Rates of reaction
Chemistry · GCSE · The School
Collision theory
A reaction happens when particles collide with enough energy. So anything that makes collisions MORE FREQUENT or MORE ENERGETIC speeds the reaction up. That single idea explains every factor below, which is why it is worth learning before the list rather than after it.
The four factors
TEMPERATURE: particles move faster, so they collide more often and harder. CONCENTRATION (or pressure for gases): more particles in the same space means more collisions. SURFACE AREA: a powder exposes far more of itself than a lump. CATALYST: provides a route needing less energy, and is not used up.
Reading the graph
Plot product against time and the line is steepest at the start, where reactants are most concentrated, then flattens as they are used up, then goes horizontal when the reaction finishes. The GRADIENT is the rate. Two experiments ending at the same height reached the same amount of product — a steeper line just got there faster.
Marble chips in acid, with the gas collected each 30 s: 0, 22, 38, 48, 54, 57, 58, 58 cm³. The curve is steepest at the start, where the acid is most concentrated and there are most successful collisions per second. It levels off at 58 cm³ because a reactant has run out. Rate over the first 30 s = 22 ÷ 30 = 0.73 cm³/s; over the last 30 s it is 0. Steepness IS rate — that is what the gradient means.