Rate of reaction: factors and graphs

The rate of a reaction is how fast reactants turn into products. Four things change it — concentration, temperature, surface area and a catalyst — and collision theory explains why. A rate graph shows the story: a steep start that levels off as reactants run out.

The factors that change rate

Reactions speed up with higher concentration (or pressure, for gases), higher temperature, greater surface area of a solid, and a catalyst. Each of these is a standard exam variable, and questions often ask you to change one while keeping the others constant.

Why — collision theory

Particles must collide, and collide hard enough, to react. More concentration or smaller pieces mean more frequent collisions; higher temperature means collisions are more frequent and more energetic, so more of them succeed. A catalyst provides an easier route that needs less energy, speeding the reaction without being used up.

Reading rate graphs

Plot product formed (or reactant used) against time and you get a curve that is steepest at the start — when concentration is highest — and gradually levels off as reactants are consumed. The gradient is the rate. A faster reaction has a steeper initial slope and reaches its plateau sooner; the plateau height shows the total amount produced.

Comparing experiments

When two curves are drawn together, compare two things: which is steeper at the start (the faster reaction) and where each plateaus (the amount of product). Two reactions can finish at the same height but at different speeds, or at different heights if the amounts of reactant differ.