Reversibility and dynamic equilibria
3 marks · EasyThe table below shows the amount of solid and gas present in a sealed container at different times during a chemical reaction.
| Time (min) | Solid (g) | Gas (g) |
|---|---|---|
| 0 | 5.0 | 0.0 |
| 2 | 4.2 | 0.8 |
| 4 | 3.9 | 1.1 |
| 6 | 3.9 | 1.1 |
Between which times does the reaction reach equilibrium?
Give one piece of evidence from the table for your answer to (a).
Write a word or chemical equation for the reversible reaction that could be occurring.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Between 4 and 6 min / after 4 min / at 4 min
(b) Amounts of solid and gas no longer change / values stay the same
(c) \ce{CaCO3(s) \rightleftharpoons CaO(s) + CO2(g)} or "calcium carbonate ⇌ calcium oxide + carbon dioxide" · must show reversible arrow, correct substances
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly identifies when equilibrium is reached (between 4 and 6 min, or 'after 4 min').
B1 (b) · 1 mark · Gives a valid reason based on data remaining unchanged for both solid and gas.
B1 (c) · 1 mark · Writes a correct word or chemical equation for a reversible reaction involving a solid and a gas (e.g. calcium carbonate decomposition).
Worked solution
a) The reaction reaches equilibrium after 4 minutes (between 4 and 6 min).
b) Both solid and gas amounts stay the same from 4 minutes onwards.