How concentration changes affect equilibrium Revision Notes | GCSE AQA Higher Chemistry

1. Why Concentration Changes Matter

Picture a busy motorway junction where cars enter and leave at exactly the same rate. The traffic density stays constant - that's equilibrium. But what happens if you suddenly open three new slip roads? More cars flood in, disrupting the balance. Chemical equilibrium works the same way.

When you change the concentration of any substance in an equilibrium mixture, you break the delicate balance. The system must respond to restore equilibrium - but it won't go back to the original concentrations. Understanding this response is crucial for controlling industrial processes and predicting reaction outcomes.

2. The Equilibrium Response System

Think of equilibrium like a perfectly balanced see-saw. When you add weight to one side, the see-saw tips. The system tries to restore balance by moving the fulcrum - but the new balance point is different from before.

In chemical terms, if you increase the concentration of a reactant, you're adding 'weight' to the left side of the equation. The equilibrium shifts right to use up the excess reactant and restore balance. The key insight: equilibrium doesn't return to the original concentrations - it finds a new balance point.

This is where students often get confused. Let's test your understanding of the basic principle:

3. The Two Golden Rules

There are two fundamental rules that govern how equilibrium responds to concentration changes. Master these, and you can predict any equilibrium shift.

Notice both rules push the equilibrium in the same direction - towards products. This makes sense: whether you add reactant or remove product, you're creating a 'shortage' of products relative to reactants.

Let's see if you can apply these rules to a tricky scenario:

4. Working Through Real Examples

Theory is one thing, but exam questions test your ability to apply these rules to specific reactions. Let's work through the classic Haber process example that appears in many GCSE papers.

Worked example: Predicting Equilibrium Shifts in the Haber Process

For the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), predict what happens when: (a) More N₂ is added (b) Some NH₃ is removed

1. N₂(g) + 3H₂(g) ⇌ 2NH₃(g) — Reactants on left, products on right
2. More N₂ added → equilibrium shifts RIGHT — Excess reactant pushes equilibrium towards products
3. More NH₃ is formed until new equilibrium reached — System uses up excess N₂ by making more ammonia
4. NH₃ removed → equilibrium shifts RIGHT — System replaces removed product
5. More N₂ and H₂ react to form more NH₃ — Forward reaction speeds up to restore NH₃ levels

Answer: (a) Equilibrium shifts right, more NH₃ formed (b) Equilibrium shifts right, more NH₃ formed to replace what was removed

Notice how both changes pushed the equilibrium in the same direction. This is Le Chatelier's principle in action - the system opposes the change you make.

5. Industrial Applications

Understanding concentration effects isn't just academic - it's the key to maximising yields in industrial processes. Chemical engineers manipulate concentrations to drive reactions towards desired products.

In the Contact process for making sulfuric acid, sulfur trioxide (SO₃) is continuously removed as it forms. This shifts the equilibrium right, converting more SO₂ to SO₃ and increasing the overall yield. It's like emptying a bucket as it fills - you can collect far more water than the bucket's capacity.

Similarly, in the Haber process, ammonia is liquefied and removed as soon as it forms. This drives the equilibrium towards more ammonia production, despite the unfavourable high temperature needed for a reasonable reaction rate.

6. Exam Technique and Common Traps

Concentration questions are mark-winners if you know the technique, but they're full of traps for the unwary. Here's how to nail them every time.

The biggest trap is thinking equilibrium returns to its original state. It doesn't - it finds a new balance point. When you add more reactant, yes, some gets used up, but the final concentration is still higher than it was originally.

Exam precision note: In How concentration changes affect equilibrium - The effect of changing concentration (HT only) - Reversible reactions and dynamic equilibrium, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

When a concentration changes, equilibrium shifts to reduce the effect of that change and then settles at a new balance point. The new equilibrium does not return all concentrations to their original values. This is why students should describe both the direction of shift and the idea of a new equilibrium being established.