GCSE · Chemistry · AQA · Spec 8462

Equilibrium

A sealed flask can look completely still while the reaction inside it keeps going.

Seal it in, then let time pass

Time after sealing: 0. State: Just sealed. Product particles: 0. static

Time after sealing0

Only reactant particles so far, sealed in so that nothing can escape. Some of them are already turning into product particles.

Reactant particles are drawn on the left and product particles on the right so you can count them. In the real vessel they are all mixed together, and any particle can change either way (⇌).

Reading the rates

Two rates, one height

Showing 3 layers: Axes, Forward reaction rate, Reverse reaction rate

Layers

Explore

tap the graph ↑

The same sealed reaction at equilibrium, drawn as two rates against time. Tap a part of the graph, or hide one line to see that the other is still there.

Rate of the forward and reverse reactions against time, for a reversible reaction in a sealed flask once it has reached equilibrium.

Predict, then check

Think about what the reverse reaction needs in order to happen.

The same reversible reaction is run in an open container, and one of its products is a gas that escapes into the air. Will the reaction reach equilibrium?

What is true at equilibrium?

Which idea do you hold?

A reversible reaction reached equilibrium in a sealed flask some time ago. Since then, the amounts of reactants and products have not changed.

Which is closest to what you think is true in the flask now?

The reaction has finished. Nothing is reacting any more, which is why nothing changes.Both reactions are still going, at the same rate as each other.The flask now holds equal amounts of reactants and products.All the reactants have been used up, so there is nothing left to change.

How sure are you?

I'm sureI think soI'm guessing

The reaction has finished. Nothing is reacting any more, which is why nothing changes.

This is a common misconception

Nothing changing is not the same as nothing happening. Reactants are still turning into products, and products are still turning back into reactants. The two reactions go at exactly the same rate, so each one undoes the other and the amounts stay the same.

Both reactions are still going, at the same rate as each other.

This matches the evidence

Yes. The forward and reverse reactions are both still happening, at exactly the same rate. Whenever some reactant turns into product, the same amount of product turns back, so the amounts stay constant.

The flask now holds equal amounts of reactants and products.

This is a common misconception

Equilibrium makes the RATES equal, not the amounts. The amounts stay constant, but they can be very different from each other. In the particle model at the top of this page, equilibrium settled at 16 product particles and 24 reactant particles.

All the reactants have been used up, so there is nothing left to change.

This is a common misconception

In a sealed flask the reactants are never all used up: as soon as products form, some of them turn back into reactants. The amounts stop changing while both reactants and products are still present.

At equilibrium both reactions are still going, at the same rate, so the amounts stay constant. Constant, not equal.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

A reaction that looks finished can still be running in both directions.

What you need to know

A reversible reaction can only reach equilibrium in apparatus that stops the reactants and products escaping.
Equilibrium is reached when the forward and reverse reactions happen at exactly the same rate.
At equilibrium both reactions are still happening; the reaction has not stopped.
At equilibrium the amounts of reactants and products stay constant, but they are not necessarily equal.

The big picture

When a reversible reaction happens in apparatus that stops reactants and products escaping, it eventually reaches equilibrium: the point where the forward and reverse reactions happen at exactly the same rate. Neither reaction stops. Because each one undoes the other at the same rate, the amounts of reactants and products stay constant — which is not the same as equal.

Key points

1Seal a reversible reaction so that nothing can escape, and it heads towards equilibrium.

2Equilibrium is the point where the forward and reverse rates become exactly equal — and they then stay equal.

3Both reactions keep going at equilibrium, so the amounts stay constant: constant, not equal.

4If a product can escape, it cannot turn back, and equilibrium is not reached.

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Worked example

Problem

A reversible reaction is carried out in a sealed flask. A graph of the amount of product against time rises steeply at first, then more slowly, then levels off and stays level. Explain what is happening in the flask once the line has levelled off.

⚠ Watch out

Saying that the reaction has stopped at equilibrium, or that there are equal amounts of reactants and products. At equilibrium both reactions are still going at the same rate, and the amounts are constant, not necessarily equal.

🧠

Memory hook

Still on the outside, busy on the inside: equilibrium is two reactions cancelling out, not one reaction stopping.

✓

Check yourself

Without looking back: what must be true of the forward and reverse reactions at equilibrium, what apparatus does it need, and why does a level amount not mean the reaction has stopped?

Flashcards

(5)

When is equilibrium reached in a reversible reaction?

When the forward and reverse reactions are happening at exactly the same rate.

What must the apparatus do for a reversible reaction to reach equilibrium?

Stop the reactants and products from escaping.

Has the reaction stopped once it reaches equilibrium?

No. Both reactions are still happening; they just cancel each other out.

Are the amounts of reactants and products equal at equilibrium?

Not necessarily. They stay constant, but they can be very different from each other.

Why can a reaction not reach equilibrium if a product escapes?

The escaped product cannot turn back into reactants, so the forward and reverse reactions never reach the same rate.

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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Seal it in, then let time pass

Which idea do you hold?

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