Nitrogen dioxide, NO₂, reacts with itself to make dinitrogen tetroxide, N₂O₄, in an exothermic reaction. This reaction is investigated at 140°C in a sealed container. A dynamic equilibrium mixture is established. 2NO₂(g) ⇌ N₂O₄(g) NO₂(g) is a dark brown gas. N₂O₄(g) is a colourless gas.

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

1. B8(a) Reversible reaction
2. B8(b) (Reaction in which) heat is released
3. B8(c) Idea that no reactants or products can enter / leave
4. B8(d) The colour becomes more brown / colour becomes darker (1) Fewer moles on right hand side so position of equilibrium moves to the left (or reverse argument) / fewer moles on product side so position of equilibrium moves to the left (1)
5. B8(e) The colour more brown / colour becomes darker (1) Exothermic reaction so position of equilibrium moves to the left / backward reaction is endothermic so equilibrium moves to left(1)
6. B8(f)(i) • Add a reactive metal / carbonate to the two acids at the same concentration (1) AND 1 mark for any one of: • Time how long it takes for the metal / carbonate to disappear (1) • Time how long it takes to produce a fixed volume of gas (1) • Count the number of bubbles over fixed time interval (1) • Weak acid has a longer reaction time (or reverse argument) / weak acid produces fewer bubbles in a given time interval (1) OR • Add universal indicator to the two acids at the same concentration (1) AND 1 mark for either one of: • Compare colour with colour chart (1) • Red with strong acid AND yellow with weak acid (1) OR • Dip pH meter into the two acids at the same concentration (1) AND 1 mark for either one of: • Record pH (1) • pH lower for strong acid / pH less for strong acid (than for weak acid) (or reverse argument) (1)
7. B8(f)(ii) KNO₂ AND KNO₃