The effect of pressure changes on equilibrium (HT only): Students should be able to interpret appropriate given data to predict the effect of pressure changes on given reactions at equilibrium.

The idea

This higher-tier skill asks you to use supplied data, not memory, to predict how changing pressure shifts a reaction at equilibrium. First count gas moles on each side of the equation, for example N₂(g) + 3H₂(g) ⇌ 2NH₃(g) has 4 mol on the left and 2 mol on the right. Increasing pressure shifts the position of equilibrium towards the side with fewer gas molecules, so here it moves right, raising ammonia yield. Decreasing pressure shifts it towards the side with more gas molecules. If gas moles are equal, pressure has no effect. Given data may include yield tables or graphs at different pressures; read the trend, link it to the mole count, and state the direction of shift and the resulting change in yield or concentration.

What earns the marks

• Count the number of gas molecules on each side of the given equation, treating only gaseous species as relevant to pressure.
• State the rule that increasing pressure shifts equilibrium towards the side with fewer gas molecules, and decreasing pressure shifts it towards the side with more gas molecules.
• Apply the rule to the specific reaction in the data, naming the direction of shift, for example forwards or to the right.
• Interpret the given yield, percentage or concentration data to confirm or predict the effect, for example higher pressure giving a greater yield when the forward reaction reduces gas moles.
• Recognise and state that if the number of gas molecules is equal on both sides, a pressure change has no effect on the position of equilibrium.
• Link the predicted shift to a measurable outcome such as changed yield, changed equilibrium concentration or changed amount of product.

Common mistakes and how to put them right

• Counting all molecules rather than only gaseous molecules.

  Put it right: Count gas moles only, since solids and liquids contribute negligibly to pressure effects.

• Assuming increased pressure always increases yield.

  Put it right: Check which side has fewer gas molecules, because the shift favours that side.

• Reversing the direction of shift.

  Put it right: State clearly that higher pressure favours the side with fewer gas molecules and lower pressure favours the side with more.

• Ignoring the data and reciting a general rule.

  Put it right: Quote or describe the trend in the given figures to support the prediction.

Exam tips

• Write the mole count for each side beside the equation before deciding the direction of shift.
• Use the phrase 'shifts to the right' or 'shifts to the left' and then state the effect on yield or concentration.
• When data are given, refer to specific values or the overall trend rather than making a general statement alone.
• Check whether the question says increase or decrease in pressure, and answer that exact change.