Catalysts in industry
5 marks · HardThe diagram below shows the energy changes during a chemical reaction.
Explain, in terms of activation energy, how a catalyst increases the rate of a reaction.
A student suggests that adding a catalyst would change the total enthalpy change (ΔH) of a reaction. Is this correct? Explain your answer.
Describe, using a reaction profile diagram, what happens to the activation energy and enthalpy change of a reaction when a catalyst is used.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) A catalyst provides an alternative reaction pathway with lower activation energy, so more particles have enough energy to react, increasing the reaction rate. · must show mention of lower activation energy, reference to alternative pathway or route
(b) No, a catalyst does not change the enthalpy change (ΔH) of a reaction. It only lowers the activation energy and does not change the energy of reactants or products. · must show state that ΔH does not change, explanation (reactants/products energy unchanged)
(c) Activation energy is lower when a catalyst is used, but the enthalpy change between reactants and products remains the same. On the diagram, the catalysed pathway shows a lower peak than the uncatalysed pathway; the vertical distance between reactants and products (ΔH) is unchanged. · must show lower activation energy peak, ΔH unchanged
Common mistakes: Thinking catalysts change ΔH.; Explaining only about rate, not energy.; Not mentioning alternative pathway.
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Marking points
M1 (a) · 1 mark · Explains that a catalyst lowers the activation energy of a reaction or provides an alternative pathway.
A1 (a) · 1 mark · Relates the lower activation energy to increased rate (e.g., more particles have enough energy to react).
B1 (b) · 1 mark · States that the enthalpy change (ΔH) does not change when a catalyst is added.
M2 (b) · 1 mark · Explains why (energy of reactants and products is unchanged, only the pathway changes).
B2 (c) · 1 mark · Describes, with reference to a profile diagram, that activation energy is lower with a catalyst, but ΔH (vertical difference between reactants and products) stays the same.
Worked solution
(a) A catalyst increases the rate of a reaction by providing an alternative reaction pathway with lower activation energy. This means that more particles will have enough energy to react.
(b) The student's suggestion is incorrect. A catalyst does not change the total enthalpy change (ΔH) of the reaction. It only lowers the activation energy. The energy of the reactants and products remains the same.
(c) On a reaction profile diagram, the activation energy with a catalyst is shown as a lower peak than without a catalyst. The enthalpy change, represented by the vertical distance between reactants and products, is not altered by the catalyst.