Equilibria (OCR Gateway GCSE Chemistry: Combined Science)

Exam Questions

28 mins16 questions
11 mark

Look at the equation for the Haber process.

N2 + 3H2 ⇌ 2NH3

What is meant by the symbol ⇌ in the equation?

  • A reaction that involves a catalyst.

  • A reaction that is exothermic

  • A reaction with 100% atom economy.

  • A reversible reaction.

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21 mark

Which statement is true for a reversible reaction when it is at dynamic equilibrium?

  • The concentration of the products is increasing.

  • The rate of the backward reaction is greater than the rate of the forward reaction.

  • The rate of the forward reaction is equal to the rate of the backward reaction.

  • The rate of the forward reaction is greater than the rate of the backward reaction.

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31 mark

Ammonia is made in the Haber process.


N2 + 3H2 rightwards harpoon over leftwards harpoon 2NH3


What happens when this reaction reaches equilibrium?

  • The backward reaction happens at a faster rate than the forward reaction.

  • The forward and backward reactions happen at the same rate.

  • The forward and backward reactions stop happening.

  • The forward reaction happens at a faster rate than the backwards reaction.

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11 mark

The Haber process is an example of a reversible reaction where the forward reaction is exothermic:

N2 (g) + 3H2 (g) rightwards harpoon over leftwards harpoon 2NH3 (g)

What is not true about this system when it is at equilibrium?

  • The energy change of the reverse reaction is endothermic

  • The equilibrium occurs within a closed system

  • The reaction has reached completion and stopped

  • The rate of the forward reaction is equal to the rate of the reverse reaction

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21 mark

Which graph shows the highest yield of products?

  • P1-dWbBs_6-2-mcq-q2a

  • 6-2-mcq-q2b

  • 6-2-mcq-q2c

  • All of the above have the same yield of products

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31 mark

The gasification of carbon is a reversible endothermic reaction:

C (s) + H2O (g) rightwards harpoon over leftwards harpoon CO (g) + H2 (g)

Which of the following conditions could be changed to increase the relative amount of H2 gas produced?

  • Increase the surface area to volume ratio of the carbon

  • Increase the pressure

  • Increase the temperature

  • Add a catalyst

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41 mark

Which statement is not true about Le Chatelier's Principle?

  • If changes to the conditions of a system are made, the system responds to counteract the change

  • If the concentration of the product is decreased, more reactants will react until an equilibrium position is reached again

  • Increasing the pressure will shift the position of the equilibrium to the side with the smaller number of molecules of gas

  • If temperature is decreased for an exothermic reaction, the relative amount of products made at the equilibrium point decreases

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51 mark

Hydrogen gas and iodine vapour can react to form hydrogen iodide:

H2 (g) + I2 (g) rightwards harpoon over leftwards harpoon 2HI (g)

Which statement is not true?

  • This reaction is reversible

  • Equal amounts of reactants and product are eventually made at equilibrium

  • This reaction can reach an equilibrium point in a closed system

  • There are equal total numbers of moles of reactant and as of product

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61 mark

Some systems at equilibrium can be studies by changes in colour.

Fe3+ (aq) + SCN- (aq) rightwards harpoon over leftwards harpoon FeSCN2+ (aq)

yellow     colourless                red

This reaction starts with only the reactants, and reaches equilibrium at a point where there are equal amounts of Fe3+ and FeSCN2+ present. The solutions used are very dilute.

What colour change would you expect to see during the course of the reaction?

  • Yellow to orange to red

  • Yellow to red

  • Yellow to orange

  • Yellow to red to orange

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71 mark

Which option shows two correct statements about reversible reactions and equilibrium?

  Statement 1 Statement 2
A The sign for a reversible reaction is rightwards harpoon over leftwards harpoon Reversible reactions are slower than non-reversible ones
B At equilibrium the rate of the forward reaction and the rate of the reverse reaction are equal An endothermic reaction is exothermic if reversed
C Reversible reactions are not useful in industry as they do not go to completion To reach an equilibrium the reaction must occur in a closed system
D The rate of the reaction is zero at equilibrium The concentration of the product is constant at equilibrium

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    81 mark

    Which statement describes what happens when a reaction reaches equilibrium?

    • The forward reaction happens at a faster rate than the backwards reaction.

    • The forward and backward reactions happen at the same rate.

    • The forward and backward reactions stop happening.

    • The backward reaction happens at a faster rate than the forward reaction.

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    11 mark

    The decomposition of ammonium chloride is endothermic:

    NH4Cl (s) rightwards harpoon over leftwards harpoonNH3 (g) + HCl (g)

    Which changes to the conditions would both decrease the amount of product in this reaction?

      Change 1 Change 2
    A Increase pressure Decrease temperature
    B Decrease pressure Decrease temperature
    C Increase pressure Increase temperature
    D Decrease pressure Increase temperature

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      21 mark

      Higher Only

      Look at the equation for a reversible reaction.

      4NH3 (g) + 5O2 (g) ⇌ 4NO (g) + 6H2O (g)    ΔH = –950 kJ mol–1

      The reversible reaction forms a dynamic equilibrium in a sealed container.

      Which of the following would move the position of equilibrium to the right?

      • Decreasing the pressure and decreasing the temperature

      • Increasing the pressure and decreasing the temperature.

      • Increasing the pressure and increasing the temperature.

      • Increasing the pressure and using a catalyst.

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      31 mark

      Which graph shows a dynamic equilibrium?

      • screenshot-2024-11-06-214536

      • screenshot-2024-11-06-214635

      • screenshot-2024-11-06-214753

      • screenshot-2024-11-06-214835

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