Multiple choiceQuestion 1 · Recall
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By the end you can explain that a catalyst changes the mechanism of a reaction to one with a lower activation energy, state that it is not used up, and explain why it does not change the enthalpy change of the reaction.
Two towns on opposite sides of a mountain range. The only road goes over a high pass, and only the strongest walkers make it. Build a tunnel under the range and thousands more can cross — same two towns, same start and finish, much lower barrier.
A catalyst is the tunnel. It does not push the reactants uphill or give them extra energy. It offers a different route — a different mechanism — whose highest point is lower.
Because more collisions now have enough energy to get over that lower barrier, the reaction is faster. The reactants and products are unchanged, so the energy difference between them, , is exactly what it was.
Catalyst
A catalyst is a substance that increases the rate of a reaction by providing an alternative pathway (mechanism) of lower activation energy, and is not used up in the overall reaction.
Catalysis is the process of speeding up a reaction in this way. The catalyst may take part in the steps of the mechanism — it is used and then regenerated — but it does not appear in the overall equation.
Enthalpy change depends only on the reactants and the products, not on the route between them. The catalysed and uncatalysed reactions start at the same energy level and end at the same energy level, so
A catalyst cannot make a reaction more exothermic, and it cannot make an impossible reaction happen: if the products are at a higher energy than the reactants, the catalysed reaction is still endothermic.
Nor does a catalyst change the position of equilibrium. It lowers the barrier for the forward and the reverse reaction by the same amount, so both speeds increase and the balance point is unchanged — it is simply reached sooner.
Worked example: A catalytic converter
In a car's catalytic converter, carbon monoxide and nitrogen monoxide react on a platinum–rhodium surface:
Without the metal, the gases can sit mixed for years. Explain, in terms of activation energy, why the converter works and why it does not change .
A catalyst gives the reactant molecules more energy.
It does not add energy. It lowers the energy barrier they have to clear, so the same molecules are now able to react.
A catalyst makes the reaction more exothermic.
is fixed by the reactants and products. A catalyst changes the route, not the start and finish.
A catalyst is used up, which is why you need more of it for more product.
It is regenerated at the end of the mechanism. It can be poisoned or physically lost, but it is not consumed stoichiometrically.
The mark scheme wants a mechanism statement, not just "speeds up the reaction". Write: "provides an alternative pathway/mechanism with lower activation energy".
When asked why is unchanged, say that depends only on the reactants and products (or: the initial and final energy levels are the same).
When asked why a catalyst does not change the equilibrium yield, say that it lowers the activation energy of the forward and reverse reactions by the same amount, so both rates increase equally.
A catalyst increases the yield of an equilibrium reaction.
It speeds up the forward and reverse reactions equally, so equilibrium is reached sooner but the position of equilibrium, and therefore the yield, is unchanged.
A catalyst appears in the overall equation.
It may appear in the steps of the mechanism, but it is not written in the overall equation because it is reformed.