How does a catalyst work?
2 marks · NormalState what a reaction profile diagram shows about the effect of a catalyst on: (i) the activation energy (ii) the overall energy change (enthalpy change, ).
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(i) A catalyst provides an alternative pathway with a lower activation energy (shown as the lower peak on the diagram).
(ii) A catalyst does not change the overall energy change (), so the vertical distance between reactants and products stays the same.
Common mistakes: Saying the catalyst changes the energy of reactants or products; Saying the catalyst lowers enthalpy change
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Marking points
B1 · 1 mark · States that the catalyst lowers the activation energy (by providing an alternative pathway with lower activation energy).
B2 · 1 mark · States that the catalyst does not change the overall energy change () or the energy difference between reactants and products.
Worked solution
(i) A catalyst lowers the activation energy by providing an alternative pathway for the reaction, so the peak on the reaction profile diagram is lower when a catalyst is used.
(ii) The overall energy change () does not change when a catalyst is used, so the difference in energy between reactants and products is unaffected.