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.
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.
Finding the rate at one exact moment
The gradient of the whole graph gives the overall trend, but a reaction is not one constant rate — it is fastest at the start and slows as reactants are used up. To find the rate at one particular time, draw a TANGENT to the curve at that point: a straight line that just touches the curve without crossing it, then find its gradient from a rise divided by a run along the tangent, not along the curve. A tangent drawn near the start is steep, giving a high rate; one drawn near the end is shallow. This is the method an exam wants whenever a question asks for the rate "at 40 seconds" rather than "over the first minute".