1. 1
   A chemistry technician demonstrates a classic reversible reaction by heating blue hydrated copper sulfate crystals, which turn into white anhydrous copper sulfate powder and release water vapour: hydrated copper sulfate ⇌ anhydrous copper sulfate + water. The technician then adds water back to the white anhydrous powder.

   (a)

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

   [1 mark]

   • AThe symbol ⇌
   • BThe symbol +
   • CThe symbol =
   • DThe symbol →→

   (b)

   The technician then adds a few drops of water to the white anhydrous copper sulfate powder. What is observed, and what does this demonstrate about the reaction?

   [1 mark]

   • AThe powder catches fire, showing an entirely new reaction has started
   • BThe powder stays white, showing that the reaction cannot be reversed
   • CThe powder dissolves completely with no colour change, showing no reaction occurred
   • DThe powder turns blue again, showing that the reaction has reversed direction

   (c)

   Using this example, explain what is meant by saying the direction of a reversible reaction can be changed by changing the conditions.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A perfume manufacturer produces a fragrant ester by reacting ethanol with ethanoic acid in a sealed flask: ethanol + ethanoic acid ⇌ ethyl ethanoate + water. The forward reaction (forming the ester) is exothermic and releases 15 kJ of energy per mole.

   (a)

   What can be said about the reverse reaction (the ester reacting with water to reform ethanol and ethanoic acid)?

   [1 mark]

   • AThe reverse reaction is also exothermic
   • BThe reverse reaction is endothermic
   • CThe reverse reaction involves no energy transfer at all
   • DThe reverse reaction only occurs at absolute zero

   (b)

   If the forward reaction releases 15 kJ of energy per mole of ester formed, how much energy is absorbed per mole when the reverse reaction occurs?

   [1 mark]

   • A30 kJ
   • B7.5 kJ
   • C15 kJ
   • D0 kJ

   (c)

   Explain why the same amount of energy is transferred in each direction of a reversible reaction such as this one.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A research chemist seals a mixture of brown nitrogen dioxide gas and colourless dinitrogen tetroxide gas in a gas syringe at a constant temperature: 2NO2 ⇌ N2O4. After some time, the colour of the gas mixture stops changing, even though the reaction has not stopped.

   (a)

   Describe what is meant by dynamic equilibrium in this closed system.

   [3 marks]

   (b)

   Explain why this equilibrium is described as "dynamic", even though the colour of the gas mixture in the sealed syringe appears constant and unchanging.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A process engineer is designing a sealed industrial reactor for a reversible gas-phase reaction, having previously tried the same reaction in an open-topped vessel without success.

   (a)

   Explain why a reversible reaction such as this one can only reach dynamic equilibrium in a closed system, and explain why it fails to reach equilibrium in an open system.

   [6 marks]

   (b)

   Explain how the relative amounts of reactants and products present once equilibrium is reached can depend on the conditions used in the reactor, such as temperature and pressure, giving a general explanation of why manufacturers might choose to alter these conditions.

   [6 marks]

   Total for question 4: 12 marks

End of questions