The qualitative effect of temperature changes on the rate of reaction.

The idea

Raising the temperature makes a reaction faster, and usually much faster: as a rough guide a 10 K rise roughly doubles the rate of many reactions near room temperature. A plot of rate against temperature is not a straight line but a curve that steepens, an exponential rise. Two things change when a gas or solution is heated. The particles move faster, so collisions happen more often, and their energy per collision is greater. The second effect dominates by far. The collision frequency rises only slightly, roughly with the square root of the absolute temperature, while the fraction of particles with energy greater than or equal to Eₐ rises steeply, and it is this fraction that multiplies up the number of successful collisions.

What earns the marks

• Stating that rate increases as temperature increases.
• Describing the increase as exponential, or as rising by an ever greater factor.
• Stating that many more particles have energy greater than or equal to the activation energy.
• Identifying the increase in the energy of the particles, rather than the increase in collision frequency, as the dominant effect.

Common mistakes and how to put them right

• Explaining the increase only as more frequent collisions, which the mark scheme refuses on its own.

• Writing more successful collisions with no reference to energy or the activation energy, which is also refused alone.

• Describing the rate against temperature graph as a straight line of positive gradient.

• Saying the activation energy falls when the temperature rises.

• Claiming the particles gain activation energy rather than gaining kinetic energy.

Exam tips

• Always pair the two ideas: particles have more energy, so a greater proportion of collisions exceed Eₐ.
• Say many more particles exceed Eₐ; the word many is what conveys the exponential effect.
• When describing a curve, name the shape, then explain it; description and explanation carry separate marks.