1. 1
   A chemist studies a reversible reaction held at equilibrium inside a sealed container, and separately changes the temperature, then the pressure, of the system, recording how the relative amounts of reactants and products change each time.

   (a)

   A reversible reaction is held at equilibrium in a sealed container, and the temperature is then changed. What happens to the relative amounts of reactants and products at equilibrium?

   [1 mark]

   • AThey change to new relative amounts
   • BThey remain exactly the same as before
   • CThe reaction stops completely
   • DOnly the reactants remain, with no products

   (b)

   Which of these conditions can affect the relative amounts of reactants and products present at equilibrium, according to the chemist's results?

   [1 mark]

   • AOnly the colour of the container
   • BTemperature and pressure
   • COnly the size of the container, not temperature or pressure
   • DNone of the conditions have any effect

   (c)

   Explain why it is important for the chemist to keep the system as a closed system when investigating how conditions affect the position of equilibrium.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student studies a reversible reaction at equilibrium, in which the forward reaction is exothermic and the reverse reaction is endothermic, and considers how the position of this equilibrium might respond to a change in reaction conditions.

   (a)

   A reversible reaction at equilibrium has one direction that is exothermic and the opposite direction that is endothermic. If the temperature of this system is changed, what would be expected to happen to the relative amounts of reactants and products?

   [1 mark]

   • AThe exothermic direction would always be favoured, regardless of temperature change
   • BThey would stay exactly the same regardless of temperature
   • CThey would change, since equilibrium depends on temperature
   • DEquilibrium can no longer exist once temperature is changed

   (b)

   If the pressure of the gaseous reaction system is changed instead of the temperature, what would generally be expected to happen to the position of equilibrium?

   [1 mark]

   • AAll the gas would immediately turn into reactants only
   • BNothing would change, since only temperature affects equilibrium
   • CThe reaction would become irreversible
   • DThe relative amounts of reactants and products would change

   (c)

   State two different conditions that can affect the relative amounts of reactants and products present at equilibrium in a reversible reaction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An engineer is responsible for operating an industrial reactor in which a reversible gaseous reaction is carried out, aiming to maximise the yield of the desired product by adjusting the conditions of the reaction.

   (a)

   An industrial reaction reaches equilibrium inside a closed reactor. State what determines the relative amounts of reactants and products present at equilibrium, and explain why an engineer might want to control the conditions of this reactor carefully.

   [3 marks]

   (b)

   The engineer notices that when the reactor's conditions are changed, the amount of product collected at the new equilibrium is different from before. Explain how this observation supports the statement that the relative amounts of substances at equilibrium depend on the conditions of the reaction.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A textbook author is writing a chapter explaining how the conditions of a reversible reaction affect the position of equilibrium, using both a laboratory-scale reaction and a large industrial process as examples for students.

   (a)

   Explain fully what is meant by the statement that 'the relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction', using a reversible reaction taking place in a closed system as an example, and describe why this principle is important for chemical manufacturing.

   [6 marks]

   (b)

   A reversible reaction is carried out at equilibrium under one set of conditions, and then the conditions are changed twice in succession: first the temperature is changed, and then the pressure is changed. Explain how the position of equilibrium would be expected to respond after each change, and discuss why it is important to change only one condition at a time when investigating such effects.

   [6 marks]

   Total for question 4: 12 marks

End of questions