Why is 'the particles move faster' not enough for a rate of reaction answer?

Because it is only half the reason, and the half it leaves out is usually the larger one. Temperature increases rate by two separate mechanisms, and most mark schemes want both.

Collision theory, in full

A reaction happens when particles collide with enough energy — the activation energy — and in the right orientation. So the rate depends on two things:

1. How often collisions happen (collision frequency)
2. What proportion of them are successful (have enough energy)

Any factor that changes the rate does it through one or both of these. Say which one, and the answer is complete.

The four factors

Temperature. Particles move faster, so collisions are more frequent — that is the half most people write. But faster particles also have more kinetic energy, so a greater proportion of collisions exceed the activation energy. The second effect is much the more important of the two, and leaving it out costs the mark.

Concentration (or pressure for gases). More particles in the same volume means collisions are more frequent. The proportion that succeed is unchanged — the particles have no more energy than before. Only frequency changes here, and saying so shows you understand the distinction.

Surface area. Breaking a solid into smaller pieces exposes more particles to the other reactant, so again collisions become more frequent. A question about powdered versus lump calcium carbonate is testing this and nothing else.

Catalyst. This one is different and is where answers go wrong. A catalyst does not make particles move faster or collide more often. It provides an alternative pathway with a lower activation energy, so a greater proportion of the collisions already happening are now successful.

Reading the graph

Rate experiments usually plot volume of gas, or mass lost, against time.

The gradient is the rate. Steepest at the start, when concentration is highest, and it flattens as reactants are used up. The curve going flat means the reaction has stopped, not that it slowed down.

Two curves on one axis: the steeper one had the faster rate. If they level off at the same height, the same amount of product formed, so the same quantity of reactant was used — only the rate differed. If one levels off higher, more product formed, which means more reactant, not a faster reaction.

Reading that distinction correctly is worth more marks than the explanation underneath it, and it is the step students skip.