Reaction Rates

The rate of a reaction is an indication of how rapidly it is occurring. First of all, we need to have an
exact, quantitative way of describing the rate. Then we shall explore what the rate depends on.

DEFINITION OF RATE

Consider a reaction 2A + B → C, in which 1 mole of C is produced from every 2 moles of A and 1 mole
of B. We want to come up with some quantitative way of describing just how fast the reaction has
proceeded or is proceeding at any instant in time. The most natural way is to use either the
disappearance of reactants over time, or the appearance of products over time. The faster either of
these rates is, the faster the rate of reaction:

Because the concentration of a reactant decreases during the reaction while we want the rates to be
positive numbers, a minus sign needs to be placed before a rate that is expressed in terms of

reactants. For the reaction above, the rate of disappearance of A is the rate of disappearance

of B is and the rate of appearance of C is In this particular reaction, the three rates are
not equal. According to the stoichiometry of the reaction, A is used up twice as fast as B

and A   is  consumed    twice   as  fast    as  C   is  produced    

To show a standard rate of reaction in which the rates with respect to all substances are equal, the
rate for each substance should be divided by its stoichiometric coefficient. In this particular case,
then: