Catalysts and how they lower the activation energy, enzymes as biological catalysts, the effect of catalysts on reaction profiles, and how conditions are controlled to manage rate and yield.

A C5.2 structured question. Reward: a catalyst is a substance that increases the rate of a reaction without being used up (it is not changed chemically or in mass by the end of the reaction). A catalyst increases the rate by providing an alternative reaction pathway with a lower activation energy. Because the activation energy is lower, a greater proportion of the colliding particles have enough energy to react, so more collisions are successful and the rate increases. By the end of the reaction the catalyst is unchanged: it can be recovered and used again. Markers credit the definition (speeds up the reaction without being used up), the lower-activation-energy pathway giving more successful collisions, and the catalyst being unchanged at the end. A common error is to say a catalyst lowers the energy of the reactants rather than providing a lower-activation-energy route.

A Higher tier question on enzymes and catalysts in industry. Reward: an industrial use of an enzyme is, for example, in making biological washing powders (to break down stains), in brewing or in producing foods such as sugar syrups. Enzymes work only over a narrow temperature range because they are proteins: if the temperature is too high they denature (their shape changes) and stop working, and if it is too low they work too slowly, so there is an optimum temperature. Catalysts are used in industry because they increase the rate of reaction, which allows lower temperatures and pressures to be used, saving energy and reducing costs, while the catalyst is not used up so it can be reused. Markers credit a valid enzyme use, the explanation that enzymes denature at high temperature (and are slow at low temperature), and the point that catalysts speed up reactions and save energy and money in industry. A common error is to say enzymes are simply destroyed by cold.