The formula for half-life in chemistry depends on the order of the reaction. For a zero order reaction, the formula is t½ = [Ao] / 2k. For a first order reaction, t½ = 0.693 / k, and for a second order reaction, t½ = 1 / k [Ao]. Half-life, or t½, is the time that elapses before the concentration of a reactant is reduced to half its initial value.
If the order of a reaction is unknown, sufficient information to determine it must be deduced in order to calculate its half-life. The rate constant, K, for the chemical process or enough information to determine it must be given. In some reactions, the initial concentration [Ao] needs to be known.
With a zero order reaction, the substrate concentration does not affect the speed of reaction. Decreasing the substrate concentration of this type of reaction decreases the half-life. The original concentration and rate constant will also affect the half-life. With first order reactions, the length of half-life remains constant even if the substrate concentration changes, and only the reaction rate constant affects it. For second order reactions, a constant decrease in the substrate concentration lengthens the half-life. Original concentration and the rate constant determine the half-life in second-order reactions.