**The
definition of rate in a chemical reaction
**

Rate in chemistry is defined like rate in other scientific fields. Rate is the change in property per unit time. An everyday example is speed which is the change of position divided by travel time.

Likewise, the rate of a chemical reaction is defined as the change in concentration of a substance per unit time.

Considering the reaction,

N_{2}O_{4}_{ }
2NO_{2}

_{ }the
rate of decomposition of
N_{2}O_{4}_{ }is
given by:

the rate of formation of NO_{2}_{
}
is given by:

If we alter these expressions by replacing the words
“decomposition” and “formation” with the word “modification”, the
rate of modification of N_{2}O_{4}
would be:

and the rate of modification of NO_{2}
would be:

_{
}

The signs - and + in the two last equations imply that N_{2}O_{4}
is decomposed and NO_{2}
is produced.

As far as the rate of a reaction is concerned, it
should be defined from the modification of one of the components of a substance
per unit time.

In the former reaction we observe that for each mol of N_{2}O_{4}
that is decomposed, 2 moles of NO_{2}
are formed.

In order to equate the
rates of formation and decomposition, we have to divide the rate of modification
of NO_{2}
by the factor of 2, therefore in the reaction

N_{2}O_{4}_{ }
2NO_{2 }_{ }the
following definition of the rate results

If
we consider the general type reaction,

aA + bB cC +
dD the rate is given by:

under the condition that one of the coefficients a,b,c and d is unit.