Edexcel IGCSE Chemistry · Spec 3.17-3.18
Reversible Reactions
Reversible reactions and how they can reach a balance.
Chemistry revision video
Reversible Reactions
Prefer to watch on YouTube? Open this video on YouTube.
Explained
Reversible reactions, and the words that are not interchangeable
The products of a reversible reaction can react together to re form the reactants. It runs in both directions, and it is shown by a double arrow with a half head on each line rather than a single arrow.
Most reactions you meet go one way only. Burning magnesium gives magnesium oxide, and no amount of waiting turns it back. A reversible reaction is different in kind, not merely in degree, and the double arrow is the notation that says so.
The two examples to know
Hydrated copper sulfate is blue. Heat it and it turns white, becoming anhydrous copper sulfate, as the water of crystallisation is driven off. Add water to the white solid and it turns blue again, releasing heat as it does.
That colour change makes anhydrous copper sulfate a test for water: white to blue means water is present. Anhydrous cobalt chloride does the same job in the other direction, blue to pink.
Ammonium chloride is the second example. Heat the white solid and it decomposes into ammonia and hydrogen chloride, both colourless gases. Where those gases cool, they recombine and a white solid forms again. The equation is written NH3 plus HCl in reversible arrows with NH4Cl, and the state symbols matter here because it is a solid becoming two gases and then a solid again.
Energy runs the other way too
If the forward reaction is exothermic, the reverse reaction is endothermic, and by exactly the same amount.
Hydrating copper sulfate gives out heat, so dehydrating it takes heat in, which is why you have to keep heating it to drive the water off. The energy is not lost or created anywhere in the cycle; it goes out one way and comes back in the other.
That symmetry is worth carrying into the equilibrium topic, because it is the reason raising the temperature favours whichever direction is endothermic.
What examiners say about this topic
A principal examiner report for Edexcel International GCSE Chemistry describes a question asking what kind of reaction a double arrow shows. A large majority gained the mark for reversible reaction, and the report then lists what happened to the rest.
Some wrote equilibrium or dynamic equilibrium, and that was ignored. Some said goes both ways, and that was allowed. And some lost the mark for saying reverse reaction, which the report states plainly is not the same as a reversible reaction.
Three near misses, three different outcomes. Worth pausing on each.
A reverse reaction is one direction of the pair, the backward one. A reversible reaction is the whole thing, both directions available. One word describes half of what the arrow means, so it cannot be credited.
Equilibrium is not wrong, but it is a different idea. It describes what a reversible reaction settles into inside a closed container, once the forward and backward rates have become equal. The question asked what kind of reaction it is, and equilibrium answers what state it reaches.
Goes both ways is allowed because, however informal it sounds, it says exactly the right thing. The mark is for the meaning, not the vocabulary, which is why a plain description in your own words is a safer answer than a technical term used approximately.
Reaching equilibrium
In a closed container, where nothing can escape, a reversible reaction eventually reaches equilibrium.
At the start there are only reactants, so the forward reaction is fast and the backward one cannot happen. As products build up the backward reaction speeds up and the forward one slows down. When the two rates become equal, the amounts of everything stop changing.
This is called dynamic equilibrium, and dynamic is the word doing the work. Both reactions are still happening, at the same rate as each other, so nothing appears to change even though everything is still reacting.
Closed is essential. If a gas can escape, it cannot take part in the reverse reaction, and the equilibrium never establishes. That is why heating ammonium chloride in an open test tube looks like a one way decomposition, and why the solid re forms near the cool top of a closed one.
Spec 3.17-3.18
What you need to know
- Explain what a reversible reaction is
- Recognise the reversible arrow
- Describe an example of a reversible change
Active recall
Quick check
Answer each question before opening the answer.
What does the ⇌ symbol mean in an equation?
That the reaction is reversible — it can go in both directions.
What happens when blue hydrated copper sulfate is heated, and how is it reversed?
It turns white (anhydrous) as water is driven off; adding water turns it blue again.
Chemistry revision app
Take this topic further in the app
275 guided topics with questions marked as you answer them, section checkpoints and timed practice papers. Fifteen topics are free to try, with no card needed.