GCSE · Chemistry · Edexcel · Spec 1CH0

Dynamic equilibrium

Seal a reaction in a flask and wait. Soon it looks finished. It isn't: inside, two reactions are still running in opposite directions, perfectly matched.

Watch a sealed container reach equilibrium

Stage: 1. State: Start: only reactants. static

Stage1

The container is sealed with only reactants inside. Their concentration is as high as it will ever be, so the forward reaction (reactants → products) runs at its fastest. There are no products yet, so the backward reaction has nothing to work on.

Blue particles are reactants, orange are products, and the lid is on: nothing gets in or out. The ⇌ at the top is your reminder that both reactions keep going the whole time. The exact split shown is an example, not measured data.

What do you really think?

Is anything still happening?

A reversible reaction has been sealed in a container for a while. The amounts of reactants and products have stopped changing.

Which is closest to what you think is going on inside right now?

The reaction has stopped. Nothing is reacting any more.There must now be equal amounts of reactants and products.The forward reaction has finished, and only the backward reaction is still going.Both reactions are still going, at the same rate, so the amounts stay the same.

How sure are you?

I'm sureI think soI'm guessing

The reaction has stopped. Nothing is reacting any more.

This is a common misconception

It certainly looks that way, but no. Reactants are still turning into products, and products are still turning back into reactants. The two reactions happen at the same rate, so they cancel each other out and the amounts stay the same. That's exactly why it's called DYNAMIC equilibrium: it's still moving.

There must now be equal amounts of reactants and products.

This is a common misconception

"Equilibrium" sounds like "equal", which makes this an easy trap. What's equal is the two RATES, not the amounts. The concentrations of reactants and products are constant, but they don't have to be equal. In the container you just watched, there were more reactants than products at equilibrium.

The forward reaction has finished, and only the backward reaction is still going.

This is a common misconception

Test that idea. If only the backward reaction were running, products would keep turning into reactants and the amounts would keep changing. But they aren't changing. So the forward reaction must still be happening too, at the same rate as the backward one.

Both reactions are still going, at the same rate, so the amounts stay the same.

This matches the evidence

Spot on. Neither reaction has stopped: the forward and backward reactions are running at equal rates. So the concentrations of reactants and products stay constant, even though they needn't be equal to each other.

At equilibrium it's the RATES that are equal. The amounts are constant, not necessarily equal, and neither reaction has stopped.

Why the lid has to be on

Closed system (sealed container)vsOpen system (e.g. an open beaker)

Read the top row first: that one difference decides everything below it.

Focus

Can substances get in or out?

Closed system (sealed container)

No. Nothing can enter or leave.

Open system (e.g. an open beaker)

Yes. A gaseous product, for example, can escape.

The insight

This is the one difference. Everything else follows from it.

Where do the products go?

Closed system (sealed container)

They stay inside and build up.

Open system (e.g. an open beaker)

An escaping gaseous product doesn't build up; it leaves.

Can the backward reaction keep up?

Closed system (sealed container)

Yes. It speeds up as products build up, until it matches the forward rate.

Open system (e.g. an open beaker)

No. With product escaping, the backward reaction can't keep pace with the forward one.

Is equilibrium reached?

Closed system (sealed container)

Yes, eventually.

Open system (e.g. an open beaker)

No.

Your turn: write it

Say it in your own words

Explain what is meant by dynamic equilibrium. [4 marks]

Your answer

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

The mark scheme

The forward and backward reactions are both still happening (the reaction has not stopped). (1 mark)
The forward and backward reactions happen at the same rate. (1 mark)
So the concentrations (amounts) of reactants and products stay constant. (1 mark)
It happens in a closed system, where nothing can enter or leave. (1 mark)

Model answer

In a closed system, where nothing can get in or out, a reversible reaction reaches a point where the forward and backward reactions are still both taking place, but at the same rate. Because reactants are being used up exactly as fast as they are re-formed, the concentrations of the reactants and products stay constant, even though they are not necessarily equal.

Check your answer for all four ideas: still reacting, same rate, constant amounts, closed system. The one people drop most easily is 'still reacting'.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why a reaction that looks finished hasn't stopped at all, and what is really equal when a reaction reaches equilibrium.

What you need to know

In a reversible reaction the products can react to re-form the reactants, so the reaction can go in both directions. The symbol ⇌ shows a reversible reaction.
The forward reaction turns reactants into products. The backward reaction turns products back into reactants.
In a closed system (nothing can enter or leave), a reversible reaction eventually reaches equilibrium.
At the start the forward reaction is fastest, because the reactant concentrations are highest. As products build up, the backward reaction speeds up and the forward reaction slows down, until the two rates are equal.
Dynamic equilibrium: the forward and backward reactions are still both happening, at the same rate, so the concentrations of reactants and products stay constant. It's 'dynamic' because the reactions haven't stopped, even though nothing appears to change.
At equilibrium the concentrations are constant but not necessarily equal. Equilibrium can only be reached in a closed system: in an open system a gaseous product, for example, can escape, and the backward reaction can't keep pace.

The big picture

In a reversible reaction (shown by ⇌) the products can re-form the reactants, so the reaction runs forward and backward. In a closed system the forward reaction starts fastest; as products build up the backward reaction speeds up and the forward one slows, until the rates are equal. That is dynamic equilibrium: both reactions continue at the same rate, so the concentrations stay constant, though not necessarily equal. It needs a closed system: in an open one a gaseous product, for example, can escape and the backward reaction can't keep pace.

Key points

1⇌ means reversible: the reaction goes forward and backward.

2Forward starts fastest; the backward reaction speeds up as products build up, until the rates are equal.

3At dynamic equilibrium both reactions continue at the same rate, so the amounts stay constant.

4Constant does not mean equal: the amounts of reactants and products needn't match.

5Equilibrium needs a closed system, where nothing can enter or leave.

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Worked example

Problem

A reversible reaction is sealed in a flask. The reactants are colourless and the product is coloured, so the colour shows how much product there is. A student notices three things: at first the colour deepens quickly; later it deepens more slowly; after a while it stops changing. Explain each observation in terms of the forward and backward reactions.

⚠ Watch out

Thinking equilibrium means the reaction has stopped, or that there are equal amounts of reactants and products. Neither is true. At equilibrium the RATES of the forward and backward reactions are equal, so the amounts stay constant, and those amounts can be quite different from each other.

🧠

Memory hook

Equilibrium means equal RATES, not equal AMOUNTS. Picture it as busy, but balanced: everything is still moving, it just all cancels out.

✓

Check yourself

Without looking back: which reaction is fastest at the very start, and why? At dynamic equilibrium, what is equal and what is only constant? Why won't an open beaker reach equilibrium?

Flashcards

(8)

What does the symbol ⇌ show?

A reversible reaction: the products can react to re-form the reactants, so it can go both ways.

Forward reaction vs backward reaction

Forward: reactants → products. Backward: products → reactants.

What is a closed system?

One where nothing can enter or leave.

Right at the start, which reaction wins, and why?

The forward one. The reactants are at their highest concentration.

Products are piling up. Which rate climbs and which drops?

Backward climbs, forward drops, until they meet.

Define dynamic equilibrium.

Forward and backward reactions both still going, at the same rate, so the concentrations of reactants and products stay constant.

At equilibrium, are the amounts of reactants and products equal?

Not necessarily. They're constant, not necessarily equal.

Why can't an open system reach equilibrium?

A product (for example a gas) can escape, so the backward reaction can't keep pace with the forward one.

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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Watch a sealed container reach equilibrium

Is anything still happening?

Say it in your own words

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