The only unfamiliar aspect about this rate equation is the dx/dt style of writing rates. This equation should be translated as: "The rate of disappearance of H2 and the rate of disappearance of I2, both are half the rate of appearance of HI (since two HI molecules are produced), and each of these is proportional to the product of H2 and I2 concentrations.

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.

Example. The decomposition of N205 vapor,

has been observed experimentally to follow the rate law

What is the order of this reaction?