The differential rate law (equation) has the general form: rate = k[A]x[B]y. 
The magnitudes of exponents x and y are important clues as to the pathway for the reaction.

One way of determining
  • the exponent x of [A] is to measure the rate at the beginning of the reaction (before product buildup affects the rate) for different initial concentrations of A.

    If doubling [A] while holding [B] constant
    does not change reaction rate, then x = 0
    doubles reaction rate, then x = 1
    quadruples reaction rate, x = 2

     
  • the exponent y of [B] is to vary the initial concentration of B while keeping [A] constant and apply the above.

     
  • the magnitude of the rate constant k once x and y are known, is to substitute an experimental rate and the concentration(s) of A and B at that rate into the rate law.
The magnitude of k is equal to the rate of the reaction when the concentration of each substance in the rate law is 1 mol L–1.
The unit of k depends on the order of the reaction and is (L mol–1)n–1 s–1 where n equals x + y.
Dr Sheila Woodgate
© The University of Auckland 2002