In order to introduce the idea of rate, let's suppose we wish to know
how fast the following
reaction is going:

One way to do this is to define rate as the change in concentration of some
species with respect to
time, and then measure the concentrations of all species at multiple times to
determine the rate. The
results of such a hypothetical experiment is given in the for
the
reaction of hydrogen and iodine. The initial concentrations of
H_{2} and I_{2}
are equal at all times and the initial concentration of product is zero:

As you can see, the rate of formation of HI is twice the rate of
disappearance of H_{2} or
I_{2} at any given time. Also, note that the rate slows in time
due to decreasing
concentrations of the reactants. Stated mathematically, the relationship
between the formation of
products and the disappearance of reactants for this reaction is:

In general, for the reaction below:

The rate is expressed as follows:

Another expression for a rate is called the differential rate law, or
simply, the rate law. It
expresses the rate of a reaction in terms of the concentrations of the
reactants raised to an
experimentally determined power. The exponent on each concentration
term is called the
order of the reaction in that particular reactant. The sum of the
exponents in the rate law is called
the order of the reaction. The powers on the concentration terms in a
rate law are NOT the
stoichiometric coefficients from the balanced equation! For example,
the rate law for the
will have the following form: