Reaction rate law
Reaction rates usually depend on concentrations. The rate law is a mathematical equation which relates the reaction rate to the reactant concentrations. Consider the general reaction
where ‘a’ and ‘b’ are the coefficients in the balanced chemical equation, X and Y are the reactants.
The rate of a reaction is proportional to the products of the reactants each raised to some power.
- rate = k [X]n [Y]m
where k, m, and n are numbers that are determined experimentally!!
The reaction is determined to be:
- mth order with respect to X
- nth order with respect to Y
- The overall reaction order is m + n.
m and n are usually small whole numbers but may be fractional, negative or zero. They are often not related to a and b. m and n must be determined experimentally.
For example, the reaction between HgCl2 and C2O42- is found to have the rate law
- Rate = k [HgCl2]2 [C2O42-]1
So the reaction is
- 2nd order wth respect to HgCl2
- 1st order with respect to C2O42-
- 3rd order over all (2 + 1 = 3)
k is the rate constant.
- The bigger the value of rate constant, k, the faster the reaction.
- k is temperature dependent.
- k is reaction dependent.
- k has units that depend on the order of the reaction.
If concentration is in the units of M and time in the units of s then the rate of reaction has units of M/s. If the reaction has an overall order of p then considering the units in the rate law gives (units in brackets)
- Rate(M/s) = k (M)p
or units for k = (M/s) / Mp = M1-p s-1
The following table gives the units of k for some orders of reaction.
Overall Reaction Order Units for k 0 M s-1 1 s-1 2 M-1 s-1 3 M-2 s-1
Knowing the rate law, the effect on the rate of the reaction when the concentrations are changed can be predicted.