Reversible reactions and equilibriumIB MYP Chemistry: Subtopic test

1. 1
   A technician in a school laboratory in Nairobi heats blue crystals of hydrated copper(II) sulfate in a test tube. The crystals turn into a white powder and water vapour condenses near the top of the tube. When the tube has cooled, she adds a few drops of water to the white powder. It turns blue again and the tube feels warm.

   (a)

   Which symbol is used in a chemical equation to show that a reaction is reversible?

   [1 mark]

   • A→
   • B⇌
   • C=
   • D↑

   (b)

   Which observation shows that the change on heating can be reversed?

   [1 mark]

   • AThe white powder turns blue again when water is added
   • BWater vapour condenses near the top of the tube
   • CThe blue crystals turn white when they are heated
   • DSteam is given off when the crystals are heated

   (c)

   Describe what is meant by a reversible reaction, using this experiment as an example.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student seals brown nitrogen dioxide gas, NO₂, in a glass tube kept at a constant temperature. Over several minutes some of the brown gas changes into colourless dinitrogen tetroxide, N₂O₄, as shown by the equation 2NO₂(g) ⇌ N₂O₄(g). After ten minutes the colour stops getting paler and stays the same shade of brown, although both gases are still present.

   (a)

   Why must the system be closed for the reaction to reach dynamic equilibrium?

   [1 mark]

   • ASo that the pressure falls as the reaction proceeds
   • BSo that the reaction stops once the gases are mixed
   • CSo that a catalyst cannot be added
   • DSo that no reactants or products can escape

   (b)

   Which statement describes the mixture once the colour has stopped changing?

   [1 mark]

   • AThe forward reaction has stopped
   • BThe reverse reaction has stopped
   • CBoth reactions are still happening at the same rate
   • DAll of the NO₂ has been used up

   (c)

   Explain why the colour of the gas mixture stays the same even though the reactions have not stopped.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A group of students in a school in Singapore investigate how temperature affects the equilibrium 2NO₂(g) ⇌ N₂O₄(g). NO₂ is a dark brown gas and N₂O₄ is colourless. The forward reaction is exothermic. They seal the same mixture of the gases in three identical tubes and place one tube in a beaker of ice water, one in a beaker of water at room temperature and one in a beaker of water at about 80 °C. After ten minutes the tube in ice water is pale brown, the tube at room temperature is medium brown and the tube in hot water is dark brown.

   (a)

   Identify the independent variable, the dependent variable and one control variable in this investigation.

   [3 marks]

   (b)

   Describe the trend in the results and explain it in terms of the position of equilibrium. Suggest one way to make the comparison of the colours more reliable.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A fertiliser company in Morocco makes ammonia by the Haber process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic. The gases are passed over an iron catalyst at about 450 °C and 200 atmospheres pressure. The mixture leaving the reactor is cooled so that the ammonia condenses and is removed as a liquid, and the unreacted nitrogen and hydrogen are recycled back into the reactor. Most of the ammonia is used to make fertilisers. The hydrogen comes from natural gas, and the plant uses large amounts of energy.

   (a)

   Explain why the company uses a temperature of about 450 °C and a pressure of 200 atmospheres for this reversible reaction.

   [6 marks]

   (b)

   Discuss the benefits and drawbacks of making ammonia fertiliser in this way, and evaluate whether the process should continue to be used.

   [6 marks]

   Total for question 4: 12 marks

End of questions