Chemistry 9701/11 — May/June 2020

Background Concept

This question is about the equilibrium position and Le Chatelier's principle. When a reaction at equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium shifts to oppose that change. For temperature, if the forward reaction is exothermic ($\Delta H < 0$), increasing temperature favours the reverse (endothermic) reaction; decreasing temperature favours the forward (exothermic) reaction. For pressure, only gaseous species matter: increasing pressure shifts equilibrium towards the side with fewer moles of gas.

The reaction is:

It is exothermic and there is a decrease in total gas moles from 1.5 to 1.

Understanding the Question

The question asks which combination of pressure and temperature gives the greatest equilibrium yield of epoxyethane. "At equilibrium" means we are not concerned with rate; we only need to know which conditions push the position of equilibrium towards products. The options give high/low pressure and 100 or 200 °C.

Approach

Treat pressure and temperature independently. For each condition, decide which direction the equilibrium shifts. Then choose the option that combines the favourable pressure with the favourable temperature.

Step-by-Step Reasoning

1. Temperature: $\Delta H$ is negative, so the forward reaction is exothermic. Lower temperature favours the exothermic direction, i.e. formation of epoxyethane. Between 100 °C and 200 °C, 100 °C is lower, so 100 °C gives greater yield.
2. Pressure: Count gas moles on each side. Left: 1 mol $\text{C}_2\text{H}_4$ + 0.5 mol $\text{O}_2$ = 1.5 mol gas. Right: 1 mol $\text{C}_2\text{H}_4\text{O}$ = 1 mol gas. High pressure shifts equilibrium to the side with fewer gas moles, which is the product side. So high pressure increases yield.
3. Combine: high pressure and 100 °C corresponds to option A.

Key Takeaways

• For an exothermic reaction, lower temperature increases equilibrium yield.
• Pressure affects equilibrium only through gaseous moles; high pressure favours fewer gas moles.
• Yield at equilibrium and rate are different: high temperature may speed up the reaction but reduces equilibrium yield here.

Common Mistakes

• Choosing high temperature because reactions are faster; the question asks equilibrium yield, not rate.
• Forgetting to count the $\frac{1}{2}\text{O}_2$ as 0.5 mol of gas, leading to a wrong mole comparison.
• Thinking pressure affects solids or liquids; only gases matter.
• Confusing the sign of $\Delta H$: negative means exothermic, so lower temperature favours products.

Things to Be Careful About

• Use the $\Delta H$ sign correctly: negative = exothermic.
• Count all gaseous reactants, including fractional coefficients, when applying the pressure effect.
• The condition "at equilibrium" means Le Chatelier's principle applies to the position, not to the speed of reaching equilibrium.
• In the options, 100 °C is the lower temperature, so it is the favourable one for this exothermic reaction.