Collision theory, the factors affecting the rate of reaction (concentration, pressure, surface area, temperature and catalysts), and explaining each factor in terms of the frequency and energy of collisions.

A six-mark Level of Response question. Reward explaining each factor through collision theory (reactions happen when particles collide with at least the activation energy): increasing the temperature gives the particles more kinetic energy so they move faster, collide more frequently, and a greater proportion of collisions have enough energy (exceed the activation energy) to react. Increasing the concentration means there are more particles in the same volume, so collisions are more frequent. Increasing the surface area (using smaller pieces or a powder) exposes more particles at the surface, so there are more frequent collisions with the other reactant. Markers reward, for each factor, the link to more frequent collisions, and for temperature also the point that more collisions are successful (have enough energy). A common error is to say particles "get bigger" or to forget the energy point for temperature.

A Higher tier application question. Reward: using a powder instead of large chips increases the surface area (for the same mass), so more marble particles are exposed at the surface and there are more frequent collisions with the acid particles per second, which increases the rate of reaction (the reaction is faster). However, the total volume of gas produced does not change, because the same mass (and so the same number of moles) of marble reacts with the acid; only the speed changes, not the amount of product. Markers credit the larger surface area giving more frequent collisions and a faster rate, and the point that the total gas volume stays the same because the amount of reactant is unchanged. A common error is to say more gas is produced.