Homogeneous and heterogeneous catalysts

AS Level Chemistry · Reaction kinetics. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

Test this topic freeAll AS Level Chemistry topics

The idea

A catalyst increases the rate of a reaction and is chemically unchanged at the end. It works by giving the reaction a different mechanism, one with a lower activation energy.

On a Boltzmann distribution the curve itself does not change, because the temperature is the same. The activation energy line moves to the left, so a larger proportion of molecules have enough energy to react. On a reaction pathway diagram the catalysed route has a lower peak, but the reactants, the products and ΔH are the same.

A homogeneous catalyst is in the same phase as the reactants. A heterogeneous catalyst is in a different phase, usually a solid with gaseous or liquid reactants.

Rules to remember

Common mistake

Students say a catalyst gives the molecules more energy or changes ΔH. It changes neither; it only lowers the energy barrier by offering a different route.

Worked example

A reaction has ΔH = −40 kJ mol−1 and a forward activation energy of 120 kJ mol−1. A catalyst lowers the forward activation energy to 75 kJ mol−1. What is the activation energy of the reverse reaction with the catalyst?

  1. The reaction is exothermic, so the products lie 40 kJ mol−1 below the reactants.
  2. The reverse reaction must climb from the products to the same peak: reverse EA = forward EA + 40.
  3. With the catalyst: reverse EA = 75 + 40 = 115 kJ mol−1.
  4. Check: without the catalyst it is 120 + 40 = 160, so both directions are lowered by 45 kJ mol−1.

Answer: 115 kJ mol−1

Practice questions

Try each one, then open the answer.

1. How is the effect of a catalyst shown on a Boltzmann distribution curve?

  1. A
    the curve moves to the right and its peak becomes lower
  2. B
    the curve moves to the left and becomes taller
  3. C
    the curve is unchanged, but the activation energy is marked at a lower energy, so a larger area lies beyond it
  4. D
    the total area under the curve increases
Show answer

Answer: C. A catalyst does not change the energies of the molecules, so the curve stays the same. It lowers EA, so more molecules lie to the right of the new EA line.

2. Which statement about a catalyst is correct?

  1. A
    it increases the yield of product
  2. B
    it gives the reactant molecules more kinetic energy
  3. C
    it takes no part in the reaction
  4. D
    it is chemically unchanged at the end, although it may take part in intermediate steps
Show answer

Answer: D. A catalyst provides a different mechanism, for example by forming intermediates or adsorbing reactants, and is regenerated. It does not change molecular energies or the equilibrium yield.

3. Why does adding a catalyst to a system at equilibrium not change the position of equilibrium?

  1. A
    it increases the rates of the forward and reverse reactions by the same factor
  2. B
    it only increases the rate of the forward reaction
  3. C
    it is used up before equilibrium is reached
  4. D
    it changes the value of ΔH
Show answer

Answer: A. The catalyst lowers the activation energy for both directions by the same amount, so both rates increase equally. Equilibrium is reached sooner, but the composition is the same.

More questions on this topic

Read the full notes

Keep going

← Effect of temperature on reaction rates and the concept of activation energyPeriodicity of physical properties of the elements in Period 3 →All rules on one pageStuck? Ask a question