A LevelChemistryOCRExam Questions5. Physical Chemistry & Transition ElementsEquilibria

Equilibria (OCR A Level Chemistry A): Exam Questions

Exam code: H432

4 hours32 questions

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Equilibria

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1
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1 mark

The reversible reaction of sulfur dioxide and oxygen to form sulfur trioxide is shown below.

2SO2 (g)   +   O2 (g)⇌  2SO3 (g)

An equilibrium mixture contains 2.4 mol SO2, 1.2 mol O2 and 0.4 mol SO3.

The total pressure is 250 atm.

What is the partial pressure of SO3?

  • 15 atm

  • 25 atm

  • 100 atm

  • 200 atm

Choose your answer

2
1 mark

Ethyl ethanoate is formed in an esterification reaction.

ethanol + ethanoic acid ⇌ ethyl ethanoate + water 

The forward reaction is exothermic.

How can the value of the equilibrium constant Kc be increased? 

  • Increasing the temperature

  • Decreasing the temperature

  • Increasing the pressure

  • Adding a catalyst

Choose your answer

3
1 mark

What is the correct Kp expression for the following reaction?

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)

  • Kp = p2 SO3p2 SO2 · pO2

  • Kp = p2 2SO3p2 2SO2·pO2

  • Kp = p2 SO2·pO2p2 SO3

  • Kp = p2 2SO2·pO2p2 2SO3

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4
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1 mark

3.00 mol of gas A reacts with 4.00 moles of gas B in a sealed vessel of volume 1.0 dm3 and total pressure of 1.50 kPa an equilibrium is established which contains 2.00 moles of gas C.  

A + B ⇌ 2C

What is the mole fraction of gas B at equilibrium?

  • 0.29

  • 0.40

  • 0.43

  • 0.75

Choose your answer

5
1 mark

The Kp expression for a reaction is Kp = p(CH3OH)p(CO)·p(H2)2

What is the correct equation for the reaction?

  • CH3OH (g)  ⇋ CO (g) + 2H2 (g)

  • 2CO (g) + 4H2 (g) ⇋ 2CH3OH (g)

  • CO (g) + 2H2 (g) ⇋ CH3OH (g)

  • 2CO (g) + H2 (g) ⇋ 2CH3OH (g)

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1
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1 mark

When 5.00 mol of A reacts with 5.00 mol of B in a sealed vessel of volume 1.0 dm3 and total pressure of 1.5 kPa an equilibrium is established which contains 3.00 moles of C.  

A + B ⇌ 2C

What is the equilibrium partial pressure of A?

  • 0.350 kPa

  • 0.450 kPa

  • 0.525 kPa

  • 2.00 kPa

Choose your answer

2
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1 mark

When dinitrogen tetroxide gas is heated, it dissociates into nitrogen dioxide gas:

N2O4 (g)  ⇌  2NO2 (g) 

At a particular temperature, the equilibrium partial pressure of nitrogen dioxide was 0.67 kPa and dinitrogen tetroxide was 0.33 kPa. 

What is the numerical value of Kp at this temperature? 

  • 0.49

  • 0.74

  • 1.36

  • 2.03

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3
1 mark

The temperature of the gaseous equilibrium below is increased.

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)   ΔH = -197 kJ mol-1

Which statement is correct?

  • The mole fraction of SO2 decreases

  • The partial pressure of SO3 increases

  • The position of equilibrium moves to the right

  • The value of Kp decreases

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4
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1 mark

NO (g), H2 (g), N2 (g) and H2O (g) exist in equilibrium.

2NO (g) + 2H2 (g) ⇌ N2 (g) + 2H2O (g)

At room temperature and pressure, the equilibrium lies well to the right-hand side.

What is the value of the equilibrium constant, K?

  • 1.54 × 10-3 dm3 mol-1

  • 6.50 × 102 mol dm-3

  • 1.54 × 10-3 mol dm-3

  • 6.50 × 102 dm3 mol-1

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5
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1 mark

A mixture of N2 and O2 gases has a total pressure of 1.42 atm. The mole fraction of N2 is 0.700.

What is the partial pressure, in atm, of O2 in the mixture?

  • 0.210

  • 0.426

  • 0.710

  • 0.994

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1
1 mark

Two students set up the equilibrium system below.

CH3COOC2H5 (l) + H2O (l) ⇌ C2H5OH (l) + CH3COOH (l)

The students titrated samples of the equilibrium mixture with sodium hydroxide, NaOH(aq), to determine the concentration of CH3COOH. The students used their results to calculate a value for Kc. The students’ values for Kc were different.

Which statement(s) could explain why the calculated values for Kc were different?

  1. Each student carried out their experiment at a different temperature.

  2. Each student used a different concentration of NaOH (aq) in their titration.

  3. Each student titrated a different volume of the equilibrium mixture.

  • 1, 2 and 3 

  • 1 and 2

  • 2 and 3

  • 1 only 

Choose your answer

2
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9.20 g of dinitrogen tetroxide, N2O4 (g), was heated to a temperature of 350 K at a pressure of 13.5 kPa and dissociated into nitrogen dioxide, NO2 (g). 

N2O4 (g) ⇌ 2NO2 (g)

At equilibrium, 70 % of N2O4 (g) had dissociated. 

What is the numerical value of Kp at 350 K?

  • 0.02

  • 4.67

  • 22.05

  • 51.96

Choose your answer

3
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1 mark

Chloromethane can be made from the following reaction. 0.80 moles of methanol and hydrogen chloride are added to the reaction container. 

CH3OH (g) + HCl (g) ⇌ CH3Cl (g) + H2O (g) 

At 120 ℃, the value for Kp is 6.00 x 103 

Which statement about the number of moles of reactants and products at equilibrium is correct?

  • The number of moles of CH3Cl and H2O is 0.79

  • The number of moles of CH3OH and HCl is 0.79

  • The number of moles of CH3Cl and H2O is 1.26

  • The number of moles of CH3OH and H2O is 0.79

Choose your answer

4
1 mark

Methanol can be produced by a multi-step process using methane and water as the starting materials. The first step is the production of synthesis gas, which is a mixture of carbon monoxide and hydrogen that is used to synthesise many chemicals including methanol. 

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

Which of the following changes would increase the value of Kp for this equilibrium?

  • Increasing temperature

  • Increasing pressure

  • Decreasing temperature

  • Decreasing pressure

Choose your answer

5
1 mark

What are possible Kp units for the decomposition of calcium carbonate?

  • No units

  • kPa-1

  • kPa-2

  • MPa

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