To be more specific, it can be expressed in
terms of:
(i) the rate of decrease in concentration of any one of the
reactants, or
(ii) the rate of increase in concentration of any one of the products Consider a hypothetical reaction, assuming that the volume of the
system remains constant R ® P
One mole of the reactant R produces one mole of the product P If
[R]1  and [P]1 are the concentrations of R and P respectively at time t1
and [R]2 and [P]2 are their concentrations at time  t2 then,
Dt
=
t2 – t1
D[R]
=
[R]2 – [R]1
D [P] =
[P]2 – [P]1
The square brackets in the above expressions are used to express
molar concentration