Quick questions on Rate of reaction and collision theory explained: O-Level Combined Science

Increasing the concentration of a dissolved reactant puts more particles in the same volume. The particles collide more frequently, so there are more successful collisions per second and the rate increases. (For gases, increasing the pressure does the same thing.)

Increasing the temperature has two effects. The particles move faster, so they collide more often. More importantly, a greater fraction of particles have energy above the activation energy, so a much greater proportion of collisions are successful. The second effect dominates, which is why a small rise in temperature gives a large rise in rate.

Breaking a solid into smaller pieces, or grinding it to a powder, increases its surface area. More of the solid's particles are exposed for the other reactant to collide with, so the frequency of successful collisions increases and the rate goes up.

A catalyst speeds up a reaction without being used up. It works by providing an alternative pathway with a lower activation energy, so a greater fraction of collisions have enough energy to react. The catalyst is chemically unchanged at the end and can be reused.

On a graph of product (volume or mass change) against time, the steeper the curve, the faster the rate. The curve is steepest at the start (most reactant present) and flattens as reactants run out; it becomes horizontal when the reaction has finished.

State three ways to increase the rate of a reaction between a solid and a solution. [3 marks]

Explain, using collision theory, why a powdered solid reacts faster than a single lump of the same mass. [2 marks]

State what a catalyst does to the activation energy and whether it is used up. [2 marks]