Let's once again look at the reaction we've examined on the previous page:\[3A + 2B \rightarrow C + 4D\]
Its reaction rate was defined as \(v = \frac{\Delta [C]}{\Delta t} = \frac{1}{4}\cdot \frac{\Delta [D]}{\Delta t} \:\)\(= -\frac{1}{2}\cdot \frac{\Delta [B]}{\Delta t} = -\frac{1}{3}\cdot \frac{\Delta [A]}{\Delta t}\)
The rate function we have found here depends on the concentration of the reactants in the reaction mixture. In our general example above, the rate function would thus be noted as follows: \(v = \frac{\Delta [C]}{\Delta t} = \frac{1}{4}\cdot \frac{\Delta [D]}{\Delta t} \:\)\(= -\frac{1}{2}\cdot \frac{\Delta [B]}{\Delta t} = -\frac{1}{3}\cdot \frac{\Delta [A]}{\Delta t} = k\times [A]^a\times[B]^b\). Here, a and b are whole numbers, and k is a temperature-dependent constant called the rate constant. The exponents can be equal to the stoichiometric coefficients that correspond to that molecule in the reaction, but often this is not the case.
The order of a reaction is determined by the sum of all exponents present in this form of the rate equation, each exponent being called the partial order with respect to the respective reactant.
This then gives the following formulas for different reaction orders:
Partial reaction orders can sometimes in special, complex cases be fractional non-integer numbers.
Written by Imre Bekkering