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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.
Measuring a rate two ways
Either follow a product being made — gas volume by syringe, or mass lost on a balance — or follow a reactant disappearing, such as the cross vanishing under a cloudy solution. Each has a weakness worth naming: gas escapes before the bung is in, and the disappearing cross depends on the person watching.
Reading the gradient, not the height
Rate at any moment is the gradient of the curve, so it is steepest at the start when concentration is highest, and flattens as reactants run out. A horizontal line means the reaction has finished, not that it is slow. For a rate at a given time, draw a tangent and take its gradient.