GCSE · Chemistry · AQA · Spec 8462

Effect of temperature on equilibrium (HT)

Heat a reaction at equilibrium and you can end up with LESS product. More heat, less stuff. Here's how to predict which way it goes, and why.

Turn up the heat: where does the balance settle?

Temperature: 300 °C. State: Starting point. static

Temperature300 °C

The system is at equilibrium. A is still turning into B and B is still turning back into A, but the amounts stay steady. The amounts aren't equal: there's more B than A. Now drag the temperature up or down.

A ⇌ B is a made-up reaction, so the temperatures are just a scale. Its forward reaction, A → B, is exothermic: it gives out energy. That makes the reverse reaction, B → A, endothermic: it takes energy in. Drag the temperature and watch the mix.

The reason behind the shift

Reason it through

Why does raising the temperature favour the endothermic direction?

Link 1 of 4

First link · your turn

You raise the temperature of a system at equilibrium. What have you actually done to it?

Locked — reveal the link above first

What do you really think?

Heat it up, and then what?

A reversible reaction is at equilibrium. Its forward reaction is exothermic. You raise the temperature and wait for it to settle again.

Which is closest to what you think happens?

You get more product. Heating a reaction always gives you more product.The forward reaction is favoured, because heat goes with the exothermic direction: that's the one that makes heat.Nothing changes. Once a reaction has reached equilibrium, the amounts are fixed.The endothermic direction is favoured. Here that's the reverse reaction, so there's less product.

How sure are you?

I'm sureI think soI'm guessing

You get more product. Heating a reaction always gives you more product.

This is a common misconception

It's a natural thought, because heat makes things happen faster. But this question is about where the balance settles, not about speed. Heating favours the endothermic direction. Here that's the reverse reaction, so the relative amount of product goes DOWN. Heating gives more product only when the forward reaction is endothermic.

The forward reaction is favoured, because heat goes with the exothermic direction: that's the one that makes heat.

This is a common misconception

This flips the rule. The system counteracts the change. You've added heat, so the direction that takes heat IN, the endothermic direction, is favoured. Favouring the exothermic direction would push the temperature even higher, which is the opposite of counteracting.

Nothing changes. Once a reaction has reached equilibrium, the amounts are fixed.

This is a common misconception

The amounts stay steady only while the conditions stay the same. The relative amounts at equilibrium depend on the conditions, so change the temperature and the system shifts to a new mix.

The endothermic direction is favoured. Here that's the reverse reaction, so there's less product.

This matches the evidence

Exactly right. You found which way is endothermic first (the reverse, because the forward is exothermic), then applied the rule. The system counteracts the temperature rise by favouring the direction that takes in energy, so the relative amount of product falls.

Heating favours the endothermic direction, whichever way that runs. Always find the endothermic direction first, then decide what happens to the products.

Using the information you're given

Problem

(a) For the reaction C ⇌ D, the forward reaction takes in energy from the surroundings. Predict what happens to the relative amount of D at equilibrium when the temperature is increased. (b) For the reaction F ⇌ G, cooling the equilibrium mixture increases the proportion of G. Is the forward reaction exothermic or endothermic?

Your turn

Predict it, explain it, then check it

Gases P and Q react in a sealed container and reach equilibrium: P + Q ⇌ R. The forward reaction is endothermic. The temperature of the container is lowered. Predict and explain what happens to the relative amount of R at equilibrium. [3 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

The reverse reaction (the exothermic direction) is favoured. (1 mark)
The relative amount of R decreases. (1 mark)
Reason: the system counteracts the fall in temperature, because the exothermic direction gives out energy. (1 mark)

Model answer

The forward reaction is endothermic, so the reverse reaction, R → P + Q, is exothermic. When the temperature is lowered, the system responds to counteract the change, so the exothermic direction is favoured because it gives out energy. That's the reverse reaction, so the relative amount of R at equilibrium decreases.

A complete answer makes three moves: which direction is favoured, what happens to the product, and why (the system counteracts the temperature change).

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

More heat doesn't always mean more product. Sometimes it means less, and you can predict which.

What you need to know

The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction, including the temperature.
If a condition of a system at equilibrium changes, the system responds to counteract the change. You can use Le Chatelier's Principle to predict the effect.
A reversible reaction that is exothermic in one direction is endothermic in the other.
Raising the temperature favours the endothermic direction. Lowering the temperature favours the exothermic direction.
So if the forward reaction is endothermic, raising the temperature increases the relative amount of products and lowering it decreases it. If the forward reaction is exothermic, it's the other way round.
You can work backwards from data: if raising the temperature lowers the amount of product, the forward reaction must be exothermic.

The big picture

At equilibrium, the relative amounts of reactants and products depend on the conditions, and temperature is one of them. Change the temperature and the system responds to counteract the change. Raise it, and the endothermic direction (the one that takes in energy) is favoured. Lower it, and the exothermic direction (the one that gives out energy) is favoured. Whether you get more or less product depends on which way the endothermic direction runs. Predicting effects like this is using Le Chatelier's Principle.

Key points

1Find the energy direction first: which way is exothermic and which way is endothermic? Everything else follows from that.

2If a question describes the energy instead of naming it, translate it first. Energy taken in from the surroundings means endothermic. Energy given out to the surroundings means exothermic.

3A full prediction has three parts: the direction that's favoured, what happens to the relative amount of products, and the reason (the system counteracts the temperature change).

4After a temperature change, both reactions keep going. The system settles at a new equilibrium with a different mix. It doesn't drift back to the old one.

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Worked example

Problem

For the reaction X ⇌ Y, the forward reaction is exothermic. A chemist can keep the equilibrium mixture at either 250 °C or 450 °C. Which temperature gives the greater relative amount of Y at equilibrium?

⚠ Watch out

Assuming that raising the temperature always gives more product. It does only when the forward reaction is endothermic. When the forward reaction is exothermic, raising the temperature gives less product.

🧠

Memory hook

Hot favours endo, cold favours exo. Whatever you do to an equilibrium, it pushes back.

✓

Check yourself

Cover the page. In two sentences, explain why heating an equilibrium favours the endothermic direction. Then say what result would tell you that a forward reaction is endothermic.

Flashcards

(11)

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

The conditions of the reaction, such as the temperature.

A system is at equilibrium and one of its conditions is changed. How does the system respond?

It responds to counteract the change.

What is the name of the principle used to predict the effect of changing conditions on a system at equilibrium?

Le Chatelier's Principle.

Temperature up or temperature down: which direction does each favour?

Up favours the endothermic direction. Down favours the exothermic direction.

What does it mean for the amounts when one direction of a reaction is 'favoured'?

The mix shifts towards what that direction makes, so there is relatively more of it at the new equilibrium.

The forward reaction of a reversible reaction is endothermic. What is the reverse reaction?

Exothermic. A reversible reaction that is endothermic in one direction is exothermic in the other.

A forward reaction gives out energy to the surroundings. What does that tell you?

The forward reaction is exothermic (and the reverse is endothermic).

Why does taking in energy counteract a rise in temperature?

The endothermic direction takes in some of the extra energy that was added, which works against the rise.

Does raising the temperature always increase the relative amount of products?

No. Only when the forward reaction is endothermic. If it's exothermic, the relative amount of products falls.

Raising the temperature increases the proportion of product at equilibrium. What does that tell you about the forward reaction?

It's endothermic, because heating favours the endothermic direction.

After the temperature of an equilibrium is changed and then held steady, do the reactions stop?

No. Both directions keep going, and the system settles at a new equilibrium with a different mix.

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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Turn up the heat: where does the balance settle?

Heat it up, and then what?

Predict it, explain it, then check it

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