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)rightwards harpoon over leftwards harpoon  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 = fraction numerator p to the power of 2 space end exponent SO subscript 3 over denominator p squared italic space SO subscript 2 space times space p straight O subscript 2 end fraction

  • Kp = fraction numerator p squared space 2 SO subscript 3 over denominator p squared space 2 SO subscript 2 times p straight O subscript 2 end fraction

  • Kp = fraction numerator p squared space SO subscript 2 times p straight O subscript 2 over denominator p squared space SO subscript 3 end fraction

  • Kp = fraction numerator p squared space 2 SO subscript 2 times p straight O subscript 2 over denominator p squared space 2 SO subscript 3 end fraction

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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

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5
1 mark

The Kp expression for a reaction is Kp = fraction numerator p left parenthesis text CH end text subscript 3 text OH end text right parenthesis over denominator p left parenthesis text CO end text right parenthesis times p left parenthesis text H end text subscript 2 right parenthesis squared end fraction

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

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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

Choose your answer

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

Choose your answer

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) rightwards harpoon over leftwards harpoon 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 

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2
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1 mark

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

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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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