Answers · Reversible reactions and equilibrium
Reversible reactions and equilibrium — final check key
Check meaning, not exact wording. These are teaching points, not official Cambridge marks.
1. XK1 · Final check
Model answer. Water turns white anhydrous copper(II) sulfate blue, and turns blue cobalt chloride paper pink. The other results are the wrong way round, or show no water.
A full answer includes
- Anhydrous copper(II) sulfate turns from white to blue.
Also accept
- White copper(II) sulfate turns blue
Watch for
- “Blue copper(II) sulfate turning white shows water.” — That happens when water is driven out by heating. The test adds the liquid to the white powder.
2. XK2 · Final check
Model answer. (a) The white powder turned blue, so there is water in the liquid. (b) The test can't show whether other substances are dissolved in the water: salty water would turn it blue too. Measure the boiling point: pure water boils at exactly 100 °C (and melts at 0 °C).
A full answer includes
- (a) The liquid contains water.
- (b) Other substances could be dissolved in it; pure water boils at 100 °C (or melts at 0 °C).
Also accept
- Contains water; could have dissolved substances; boiling point 100 °C
Watch for
- “If the test turns blue (or pink), the liquid is pure water.” — The test shows that water is present, not that nothing else is dissolved in it. Check the boiling point (100 °C) or the melting point (0 °C) for purity.
3. XK3 · Final check
Model answer. In a dynamic equilibrium both reactions continue at equal rates, so the concentrations stay the same. They haven't stopped, the amounts needn't be equal, and it needs a closed system.
A full answer includes
- The forward and reverse reactions go on at the same rate.
Also accept
- The forward and reverse reactions go on at the same rate
Watch for
- “At equilibrium the reactions have stopped.” — Both reactions are still going, at the same rate, so the amounts stay the same.
- “At equilibrium there are equal amounts of reactants and products.” — The concentrations are steady, not equal: either side can be larger.
4. XK4 · Final check
Model answer. (a) The forward reaction takes in heat, so a higher temperature moves the position to the right: more hydrogen. (b) 1 + 1 = 2 gas molecules on the left, 1 + 3 = 4 on the right. A higher pressure moves the position to the side with fewer gas molecules, the left: less hydrogen.
A full answer includes
- (a) More hydrogen: heating favours the endothermic forward reaction.
- (b) Less hydrogen: 2 gas molecules on the left, 4 on the right; higher pressure favours the side with fewer.
Also accept
- More (endothermic forward); less (fewer gas molecules on the left)
Watch for
- “A higher pressure always moves the position to the right.” — A higher pressure moves it to the side with fewer gas molecules, which may be either side; with the same number on each side it makes no difference.
5. XK5 · Final check
Model answer. The Haber process runs at about 450 °C and 200 atm with an iron catalyst. 2 atm with vanadium(V) oxide is the Contact process.
A full answer includes
- 450 °C, 20 000 kPa (200 atm), iron catalyst.
Also accept
- 450 °C, 200 atm, iron
Watch for
- “Both processes use the same conditions.” — They share 450 °C, but the Contact process uses 2 atm and vanadium(V) oxide.
6. XK6 · Final check
Model answer. (a) Air and methane. (b) The forward reaction is exothermic, so a lower temperature would move the position to the right, but the reaction would be too slow; at 450 °C ammonia is made fast enough with an acceptable yield. (c) Higher pressures need stronger, more expensive equipment and more energy, and are more dangerous.
A full answer includes
- (a) Nitrogen from the air; hydrogen from methane.
- (b) A lower temperature gives a higher yield (exothermic forward reaction) but is too slow; 450 °C is a compromise.
- (c) Cost of equipment and energy, or the danger of very high pressure.
Also accept
- Air, methane; compromise of yield and rate; cost or safety
Watch for
- “450 °C is used because it gives the highest yield.” — A lower temperature gives a higher yield, because the forward reaction is exothermic. 450 °C is a compromise: a lower yield, but made fast enough.