Kinetic energy

Kinetic energy is the energy a particle has because of its motion. At a higher temperature particles have more kinetic energy, so more of their collisions exceed the activation energy. Malay: tenaga kinetik · Chinese: 动能.

In a mixture, particles are always moving, and each carries kinetic energy that depends on how fast it moves. Temperature is a measure of the average kinetic energy of the particles: heating the mixture makes the particles move faster, so their average kinetic energy rises. This matters for rate in two linked ways. First, faster particles collide more often, so the collision frequency goes up. Second, and more importantly, a larger fraction of the particles now has kinetic energy at or above the activation energy, so a greater share of collisions is effective.

Example. When dilute hydrochloric acid reacts with sodium thiosulfate, warming the mixture from room temperature makes the cross under the flask disappear much sooner. The particles have gained kinetic energy, so effective collisions happen more often and the rate is higher. Cooling the mixture has the opposite effect.

Confusion to avoid. Do not say that raising the temperature works only by making particles collide more often. The bigger reason is that more particles now have enough kinetic energy to get over the activation energy barrier, so a larger fraction of collisions is effective. Quoting only the collision frequency misses the main point.

Kinetic energy is the idea that explains why temperature is such a powerful rate factor, linking the collision theory to activation energy, so getting this explanation right is important for SPM Chemistry. A useful check is to name both effects in order: particles move faster, so collisions are more frequent, and a larger fraction now reaches the activation energy, so more of those collisions are effective. Writing the two effects in that sequence keeps the answer both correct and complete.