Rates of reaction

The rate of a reaction is how fast reactants are used up or products formed: mean rate = quantity of product ÷ time. It can be followed by measuring gas volume, mass loss or how long a solution takes to turn cloudy. On a graph of product against time, the gradient is the rate; it is steepest at the start and becomes zero when the reaction stops.

Collision theory says particles react only when they collide with enough energy (the activation energy). Rate increases with higher temperature (particles move faster, collide more often and with more energy), higher concentration or pressure (more particles in the same space), and larger surface area (smaller pieces expose more particles). A catalyst speeds up a reaction without being used up by providing a route with a lower activation energy.