3.3 Reversible Reactions & EquilibriaEdexcel IGCSE Chemistry: Revision notes
Section 1
What is a reversible reaction?
A reversible reaction is one in which the products can react to re-form the original reactants. This is shown in equations using the symbol ⇌ instead of a single arrow:
A + B ⇌ C + D
This means the reaction can proceed in the forward direction (A + B → C + D) and in the reverse direction (C + D → A + B).
Section 2
What are some example reversible reactions?
- Dehydration of hydrated copper(II) sulfate: blue hydrated copper(II) sulfate crystals, when heated, lose their water of crystallisation to form white anhydrous copper(II) sulfate. Adding water back to the white anhydrous solid reverses the reaction, turning it blue again and releasing heat.
CuSO4·5H2O ⇌ CuSO4 + 5H2O
- Effect of heat on ammonium chloride: solid white ammonium chloride, when heated, decomposes (sublimes) into ammonia gas and hydrogen chloride gas. On cooling, these gases recombine to re-form solid ammonium chloride.
NH4Cl ⇌ NH3 + HCl
The colour change of hydrated/anhydrous copper(II) sulfate (blue ⇌ white) is a classic way examiners test whether water is present in a substance, as well as demonstrating reversibility.
Section 3
What is dynamic equilibrium?
If a reversible reaction takes place in a sealed (closed) container, it can reach dynamic equilibrium. At dynamic equilibrium:
- The forward and reverse reactions occur at the same rate.
- The concentrations of reactants and products remain constant (though not necessarily equal to each other).
It is called 'dynamic' because both reactions are still actively happening — equilibrium does not mean the reactions have stopped, just that they are perfectly balanced.
Dynamic equilibrium does NOT mean the amounts of reactants and products are equal — it means they are constant, which can be at very different fixed levels.
Section 4
Does a catalyst affect the position of equilibrium?
A catalyst does not affect the position of equilibrium. This is because a catalyst speeds up both the forward and the reverse reaction by exactly the same amount (by lowering the activation energy of both pathways equally). Equilibrium is therefore reached faster with a catalyst, but the final equilibrium position (the relative amounts of reactants and products) is unchanged.
Section 5
How do temperature and pressure affect the position of equilibrium?
Changing conditions shifts the position of equilibrium, favouring either the forward or reverse reaction:
- Increasing temperature shifts equilibrium in the endothermic direction (to absorb the extra heat); decreasing temperature shifts it in the exothermic direction.
- Increasing pressure (for gas reactions) shifts equilibrium towards the side with fewer gas molecules; decreasing pressure shifts it towards the side with more gas molecules.
These shifts are examples of the system responding to oppose the change that was made to it.
When asked to explain a shift, always link it to the direction that opposes the applied change — for temperature, name whether that direction is exothermic or endothermic.
Must Know
- A reversible reaction can go forwards and backwards, shown using the ⇌ symbol.
- Examples: dehydration of hydrated copper(II) sulfate (blue ⇌ white); heating ammonium chloride (solid ⇌ ammonia + hydrogen chloride gases).
- In a sealed container, a reversible reaction can reach dynamic equilibrium.
- At dynamic equilibrium, forward and reverse reactions occur at the same rate, and concentrations of reactants/products stay constant (not necessarily equal).
- A catalyst speeds up both forward and reverse reactions equally, so it does NOT affect the position of equilibrium — only how fast it is reached.
- Increasing temperature shifts equilibrium in the endothermic direction; increasing pressure shifts equilibrium towards the side with fewer gas molecules.
That's the notes covered.
Carry on to the next subtopic.