Topic 9 · page 8 of 15

Catalysts and reaction profiles

A catalyst gives the reaction an alternative route with a lower activation energy. On a reaction profile that is two lower humps with a dip between them, while the reactants, products and ΔH stay exactly where they were.

14 min read9.59.6

Read first: Collisions and activation energy

What a catalyst does 9.5

Three points about the definition matter in the exam:

A catalyst does not change the reactants, the products, the enthalpy change ΔH, or the amount of product eventually made. It only changes how quickly the reaction gets there.

The profile without a catalyst 9.6

A reaction profile (an enthalpy profile) plots enthalpy against the progress of the reaction. Reactants start on one level and products end on another. In between there is a hump: the barrier that the activation energy measures. The version for an exothermic reaction is on page 4.

For an endothermic reaction the products are on a higher level than the reactants and ΔH is positive, but the hump and the activation energy work in the same way.

The profile with a catalyst 9.6

A catalyst usually works by forming a bond or interaction with the reactants, creating a new species, then releasing the products. That new species is an intermediate, and it sits in a dip on the profile between two humps.

Reaction profile for an exothermic reaction with and without a catalyst. The uncatalysed route is a dashed grey curve with one high hump. The catalysed route is a solid teal curve with two lower humps and a dip between them labelled intermediate. Both routes start at the same reactants level and end at the same products level, and the delta H arrow is the same. Two vertical arrows show the activation energy without a catalyst, which is large, and with a catalyst, which is smaller.
Exothermic reaction. The catalysed route has two lower humps with the intermediate in the dip. The reactant and product levels are the same, so ΔH is unchanged.

Draw it in this order:

  1. Draw the axes: enthalpy (vertical) and progress of reaction (horizontal).
  2. Draw the reactants and products levels, and the uncatalysed hump. Mark EaE_\text{a} and ΔH.
  3. Draw the catalysed route from the same reactants level to the same products level, with two humps, both lower than the uncatalysed hump.
  4. Put the intermediate in the dip between the two humps, and label it.
  5. Mark the activation energy of the catalysed route from the reactants level to the top of the higher hump.
Reaction profile for an endothermic reaction with and without a catalyst. The products level is above the reactants level, and the delta H arrow points upward and is positive. The uncatalysed route is a dashed grey curve with one high hump. The catalysed route is a solid teal curve with two lower humps and a dip between them labelled intermediate. Both routes start and end at the same levels.
Endothermic reaction. Same rules: same reactant and product levels, two lower humps, intermediate in the dip.

The tops of the humps are the highest-energy points on the route. The intermediate is not at a top. It is a species that exists, even if only briefly, in the dip.

Worked exampleReading a catalysed profile

The enthalpy levels, in kJ mol⁻¹ relative to the reactants (illustrative): reactants 0; products −90; uncatalysed peak +120; catalysed peaks +60 and +45; intermediate −30.

  1. ΔH = products − reactants = −90 − 0 = −90 kJ mol⁻¹, with and without the catalyst.
  2. EaE_\text{a} without catalyst = +120 − 0 = 120 kJ mol⁻¹.
  3. EaE_\text{a} with catalyst = the higher peak above the reactants = +60 − 0 = 60 kJ mol⁻¹.
  4. The intermediate is at −30 kJ mol⁻¹, lower than both peaks.
  5. For the reverse reaction (from the products), EaE_\text{a} = peak − products: 120 − (−90) = 210 without the catalyst and 60 − (−90) = 150 with it. Both fall by 60, so the catalyst speeds up the reverse reaction as well.

Check yourself

  1. Define a catalyst.
  2. List three quantities that a catalyst does not change.
  3. Describe how to show on a profile that a catalyst forms an intermediate.
  4. An endothermic reaction has these enthalpy levels in kJ mol⁻¹ relative to the reactants: products +50; uncatalysed peak +140; catalysed peaks +100 and +90; intermediate +60. Find ΔH, and EaE_\text{a} for the forward reaction with and without the catalyst.
  5. For the reaction in question 4, find EaE_\text{a} for the reverse reaction with and without the catalyst. What do you notice?
  6. A student draws the catalysed profile with the intermediate at the top of a hump and the products lower than for the uncatalysed route. Describe the two errors.

Answers cover these until you have tried the questions

  1. A substance that increases the rate of a reaction by providing an alternative reaction route with a lower activation energy, and which is not used up.
  2. The enthalpy change ΔH, the reactants and products, and the amount of product finally formed (the position of equilibrium). The reactant and product levels on the profile do not change.
  3. Draw the catalysed route with two humps, both lower than the uncatalysed hump, and a dip between them. Label the dip intermediate.
  4. ΔH = +50 − 0 = +50 kJ mol⁻¹ (unchanged by the catalyst). EaE_\text{a} without catalyst = 140 kJ mol⁻¹. EaE_\text{a} with catalyst = the higher peak, +100, so 100 kJ mol⁻¹.
  5. Reverse EaE_\text{a} = peak − products: 140 − 50 = 90 kJ mol⁻¹ without; 100 − 50 = 50 kJ mol⁻¹ with. Both are 40 kJ mol⁻¹ lower, so the catalyst lowers the activation energy of the reverse reaction by the same amount as the forward one.
  6. The intermediate is at the top of a hump when it should be in a dip between the two humps. Lowering the products changes ΔH, which a catalyst never does: the products must be at the same level as without the catalyst.