GCSE · Chemistry · AQA · Spec 8462

Effect of pressure on equilibrium (HT)

Pressure doesn't favour products. It favours whichever side has fewer molecules, and the equation tells you which side that is.

Higher

Squeeze the mixture. Watch it rebalance.

Pressure: 1 × start. State: Starting mixture. static

Pressure1 × start

At equilibrium the forward and backward reactions run at the same rate, so the amounts hold steady. Read the equation: 1 molecule on the left, 2 on the right. Now squeeze it.

N₂O₄(g) ⇌ 2NO₂(g). Each dot is one molecule in a sample of the mixture, so you are watching the share of each gas change. The picture shows which way the balance moves, not measured amounts. The temperature stays the same throughout.

Higher

Why fewer molecules?

Reason it through

Why does increasing the pressure shift the position of equilibrium towards the side with fewer molecules?

Link 1 of 4

First link · your turn

The mixture has just been squeezed. What does a system at equilibrium do when one of its conditions is changed?

Locked — reveal the link above first

Higher

Count before you predict

View: 3 · Compare. Showing 3 layers: Equation, Molecules, Totals

View

Layers

Explore

Add up each side: 3 molecules on the left, 2 on the right. The right is the fewer-molecules side. That's all you need before you make any prediction.

Step through the views, then tap each part.

Higher

Which way will it shift?

Sort each equilibrium by what the pressure change does to its position.

The pressure goes UP. Which way does each equilibrium shift?

Still to sort

Shifts right (towards products) (0)

The change favours the right-hand side of the equation.

Shifts left (towards reactants) (0)

The change favours the left-hand side of the equation.

No shift (0)

Pressure can't favour either side.

Where the line is: If both sides have the same number of molecules, neither direction changes the number of molecules, so a pressure change has nothing to counteract with.

5 of 5 still to sort.

Count the molecules on each side, then sort. Then flip the change and sort again.

Higher

Spot the slip

CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g). Predict the effect of increasing the pressure on the amount of hydrogen at equilibrium.

A student's answer — which line goes wrong?

Increasing the pressure means the system will respond to counteract the change.
So the position shifts towards the side with fewer molecules.
Left side: CH₄ and H₂O, so 2 molecules. Right side: CO and H₂, so 2 molecules.
Both sides have the same number of molecules, so the position doesn't move.
The amount of hydrogen at equilibrium stays the same.

Higher

What do you really think?

A sealed mixture of gases is at equilibrium. It is squeezed into a smaller space, so the pressure goes up.

Which is closest to what you think happens?

Raising the pressure always makes more product.Count the atoms on each side, or the number of different substances. The side with fewer is favoured.The position shifts towards whichever side of the equation has fewer molecules. That could be either side.The reaction shifts and then stops. The mixture has finished reacting.

How sure are you?

I'm sureI think soI'm guessing

Raising the pressure always makes more product.

This is a common misconception

Only when the product side has fewer molecules. In N₂O₄ ⇌ 2NO₂, squeezing made LESS product, because the reactant side had fewer molecules. Pressure picks a side by molecule count, not by which side is called 'products'.

Count the atoms on each side, or the number of different substances. The side with fewer is favoured.

This is a common misconception

Atoms can't tell you anything, because a balanced equation always has the same atoms on both sides. And counting different substances ignores the big numbers. That's exactly the slip in the student's answer you just checked: CH₄ + H₂O ⇌ CO + 3H₂ has two substances on each side but 2 molecules against 4. Count molecules, using the big numbers.

The position shifts towards whichever side of the equation has fewer molecules. That could be either side.

This matches the evidence

Yes. The system counteracts the rise in pressure by favouring the side with fewer molecules, and the equation tells you which side that is before anything happens.

The reaction shifts and then stops. The mixture has finished reacting.

This is a common misconception

Both reactions keep going. At the new equilibrium they run at the same rate again. What has changed is the relative amounts of each substance, not whether reacting happens.

Count the molecules on each side of the equation. More pressure favours the side with fewer. That can be the reactants or the products.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Squeeze a gas mixture at equilibrium and it rearranges itself. The surprise: you can predict exactly how, just by counting.

What you need to know

What an increase in pressure does to the position of a gaseous equilibrium.
What a decrease in pressure does to the position of a gaseous equilibrium.
How to count the molecules on each side of a symbol equation.
Why the system shifts that way (Le Chatelier's Principle).
How to predict the effect of a pressure change on a given reaction.

The big picture

When the pressure on a gaseous reaction at equilibrium changes, the system responds to counteract the change. An increase in pressure shifts the position of equilibrium towards the side of the symbol equation with fewer molecules. A decrease shifts it towards the side with more molecules. Count the molecules from the big numbers in front of each gas formula, and you can predict the shift, and so how the relative amounts of reactants and products change, before anything happens.

Key points

1For gaseous reactions at equilibrium, increasing the pressure shifts the position towards the side of the equation with fewer molecules.

2Decreasing the pressure shifts the position towards the side with more molecules.

3Count molecules from the big numbers in front of each gas formula. No number means 1. Don't count atoms or the number of different substances.

4Le Chatelier's Principle: if a condition of a system at equilibrium is changed, the system responds to counteract the change.

5The relative amounts of reactants and products at equilibrium depend on the conditions, so a pressure change alters them.

6If both sides have the same number of molecules, changing the pressure doesn't shift the position.

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Worked example

Problem

N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The pressure on this equilibrium mixture is decreased. Predict the effect on the amount of ammonia (NH₃) at equilibrium, and explain your prediction.

⚠ Watch out

Assuming higher pressure always makes more product, or deciding the direction by counting atoms or different substances. Pressure favours the side of the symbol equation with fewer MOLECULES, counted from the big numbers, and that can be the reactant side.

🧠

Memory hook

Squeeze → fewer. Release → more. And always count the big numbers first.

✓

Check yourself

Flip the particle-model reaction: 2NO₂(g) ⇌ N₂O₄(g). Raise the pressure. Which substance increases now, and why is that the same answer as before, even though N₂O₄ is now the product?

Flashcards

(10)

Pressure goes UP on a gaseous equilibrium. Pressure goes DOWN. Which side does each favour?

Up → the side of the symbol equation with FEWER molecules. Down → the side with MORE molecules.

How do you count the molecules on one side of an equation?

Add up the big numbers in front of each gas formula. A formula with no number counts as 1.

State Le Chatelier's Principle.

If a condition of a system at equilibrium is changed, the system responds to counteract the change.

Why does having fewer gas molecules lower the pressure?

Pressure comes from molecules hitting the container walls. Fewer molecules in the same space at the same temperature means fewer hits.

Both sides of the equation have the same number of molecules. What does a pressure change do?

Nothing to the position. Neither direction changes the number of molecules, so there is no shift.

Does raising the pressure always increase the amount of product?

No. It only does if the product side has fewer molecules. If the reactant side has fewer, you get less product.

Why is counting atoms no use for a pressure prediction?

A balanced equation always has the same atoms on each side, so atoms can never show you a 'fewer' side.

The position of equilibrium shifts to the right. What happens to the relative amounts?

The amounts of products at equilibrium increase and the amounts of reactants decrease.

What decides the relative amounts of reactants and products at equilibrium?

The conditions of the reaction, such as the pressure.

After the position has shifted, have the reactions stopped?

No. The forward and backward reactions carry on at equal rates again. Only the balance of amounts has changed.

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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Squeeze the mixture. Watch it rebalance.

Which way will it shift?

Spot the slip

What do you really think?

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