The order of an experimental rate equation describes how the rate depends on the power of the concentration terms. The rate of breakdown of carbon-14 is proportional to the first power of C-14 concentration, so the reaction is first order. The hydrogen-iodine reaction rate depends on the product of two concentrations, so the reaction is second order in overall concentration.

One also can focus on the individual reactants, and say that the HI reaction is separately first order in H₂ and I₂ concentrations, since these each occur in the rate equation with a firstpower dependence.