult to predict what would happen to the position of equilibrium after these changes in temperature and pressure.

Nitric acid, HNO₃, is manufactured in large quantities. The main use of nitric acid is in the manufacture of fertilisers.

In its industrial preparation, nitric acid is produced in three main stages.

1. Ammonia is heated with oxygen in the air to form nitrogen monoxide, NO.

2. The hot nitrogen monoxide gas is then mixed with air and cooled under pressure. Nitrogen dioxide, NO₂, forms in a reversible reaction.

   • 2NO (g) + O₂ (g) ⇌ 2NO₂ (g)    ΔH = –115 kJ mol^–1 

3. The nitrogen dioxide is reacted with water in a series of reactions to form nitric acid, HNO₃. The first of these reactions forms a mixture of nitric acid, HNO₃, and nitrous acid, HNO₂.

In step 2, explain why the equilibrium mixture is both cooled and put under pressure.

Construct an equation for:

i) The reaction that takes place in step 1

ii) The first reaction that takes place in step 3.

An industrial chemist carries out the following procedure to reserach the NO / O₂ / NO₂ equilibrium used in step 2 of the manufacture of nitric acid.

• The chemist mixes together 0.80 mol NO (g) and 0.70 mol of O₂ (g) in a container with a volume of 2.0 dm³.

• The chemist heats the mixture and allows it to stand at constant temperature to reach equilibrium.

• The container is kept under pressure so that the total volume is maintained at 2.0 dm³.

• At equilibrium, 75% of the NO has reacted.

Write an expression for K_c for this equilibrium  

Calculate the equilibrium constant, K_c, including units, for this equilibrium.

Dinitrogen tetroxide, N₂O₄(g), and nitrogen dioxide, NO₂(g), coexist in the following equilibrium.

2NO₂ (g) ⇌ N₂O₄ (g)   ΔH = –57 kJ mol^–1

A chemist adds 4.00 mol NO₂ to a container with a volume of 2.00 dm³. The container is sealed, heated to a constant temperature and allowed to reach equilibrium. The equilibrium mixture contains 3.20 mol NO₂.

Calculate the value for K_c under these conditions

The experiment is repeated but the pressure in the container is doubled.

Explain in terms of K_c the effect on the concentrations of NO₂ and N₂O₄ when the mixture has reached equilibrium.

This question is about equilibrium reactions.

Hydrogen gas is manufactured by the chemical industry using the reversible reaction of methane and steam shown below.

CH₄(g) + H₂O(g) → 3H₂(g) + CO(g)   ΔH = +210 kJ mol^–1

Explain, in terms of le Chatelier’s principle, the conditions of pressure and temperature for a maximum yield of hydrogen from the equilibrium and explain why the operational conditions used by the chemical industry may be different.  

The following equation shows the equilibrium reaction between hydrogen and nitrogen to form ammonia as part of the Haber process. The enthalpy value for this reaction is -91 kJ mol^-1.

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g) 

Explain the effect, if any, of an increase in temperature on the value of K_p for this reaction if the volume remains constant and justify your answer.

Nitrogen monoxide, NO, and oxygen, O₂, react to form nitrogen dioxide, NO₂, in the reversible reaction shown.

2NO (g) + O₂ (g) → 2NO₂ (g)

Write an expression for K_c for this equilibrium and state the units

A chemist mixes together nitrogen and oxygen and pressurises the gases so that the total gas volume is 4.0 dm³.

The equilibrium mixture is allowed to reach equilibrium at constant temperature and volume and contains 0.40 mol NO and 0.80 mol O₂. Under these conditions, the numerical value of K_c is 45.

Calculate the amount, in mol, of NO₂ in the equilibrium mixture

The values of K_p for the following equilibrium at 298 K and 1000 K are shown below.

2NO (g) + O₂ (g) → 2NO₂(g)

Predict, with a reason, whether the forward reaction is exothermic or endothermic.

The chemist increases the pressure of the equilibrium mixture at the same temperature.
State, and explain in terms of K_p, how you would expect the equilibrium position to change.

Ammonia can be manufactured from nitrogen and hydrogen gases in the Haber process.

The equilibrium is shown below.

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)   ΔH = –91 kJ mol^–1

Write an expression for K_c for this equilibrium.

A chemist carries out a series of experiments to investigate the conversion of nitrogen and hydrogen into ammonia under different conditions.

The chemist mixes together 10.40 mol N₂ and 22.50 mol H₂ and pressurises the gases so that the total gas volume is 5.00 dm³.

The mixture is allowed to reach equilibrium at constant temperature and without changing the total gas volume. The equilibrium mixture contains 5.60 mol NH₃.

Calculate K_c, including units, at this temperature. Give your answer to three significant figures.

The chemist repeats the experiment several times. In each experiment, the chemist makes one change.

The chemist heats the mixture to a higher temperature at constant pressure.

Explain whether the value of K_c would be greater, smaller or the same.

The chemist increases the pressure of the mixture at constant temperature.

Explain whether the value of K_c would be greater, smaller or the same.

This question is about thermodynamics and chemical equilibrium.

Ammonia, NH₃, is synthesised in industry by the reaction between nitrogen and hydrogen.

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

ΔH = −92 kJ mol⁻¹

A chemist mixes 12.0 mol of N₂ and 36.0 mol of H₂​ in a sealed container. The mixture is heated to a constant temperature and allowed to reach equilibrium.

The equilibrium mixture is found to contain 8.00 mol of NH₃.

The total pressure of the equilibrium mixture is 5.00 × 10⁶ Pa.

i) Calculate the mole fraction of each gas in the equilibrium mixture.

[3]

ii) Calculate the partial pressure, in Pa, of each gas in the equilibrium mixture.

[1]

iii) Calculate the value of the equilibrium constant, K_p, for this reaction at this temperature.

Include the units in your answer.

Give your answer to 3 significant figures.

[3]

The standard entropy change, ΔS^θ, for this reaction is −199 J K⁻¹ mol⁻¹.

Calculate the minimum temperature, in K, above which this reaction becomes non-feasible (not spontaneous).

The industrial synthesis of ammonia is carried out at a temperature of 400–450 ^oC and a pressure of 200 atm, using an iron catalyst.

Explain the choice of these conditions.

In your answer, you should consider:

• The effect of temperature and pressure on the equilibrium yield.

• The effect of temperature and pressure on the rate of reaction.

• The compromise between yield, rate, and cost.

This question is about the Contact Process.

Sulfur dioxide and oxygen were mixed in a 2:1 mole ratio and left to reach equilibrium in a flask at a temperature of 750 K. The equation for the reaction between sulfur dioxide and oxygen is shown.

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)     ∆H = -196 kJ mol^-1

At equilibrium, the mole fraction of SO₃ (g) in the mixture was 0.70 and the total pressure in the flask was 150 kPa.

Calculate the partial pressure of each gas in this equilibrium mixture.

Calculate the value of the equilibrium constant (K_p) for the reaction shown in part (a).

Give your answer, to the appropriate number of significant figures, in standard form and state the units.

Explain the effect, if any, of decreasing the temperature on the yield and rate of reaction for the Contact Process.

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)    ∆H = -196 kJ mol^-1

Explain the effect, if any, of decreasing the temperature on the value of K_p for the Contact Process.

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)     ∆H = -196 kJ mol^-1

This question is about the dissociation of sulfur trioxide.

The total pressure of the following equilibrium mixture is 8.2 MPa.

2SO₃ (g) ⇋ 2SO₂ (g) + O₂ (g) 

Calculate the value of K_p when 75% of sulfur trioxide dissociates at a given temperature.

In the reverse reaction, as shown, 2.50 moles of sulfur dioxide are mixed with 1.50 moles of oxygen. 

2SO₂ (g) + O₂ (g) ⇋ 2SO₃ (g)

The total amount of the three gases at equilibrium is 3.20 moles.

Calculate the mole fraction of each gas in the equilibrium mixture.

The total pressure of the equilibrium mixture, shown in part (b), is 11.0 kPa.

Use your answers, from part (b), to calculate K_p for this reaction.

Sulfur trioxide used to be made using the Chamber Process involving a lead catalyst. This process had issues producing higher acid concentrations.

The Contact Process using platinum or vanadium catalysts was developed to remove this issue.

State the effect of a platinum catalyst on the value of K_p.

This question is about reactions of ammonia with oxygen.

Ammonia can react in different ways with oxygen.

Reaction 1: In the presence of a platinum catalyst, ammonia is oxidised to nitric oxide and oxygen is reduced.

Reaction 2: Ammonia is heated with oxygen in the absence of a catalyst producing nitrogen and one other gaseous product. 

Write balanced chemical equations, with whole number coefficients, for these reactions.

Give an expression for the equilibrium constant, K_p, when Reaction 1, from part (a), occurs at 320 K.

Reaction 1 used 3.0 moles of ammonia and 7.0 moles of oxygen. They reacted at 800 K and 450 kPa. At equilibrium, 2.0 moles of nitric oxide were formed.
Calculate a value for the equilibrium constant, K_p, including units, for this equilibrium.
Give your answer to an appropriate number of significant figures.

Reaction 2 used the same conditions of 3.0 moles of ammonia and 7.0 moles of oxygen at 800 K and 450 kPa. At equilibrium, 1.49 moles of nitrogen gas were formed.

i) Calculate the number of moles of each chemical in the equilibrium mixture.

Give your answers to an appropriate number of significant figures.

ii) Suggest whether the K_p for this reaction will be higher or lower than the value in Reaction 1 in part (c). Justify your answer.

This question is about various dissociation and decomposition equilibria reactions.

At high temperatures, gaseous phosphorus(V) chloride dissociates according to the following equation:

PCl₅ (g) ⇋ PCl₃ (g) + Cl₂ (g)

At 550K, the equilibrium amount of each gas is shown in Table 1.

Table 1

Calculate the total pressure of the reaction, if K_p = 526 kPa.

Give your answers to an appropriate number of significant figures.

Nitrogen dioxide decomposes according to the following equation:

2NO₂ (g) ⇋ 2NO (g) + O₂ (g)

At 700 K, the equilibrium amount of each gas is shown in Table 2.

Table 2

Calculate the total pressure of the reaction, if K_p = 9.8 x 10^-3 kPa.

Hydrogen iodide can undergo thermal decomposition according to the following equation:

2HI (g) ⇋ H₂ (g) + I₂ (g)

The value of K_p for this reaction at 700 K is 0.0185.

Calculate the total pressure when the partial pressure of hydrogen iodide, at equilibrium, is 133 kPa. Give your answer to the appropriate number of significant figures.

Dinitrogen tetroxide decomposes according to the following equilibrium:

N₂O₄ (g) ⇋ 2NO₂ (g) 

The value of K_p for this reaction at 300 K is 385 kPa.

The equilibrium mixture contained 2.40 moles of nitrogen dioxide after an initial 3.00 moles of dinitrogen tetroxide were heated at 300 K

Calculate the partial pressures of dinitrogen tetroxide and nitrogen dioxide.

Give your answer to the appropriate number of significant figures.

This question is about the multi-step process to produce methanol using methane and water as starting materials.

Step 1 is the production of synthesis gas, which is a mixture of carbon monoxide and hydrogen that is used to synthesise many chemicals including methanol. 

Step 1: CH₄ (g) + H₂O (g) ⇋ CO (g) + 3H₂ (g)    ∆H = +206 kJ mol^-1

The general industrial conditions for synthesis gas production are pressures of between 1000 - 2000 kPa, at temperatures of 1080 K and a nickel catalyst. 

Figures 1 and 2 show the percentage conversion of methane into carbon monoxide and hydrogen under different conditions.

Use the information, to discuss the industrial conditions used to form synthesis gas as an intermediate step in the production of methanol.

Explain the effects of the general industrial conditions used for temperature and pressure, as stated in part (a), on the value of K_p.

Step 2 is the production of methanol from the synthesis gas mixture.

Step 2: CO (g) + 2H₂ (g) → CH₃OH (g) ∆H = -90 kJ mol^-1 

A 52.6 mol sample of carbon monoxide was reacted with 96.3 mol of hydrogen. When equilibrium was reached at a given temperature, the total pressure of the mixture was 7.90 MPa and contained 22.1 mol of methanol.

Calculate a value for K_p. Give your answer to the appropriate number of significant figures. Give units with your answer.

The yield depends on the temperature and pressure used.

i) Suggest temperature and pressure conditions, for use in industry, to increase the yield for Step 2.

Step 2: CO (g) + 2H₂ (g) → CH₃OH (g)    ∆H = -90 kJ mol^-1 

[2]

ii) Explain the effect that the suggested conditions will have on the value of K_p.

[2]

Methanol, CH_3​OH, is an important chemical feedstock. It is produced industrially by the reaction of carbon monoxide with hydrogen. The reaction is reversible and exothermic.

CO (g) + 2H₂​ (g) ⇌ CH₃​OH (g) ΔH = −91 kJ mol⁻¹

A chemist mixes 0.800 mol of carbon monoxide and 1.20 mol of hydrogen in a sealed container with a volume of 4.00 dm³.

The mixture is heated to a constant temperature and allowed to reach equilibrium.

At equilibrium, the mixture is found to contain 0.300 mol of methanol.

i) Calculate the value of the equilibrium constant, K_c​, for this reaction at this temperature. Include the units in your answer. Show all your working.

[3]

ii) The chemist repeats the experiment at a higher temperature but keeps the initial amounts and volume the same.

State and explain the effect of this temperature change on the value of K_c​.

[2]

* In industry, the synthesis of methanol is typically carried out at a pressure of 50–100 atm and a temperature of 250 °C, using a copper catalyst.

Discuss the choice of these conditions. In your answer, you should consider both equilibrium yield and rate of reaction.