6.2.5 The effect of changing concentration
Adding a reactant pushes the equilibrium towards the products
Definition
Concentration
The mass of solute present per unit volume of solution, usually measured in g/dm3\text{g}/\text{dm}^3g/dm3 for this calculation.
- Increasing the concentration of a reactant moves the equilibrium to the right.
- More product is formed until a new equilibrium is reached.
- Removing a product as it forms also moves the equilibrium to the right.
Adding a product pushes the equilibrium back towards the reactants
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.
- Increasing the concentration of a product moves the equilibrium to the left.
- More reactant is formed until the equilibrium is restored.
- Removing a reactant also moves the equilibrium to the left.
Example
- Take the equilibrium A+B⇌C+D\text{A} + \text{B} \rightleftharpoons \text{C} + \text{D}A+B⇌C+D.
- Adding more A moves the equilibrium to the right, making more C and D.
- Adding more C moves the equilibrium to the left, making more A and B.
Exam technique
- Say the equilibrium moves to oppose the change in concentration.
- Adding a substance moves the equilibrium away from that side; removing one moves it towards that side.
- Link the shift to whether more product or more reactant is made.
Self review
- Which way does the equilibrium move when you add more reactant?
- What happens to the amount of product when a product is removed as it forms?
- Which way does the equilibrium move when the concentration of a product is increased?
- Explain why adding a reactant makes more product.
