A reversible reaction in a closed system settles into equilibrium
Definition
Reversible reaction
A reaction in which the products can react to form the original reactants.
- A closed system is one where no reactants or products can enter or leave.
- In a closed system the forward and reverse reactions both take place.
- At the start the forward reaction is fastest, because the most reactant is present.
- As the reactants are used up the forward reaction slows down and the reverse reaction speeds up.
At equilibrium the forward and reverse reactions happen at the same rate
Definition
Dynamic equilibrium
The state reached in a closed system when the forward and reverse reactions occur at equal rates, so the concentrations of reactants and products remain constant.
- Equilibrium is reached when the forward and reverse reactions occur at exactly the same rate.
- The amounts of reactants and products then stay constant.
- Both reactions are still happening, so the equilibrium is described as dynamic.
- Equilibrium can only be reached in a closed system.
Key Idea
- At equilibrium the forward and reverse rates are equal.
- The amounts of reactants and products stay constant, but both reactions continue.
Exam technique
- Say the rates are equal, not that the reactions have stopped.
- State that the amounts of reactants and products stay constant, not that they are equal to each other.
- Point out that equilibrium needs a closed system.
Self review
- What is meant by a closed system?
- When is equilibrium reached in a reversible reaction?
- Why is equilibrium described as dynamic?
- What happens to the amounts of reactants and products at equilibrium?
