The
rate law (rate equation) shows the mathematical relationship between the rate and the concentrations of substances upon which the rate depends.
The
general form of the rate law (equation) is:
rate =
k[A]
n The unit of rate is concentration per unit time. Chemists often use
for concentration and seconds for time with the rate unit being
s
–1. The unit of rate does not depend on the reaction order.
The
n in the rate law is
experimentally determined.
n is 0 for reactants where the rate of reaction does not depend on the concentration of the substance.
Usually one or more of the reactants in the overall equation appears in the rate law, but
catalysts may appear in rate laws
products may appear in rate laws (usually in the denominator)
k is the
rate constant.
k depends on the temperature.
k does not depend on concentrations of substances that appear in the rate law.
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
–1If the reaction is overall
zero order.rate =
k s
–1=
kk has the unit
s
–1