The effect of changing conditions on equilibrium (HT only): If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change.

The idea

This statement expresses Le Chatelier's principle (assessed at Higher Tier only): when a system at dynamic equilibrium is disturbed by a change in conditions, the equilibrium shifts in the direction that opposes the change. If temperature is increased, the endothermic direction is favoured to absorb the added heat; if pressure is increased, the side with fewer gas molecules is favoured to reduce pressure; if a reactant is added, the forward reaction is favoured to use it up.

For example: In N₂(g) + 3H₂(g) ⇌ 2NH₃(g), raising pressure shifts equilibrium right because four gas molecules become two, increasing the ammonia proportion.

What earns the marks

• States that a system at equilibrium responds to a change by shifting in the direction that counteracts the change.
• Applies the principle to temperature: an increase favours the endothermic direction, while a decrease favours the exothermic direction.
• Applies the principle to pressure: an increase favours the side with fewer gas molecules, while a decrease favours the side with more.
• Applies the principle to concentration: adding a substance favours the reaction that removes it, while removing a substance favours the reaction that replaces it.
• Uses a balanced equation to justify the predicted direction of shift for a named change.

Common mistakes and how to put them right

• Saying the system 'wants' to oppose the change without linking to reaction direction.

  Put it right: State which reaction, forward or reverse, is favoured and why.

• Applying pressure rules to equilibria with equal gas molecules on both sides.

  Put it right: Pressure changes have no effect on the position of equilibrium in that case.

• Confusing the effect of a catalyst with a change in conditions.

  Put it right: A catalyst does not shift equilibrium and so does not counteract a change.

Exam tips

• Name the change first, then state the direction of shift, then explain how that counteracts the change.
• For pressure questions, count gas molecules on each side of the equation before predicting the shift.
• For temperature questions, identify whether the forward reaction is exothermic or endothermic from the information given.
• Remember that applying Le Chatelier's principle is a Higher Tier only skill, so expect it in HT papers.