8.2.1The Periodic Table
Reversible Reactions — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 4 parts Lesson 6 of 23 Unit quiz
Part 1 of 4How Equilibrium Is Reached
The Haber reaction is used as the example of a reversible reaction reaching equilibrium.
1
A reversible reaction proceeds in both directions, shown by the symbol ⇌.
2
At the start of the Haber reaction only nitrogen and hydrogen are present, so the forward reaction is at its fastest.
3
As ammonia builds up, the reverse reaction speeds up while the forward reaction slows down as reactants are used.
4
Equilibrium is reached when the forward and reverse rates become equal, and this only happens in a closed system.
Part 2 of 4Features of Equilibrium
Two features describe a system at equilibrium.
Closed system
- Equilibrium can only be reached in a closed system, where no reactant or product can escape or enter.
Equal rates
- At equilibrium the rate of the forward reaction is equal to the rate of the reverse reaction.
Constant concentrations
- Once equilibrium is reached, the concentrations of all reactants and products stay unchanged.
Conditions
- If the temperature and pressure stay the same, the amounts of reactants and products stay fixed.
Part 3 of 4A Dynamic State
Equilibrium is a balance between two opposing reactions.
A Dynamic State
Dynamic
- Both reactions continue at equilibrium, so equilibrium is a dynamic state rather than a stopped reaction.
Balance
- The balance is maintained because the opposing rates cancel out exactly.
Open system
- In an open system gases can escape, so the reaction cannot reach equilibrium.
Exam points
- When asked to describe equilibrium, state that the forward and reverse rates are equal and that concentrations are constant.
Part 4 of 4Exam Checklist & Tips
8.2.1 – Key Points for the Exam
- Reversible reaction A reaction that can occur in both the forward and reverse directions.
- Equilibrium The state in which the forward and reverse reaction rates are equal in a closed system.
- Closed system A system in which no substances can enter or leave.
- Dynamic equilibrium An equilibrium in which both reactions continue at equal rates.
Common Mistakes
- Saying equilibrium means the reaction has stopped; both reactions continue at equal rates.
- Stating that the concentrations of reactants and products are equal at equilibrium;
- they are constant, not equal.
- Describing equilibrium in an open system, where gases escape and equilibrium is never reached.
- Using a single arrow instead of ⇌ for a reversible reaction.
Last updated:
\n