The
rate law (rate equation) shows the mathematical relationship between the rate and the concentrations of substances upon which the rate depends.
Rate is equal to change in concentration per unit time.
Therefore the unit of rate is the unit of concentration divided (per) unit of time.
If the concentration unit is
and the time unit is in seconds, the unit of rate is
s
–1.
The
general form of the rate law (rate equation) is: rate =
k[A]
n k is the
rate constant.
k depends on the temperature.
The
unit for
k depends on the order of the reaction because this determines the number of powers of concentration in the rate law.
The working below shows how the unit for the rate constant can be deduced by
substituting the unit for each quantity into the rate law (equation).
combining the units using standard multiplication and division operations.
If the reaction is overall
first orderrate =
k[A]
s
–1=
k ×
has the unit s
–1If the reaction is overall
second order:
rate =
k[A]
2 OR rate =
k[A][B]
s
–1 =
k ×
×
k has the unit
s
–1If the reaction is overall
third order:
rate =
k[A]
2[B]
OR rate =
k[A][B]
2 s
–1 =
k ×
×
×
k has the unit
s
–1