Changing the temperature
4 marks · NormalThe table below shows the percentage of X and Y at equilibrium for a reversible reaction at two temperatures.
| Temperature (°C) | % X at equilibrium | % Y at equilibrium |
|---|---|---|
| 350 | 30 | 70 |
| 150 | 65 | 35 |
Describe how the percentage of X changes as temperature decreases.
Which direction of reaction is favoured at lower temperature? Explain your answer using the data.
State whether the formation of X from Y is likely exothermic or endothermic. Give a reason.
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) The percentage of X increases as temperature decreases.
(b) At lower temperature, the reaction favours the production of X. The data show that percentage of X increases from 30% to 65% as temperature decreases, so the equilibrium shifts towards X.
(c) The formation of X from Y is exothermic because equilibrium shifts towards X as temperature decreases.
Common mistakes: Mixing up percentage trends; Confusing data with process direction
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Correctly states that X increases as temperature drops.
M1 (b) · 1 mark · Uses data to say equilibrium shifts towards X at lower temperature.
M2 (b) · 1 mark · Explains this means production of X is favoured at low temperature (reference to percentage values).
B1 (c) · 1 mark · States X from Y is exothermic and links to equilibrium shift at low temperature.
Worked solution
(a) As temperature decreases, the percentage of X increases (from 30% to 65%).
(b) Lower temperature favours formation of X, as the data show more X present at equilibrium (increasing from 30% to 65%).
(c) The conversion of Y to X is exothermic, because lower temperature favours the direction producing X.