r=k[A]

^{x}[B]^{y}where:

- k is the rate constant, dependent on the temperature, surface area, etc.
- [A] and [B] are the concentrations of the reactants
- x and y are values determined by experiments.

**Order of reaction**: sum of the rate law exponents

__Half Life:__For some radioactive decay, the concentration of a substance is halved consistently after a certain duration of time. For example, some radioactive material's concentration may be halved every four hours.

The time that it takes for such a substance to decrease by half is called half life, or t

_{½ .}And

k=ln(2) ÷ t

_{½}