Rates of Reaction

Rate of reaction measures how quickly reactants are converted into products. It can be calculated by measuring the amount of reactant used up or product formed over a period of time. Fast reactions (like explosions) happen in fractions of a second, while slow reactions (like rusting) take days or years.

Rate can be measured by: collecting gas volume produced over time, measuring mass lost as gas escapes, observing colour change intensity, or timing how long until a precipitate obscures a mark.

Collision theory explains that for a reaction to occur, reactant particles must collide with each other. But not every collision causes a reaction—particles must collide with sufficient energy (at least the activation energy) and with the correct orientation.

Anything that increases the frequency of collisions or the energy of collisions will increase the rate of reaction. This is the basis for understanding why the five factors affect rate.

Temperature: Higher temperature gives particles more kinetic energy. They move faster, collide more frequently, and more collisions have energy ≥ activation energy. A 10°C rise roughly doubles the rate.

Concentration: Higher concentration means more particles in the same volume. This increases collision frequency, so rate increases. For gases, increasing pressure has the same effect—particles are pushed closer together.

Surface area: Breaking a solid into smaller pieces exposes more particles at the surface. More particles can collide at the same time, increasing rate. Powder reacts faster than lumps.

Catalyst: A catalyst provides an alternative pathway with lower activation energy. More particles now have sufficient energy to react. Catalysts are not used up and can be reused.