ASChemistryOCRExam Questions3. Periodic Table & EnergyChemical Equilibrium

Chemical Equilibrium (OCR AS Chemistry A): Exam Questions

Exam code: H032

3 hours39 questions

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Chemical Equilibrium

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1
1 mark

The reversible reaction below is allowed to reach equilibrium.

H2 (g) + I2 (g) ⇌ 2HI (g) ΔH = –9.4 kJ mol–1

Which change in conditions would be expected to shift the equilibrium position towards the products?

  • Decrease the pressure

  • Decrease the temperature

  • Increase the pressure

  • Increase the temperature

Choose your answer

2
1 mark

Which statement about dynamic equilibrium is not correct?

  • The concentration of reactants and products remains constant

  • The system must be open

  • The rate of the forward and backward reactions are equal

  • The equilibrium will shift to oppose any change in conditions

Choose your answer

3
1 mark

An equilibrium mixture is established for the reversible reaction below.

H2 (g) + I2 (g) ⇌ 2HI (g) ΔH = –9.4 kJ mol–1

I2 (g) is a purple gas. H2 (g) and HI (g) are colourless gases.

Which change in conditions will cause the equilibrium mixture to become a darker purple?

  • Decrease in pressure 

  • Increase in pressure

  • Decrease in temperature 

  • Increase in temperature

Choose your answer

4
1 mark

The reversible reaction below is allowed to reach equilibrium.

AB (g) + CD (g) ⇌ E (g) + F (g)     ΔH = + 89 kJ mol-1

Which statement is not correct about this equilibrium?

  • The forward reaction is endothermic

  • Kc = fraction numerator left square bracket straight E right square bracket space left square bracket straight F right square bracket over denominator left square bracket AB right square bracket space left square bracket CD right square bracket end fraction

  • A decrease in temperature will increase the yield of AB (g) and CD (g)

  • An increase in pressure will increase the yield of E (g) and F (g)

Choose your answer

5
1 mark

The reversible reaction below is at equilibrium.

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)

What is the expression for Kc?

  • Kc = fraction numerator left square bracket SO subscript 2 right square bracket squared space left square bracket straight O subscript 2 right square bracket over denominator left square bracket SO subscript 3 right square bracket squared end fraction

  • Kc = fraction numerator left square bracket SO subscript 3 right square bracket over denominator left square bracket SO subscript 2 right square bracket space left square bracket straight O subscript 2 right square bracket end fraction

  • Kc = fraction numerator left square bracket SO subscript 2 right square bracket space left square bracket straight O subscript 2 right square bracket over denominator left square bracket SO subscript 3 right square bracket end fraction

  • Kc = fraction numerator left square bracket SO subscript 3 right square bracket squared over denominator left square bracket SO subscript 2 right square bracket squared space left square bracket straight O subscript 2 right square bracket end fraction

Choose your answer

6
1 mark

The reversible reaction below is at equilibrium.

H2 (g) + F2 (g) ⇌ 2HF (g)

What is the expression for Kc?

  • Kc = fraction numerator left square bracket HF right square bracket squared over denominator left square bracket straight H subscript 2 right square bracket space left square bracket straight F subscript 2 right square bracket end fraction

  • Kc = fraction numerator left square bracket HF right square bracket over denominator left square bracket straight H subscript 2 right square bracket space left square bracket straight F subscript 2 right square bracket end fraction

  • Kc = fraction numerator 2 left square bracket HF right square bracket over denominator left square bracket straight H subscript 2 right square bracket space left square bracket straight F subscript 2 right square bracket end fraction

  • Kc = fraction numerator left square bracket straight H subscript 2 right square bracket space left square bracket straight F subscript 2 right square bracket over denominator left square bracket HF right square bracket squared end fraction

Choose your answer

7
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1 mark

A student investigates the equilibrium below.

H2 (g) + I2 (g) ⇌ 2HI (g)

The reaction occurs in a 10 dm3 reaction vessel. The amounts of each substance in the equilibrium mixture are shown in the table.

Substance

Equilibrium amount / mol

H2 (g)

1.50

I2 (g)

0.25

HI (g)

2.34

What is the numerical value for Kc?

  • 3.1

  • 6.2

  • 14.6

  • 62.4

Choose your answer

8
1 mark

The reversible reaction of nitrogen and hydrogen to form ammonia is shown below.

N2 (g) + 3H2 (g) ⇌ 2NH3 (g) ΔH = −92 kJ mol−1

Which statement about this equilibrium is correct?

  • Increasing the pressure gives a higher yield of H2

  • Increasing the temperature gives a higher yield of N2

  • Increasing the temperature gives a higher yield of NH3

  • Decreasing the pressure gives a higher yield of NH3

Choose your answer

9
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1 mark

A student investigates the equilibrium below.

3Cl2 (g) + I2 (g) ⇋ 2ICl3 (l)

The reaction occurs in a 1.00 dm3 reaction vessel. The amounts of each substance in the equilibrium mixture are shown in the table.

Substance

Equilibrium amount / mol

Cl2 (g)

0.50

I2 (g)

0.50

ICl3 (l)

2.00

What is the numerical value for Kc?

  • 4.0

  • 16

  • 32

  • 64

Choose your answer

10
1 mark

The reversible reaction below is allowed to reach equilibrium.

2NOCl (g) ⇌ 2NO (g) + Cl2 (g) ΔH = +77.0 kJ mol−1

Which change in conditions would change the value of Kc?

  • Decreasing the pressure

  • Adding a catalyst

  • Increasing the temperature

  • Increasing the pressure

Choose your answer

1
1 mark

The reversible reaction below is at equilibrium.

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

Which changes in pressure and temperature would shift the equilibrium position towards the products?

Pressure

Temperature

A

Decrease

Decrease

B

Decrease

Increase

C

Increase

Decrease

D

Increase

Increase

    Choose your answer

    2
    1 mark

    The reversible reaction below is at equilibrium.

    N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

    What is the expression for Kc?

    • fraction numerator left square bracket straight N subscript 2 left parenthesis straight g right parenthesis right square bracket space left square bracket straight H subscript 2 left parenthesis straight g right parenthesis right square bracket cubed over denominator left square bracket NH subscript 3 left parenthesis straight g right parenthesis right square bracket squared end fraction

    • fraction numerator left square bracket NH subscript 3 left parenthesis straight g right parenthesis right square bracket squared over denominator left square bracket straight N subscript 2 left parenthesis straight g right parenthesis right square bracket space left square bracket straight H subscript 2 left parenthesis straight g right parenthesis right square bracket cubed end fraction

    • fraction numerator left square bracket straight N subscript 2 left parenthesis straight g right parenthesis right square bracket plus 3 left square bracket straight H subscript 2 left parenthesis straight g right parenthesis right square bracket over denominator 2 left square bracket NH subscript 3 left parenthesis straight g right parenthesis right square bracket end fraction

    • fraction numerator 2 left square bracket NH subscript 3 left parenthesis straight g right parenthesis right square bracket over denominator left square bracket straight N subscript 2 left parenthesis straight g right parenthesis right square bracket plus 3 left square bracket straight H subscript 2 left parenthesis straight g right parenthesis right square bracket end fraction

    Choose your answer

    3
    1 mark

    Carbon monoxide reacts with steam in the following reaction equation:

    CO (g) + H2O (g) ⇌ CO2 (g) + H2 (g) ΔH = −40 kJ mol−1

    Which change will shift the position of equilibrium to the right hand side of the equation?

    • Decrease in pressure

    • Increase in pressure

    • Decrease in temperature

    • Increase in temperature

    Choose your answer

    4
    1 mark

    Chloroethene, CH2=CHCl, is prepared in the presence of a solid catalyst using the equilibrium reaction below.

    CH2ClCH2Cl (g) ⇌ CH2=CHCl (g) + HCl (g) ΔH = +51 kJ mol−1

    Which change would result in an increased equilibrium yield of chloroethene?

    • Increasing the pressure

    • Increasing the surface area of the catalyst

    • Increasing the temperature

    • Use of a homogeneous catalyst

    Choose your answer

    5
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    1 mark

    The reversible reaction below is allowed to reach equilibrium.

    H2 (g) + I2 (g) ⇌ 2HI (g)

    The value of Kc for this reaction is 60. The equilibrium mixture contains 2.0 mol of H2 (g) and 0.30 mol of I2 (g).

    What is the amount, in mol, of HI (g) present at equilibrium?

    • 0.01

    • 0.1

    • 6

    • 36

    Choose your answer

    6
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    1 mark

    A chemist investigates the equilibrium shown below.

    2NO2 (g) ⇌ 2NO (g) + O2 (g)

    The chemist places 4.00 mol of NO2 (g) in a 1.00 dm3 container. The mixture is allowed to reach equilibrium at a constant temperature.

    At equilibrium, the mixture contains 0.800 mol of O2 (g).

    What is the numerical value for Kc?

    • 0.032

    • 0.222

    • 0.356

    • 0.853

    Choose your answer

    7
    1 mark

    The dissociation of gas X2Y4 into XY2 is represented in the equation below.

    X2Y4 (g)  ⇌  2XY2 (g)   ΔH = +60 kJ mol−1

    Which statement is correct?

    • The yield of XY2 will increase if the pressure is decreased

    • A decrease in temperature will increase the yield of XY2

    • The yield of XY2 will increase if a catalyst is added

    • An increase in temperature will shift the equilibrium to the left

    Choose your answer

    8
    1 mark

    Which row describes a chemical system at dynamic equilibrium?

    Concentrations of reactants and products

    Rates of forward and reverse reactions

    A

    Change

    Equal

    B

    Remain constant

    Different

    C

    Change

    Different

    D

    Remain constant

    Equal

      Choose your answer

      1
      1 mark

      The dynamic equilibrium that forms between chromate(VI) and dichromate(VI) ions is shown below. Solutions of chromate ions are yellow and solutions of dichromate ions are orange.

      2CrO42- (aq) + 2H+ (aq) ⇌ Cr2O72- (aq) + 2H2O (l)

      Which of the following statements is/are correct?

      1. Addition of aqueous sodium hydroxide shifts the position of the equilibrium to the left.

      2. This reaction is a reversible and redox reaction.

      3. A homogeneous catalyst can be used to shift the position of the equilibrium to the right.

      • 1, 2 and 3

      • Only 1 and 2

      • Only 2 and 3

      • Only 1

      Choose your answer

      2
      1 mark

      A chemist investigates an unknown reversible reaction at equilibrium. The table shows the effect of changing conditions on the position of equilibrium.

      Change in condition

      Effect on position of equilibrium

      Increasing the temperature

      Shifts to the right

      Decreasing the pressure

      Shifts to the left

      Adding a catalyst

      No shift

      Which statement is correct?

      • The forward reaction is exothermic

      • There are fewer moles of gas on the right-hand side

      • The enthalpy change for the backwards reaction is positive

      • The reaction must be carried out in an open system

      Choose your answer

      3
      1 mark

      An equilibrium for the transport of oxygen by haemoglobin, Hb, is shown below.

      O2 (g) + HbH+ (aq) ⇌ HbO2 (aq) + H+ (aq)

      Which statement(s) is/are correct?

      1. If the pH of the blood is too low there will be more HbO2, increasing the capacity of haemoglobin to carry oxygen.

      2. The central Fe(II) ion has electron configuration 1s22s22p63s23p63d6 and forms dative covalent bonds with oxygen.

      3. On inhalation, the equilibrium in the capillaries surrounding the lungs shifts to the right due to an increased concentration of oxygen.

      • 1, 2 and 3

      • Only 1 and 2

      • Only 2 and 3

      • Only 1

      Choose your answer

      4
      1 mark

      An equilibrium is established for the hydrolysis of ethyl ethanoate.

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

      The numerical value of Kc is 5.26. 600 cm3 of the equilibrium mixture contains 0.270 mol of ethyl ethanoate, 0.450 mol of water, and the same amount of each product.

      What is the concentration, in mol dm−3, of each product?

      • 0.639 mol dm-3

      • 0.799 mol dm-3

      • 1.33 mol dm-3

      • 1.78 mol dm-3

      Choose your answer

      5
      1 mark

      A reversible reaction is shown below.

      3A (l) + B (l) ⇌ 2C (l) + D (l)

      Which statement(s) is/are correct?

      1. If the value of Kc increases when the temperature is increased, the forward reaction has a positive enthalpy change.

      2. The units for Kc in this reaction are dm3 mol−1.

      3. If the concentration of B increases, the value of Kc will increase.

      • 1, 2 and 3

      • Only 1 and 2

      • Only 2 and 3

      • Only 1

      Choose your answer

      6
      1 mark

      The reversible reaction below is allowed to reach equilibrium in a sealed container.

      H2 (g) + I2 (g) ⇌ 2HI (g)  ΔH = −53 kJ mol-1

      The volume of the container is then decreased at a constant temperature.

      What is the effect on the concentration of I2 (g) and the position of equilibrium?

      Concentration of I2 (g)

      Position of equilibrium

      A

      Decreases

      Shifts to the right

      B

      Increases

      Does not shift

      C

      Increases

      Shifts to the right

      D

      Stays the same

      Does not shift

        Choose your answer