Half-lives and rate equations

• The half-life (t_1/2) of a reaction is the time taken for half of the reactant to be used up.

• It can be easily calculated from a concentration-time graph by seeing at what time the concentration of a reactant has halved

Working out orders from half lives

• For zero order reaction, the half-life decreases as the reaction progresses

• For a first order reaction, the half-life remains constant as the reaction progresses

• For a second order reaction, the half-life increases as the reaction progresses

Using half-lives to calculate the rate constant of a first order reaction

• The rate constant of a first order reaction can be given by, K = ln2 / t_1/2

• The units will be S^-1