GCSE · Chemistry · AQA · Spec 8462

Effect of changing conditions on equilibrium (HT)

You change the conditions of an equilibrium, and the mixture fights back. So why doesn't it end up exactly where it started?

Higher

Heat an equilibrium and watch it respond

Step: 1 of 4. State: At equilibrium, 30 °C. of the particles are B: 30 %. static

Step1 of 4

Step 1. The mixture is at equilibrium: 12 of the 40 particles are B. A is still turning into B and B is still turning back into A — at the same rate, so the amounts hold steady.

A and B are a made-up pair in a sealed container, A ⇌ B. You're told one fact: the forward reaction, A → B, takes in energy. Move the slider one step at a time.

Higher

Why the mixture shifts

Reason it through

Why does changing a condition leave an equilibrium mixture with different amounts from before?

Link 1 of 4

First link · your turn

The mixture was at equilibrium. You change one condition. Is it still at equilibrium?

Locked — reveal the link above first

Higher

Chemistry · Equilibrium

What does 'counteract' actually mean?

Back to the A ⇌ B mixture. It sat at equilibrium with 30% B, then it was heated. The system responded to counteract the change.

Which is closest to what you think 'counteract the change' means here?

It cancels the change completely, so the mixture ends up back at 30% B.It pushes against the change, so it shifts towards B and settles at a new mix.Nothing can shift — the reactions stopped when it reached equilibrium.It shifts until there are equal amounts of A and B — that's what equilibrium means.

How sure are you?

I'm sureI think soI'm guessing

It cancels the change completely, so the mixture ends up back at 30% B.

This is a common misconception

The system opposes the change, but it can't erase it: the mixture is still at 50 °C. The amounts at equilibrium depend on the conditions, and the conditions are different now, so the mix that settles is different too. That's why it settled at 60% B, not 30%.

It pushes against the change, so it shifts towards B and settles at a new mix.

This matches the evidence

Yes. The reaction that opposes the change gets ahead for a while, which reduces the effect of the change without removing it. Then the two rates match again and the mixture holds steady at a new equilibrium, with more B than before.

Nothing can shift — the reactions stopped when it reached equilibrium.

This is a common misconception

Equilibrium is dynamic: A is still turning into B and B is still turning into A, at the same rate. That's exactly why the system CAN respond — change the conditions and one direction gets ahead of the other. If the reactions had stopped, nothing could shift at all.

It shifts until there are equal amounts of A and B — that's what equilibrium means.

This is a common misconception

Equilibrium means the two RATES are equal, not the amounts. Before heating, 30% B was an equilibrium. Afterwards, 60% B was. Neither is 50:50, and nothing pushes the mixture towards 50:50.

Counteract means oppose, not undo. Both reactions keep going, one gets ahead for a while, and the mixture settles at a new equilibrium — which is almost never equal amounts.

Higher

Your turn

Make the prediction yourself

A sealed container holds X, Y and Z at equilibrium: X + Y ⇌ Z. Extra Y is added. Predict what happens to the amount of Z.

The change: the amount of Y has been increased.Always start by naming exactly what was changed.
missing step

Which line is step 2?

To counteract that, the system must make more Y.To counteract that, the system must use up some of the Y.To counteract that, the system must use up some of the X.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Change the conditions and the mixture fights the change — but it never goes back to where it started.

What you need to know

The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction.
If a system is at equilibrium and any condition is changed, the system responds to counteract the change.
Le Chatelier's Principle lets you predict the effect of changing conditions on a system at equilibrium.
You should be able to make qualitative predictions about the effect of a change when you are given the information you need about the reaction.

The big picture

The relative amounts of reactants and products at equilibrium depend on the conditions. So when a condition changes, the old mixture no longer fits. The system responds to counteract the change: the reaction that opposes it runs faster than the other until the two rates are equal again. The result is a new equilibrium with different amounts. The change has been opposed, not undone. This is Le Chatelier's Principle, and you can use it to predict, qualitatively, what a change will do when you're given the right information about the reaction.

Key points

1Equilibrium is dynamic: both reactions keep going at the same rate. That is why the system can respond at all.

2The system responds by favouring the reaction that opposes the change, until the two rates are equal again.

3The new equilibrium has different amounts from the old one. Counteracting reduces the effect of a change — it does not undo it.

4Equilibrium means equal rates, not equal amounts of reactants and products.

5These predictions are qualitative: which way the mixture shifts and whether an amount goes up or down, not by how much.

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Worked example

Problem

E and F are at equilibrium in a sealed container: E ⇌ F. You're told the forward reaction, E → F, takes in energy. The mixture is cooled. Predict what happens to the amount of F.

⚠ Watch out

Writing that the system 'cancels' the change and goes back to the amounts it had before. It only opposes the change. Once the conditions are different, the amounts at equilibrium are different too, so the mixture always settles at a new equilibrium.

🧠

Memory hook

Push it and it pushes back — but it never goes back.

✓

Check yourself

Cover the page. Explain why an equilibrium mixture can shift at all when both reactions are still going — and why it settles at new amounts instead of going back to the old ones.

Flashcards

(8)

What do the relative amounts of reactants and products at equilibrium depend on?

The conditions of the reaction.

A system is at equilibrium and one condition is changed. How does it respond?

It responds to counteract the change.

What is Le Chatelier's Principle used for?

Predicting the effect of changing conditions on a system at equilibrium.

After a change, does the mixture return to its original amounts?

No. It settles at a new equilibrium with different amounts. The change is opposed, not undone.

Which reaction is favoured after a change?

The one that opposes the change.

Are both reactions still running while the mixture shifts?

Yes. The favoured one runs faster than the other until the rates are equal again.

Does equilibrium mean equal amounts of reactants and products?

No. It means the forward and reverse reactions happen at the same rate.

What kind of prediction does Le Chatelier's Principle give you?

A qualitative one: which way the mixture shifts and whether each amount rises or falls — not by how much.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Heat an equilibrium and watch it respond

What does 'counteract' actually mean?

Make the prediction yourself

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