Rates of reaction: why temperature speeds up a reaction, mark by mark

THE QUESTION

3 marks, and each one is for a different sentence

Almost every student knows the answer to this one, and many still score two out of three. The marks are for three separate statements, and the one that gets skipped is nearly always the same.

WHAT EARNS THE MARKS

The sentence each mark is for

1. Mark 1 of 3The particles gain kinetic energy and move faster, so they collide more frequently.This mark is for collisions. Saying the particles have more energy is only half of it. The scheme wants what that energy does: the particles move faster and meet more often.
2. Mark 2 of 3More particles have energy greater than or equal to the activation energy.This is the mark most answers miss. The activation energy has to be named. A reaction only happens when the colliding particles have at least that much energy, and at a higher temperature a larger share of them do.
3. Mark 3 of 3The frequency of effective collisions increases, so the rate of reaction increases.The last mark joins the two ideas. The word that matters is effective, or successful. More collisions does not earn it, because most collisions do not lead to a reaction.

WHAT LOSES THEM

Answers that sound right and score less

• A student writesThe particles have more energy, so the reaction is faster.It restates the question. No collisions, no activation energy, no marks.
• A student writesMore of the collisions have energy above a certain amount.A certain amount is not the activation energy. The idea is right and the mark is lost on the missing term.
• A student writesHeating lowers the activation energy.It does not. The activation energy of a reaction stays the same when you heat it. Lowering it is what a catalyst does, by providing a different route.
• A student writesThere are more collisions, so the rate increases.It earns the first mark at most. Without effective or successful, the final mark is not given.

THE REST OF THE CHAPTER

The same pattern on the other factors

Questions on the other factors are marked the same way: a statement about the particles, then a statement about effective collisions. Only temperature and catalysts bring the activation energy into it.

• Higher concentrationMore particles in the same volume, so collisions are more frequent and effective collisions are more frequent.
• Smaller pieces of a solidA larger surface area is exposed, so more particles can collide with it each second.
• Higher pressure, for gasesThe particles are closer together, so they collide more frequently.
• A catalystIt provides a different route with a lower activation energy, so more collisions are effective.