Reaction Rate Law

Law of Mass Action: The rate of a chemical reaction is proportional to the product of the concentrations of the reactants.

For any general reaction

aA + bB ------->

the rate law expression is: r [A]m[B]n

where A and B represent the molar concentrations of A and B. m and n are the powers to which the concentrations must be raised. k is a constant of proportionality known as the rate constant. Data show that the rate constant is not affected by [] (concentration) changes but does vary with temperature changes. The values of 'm' and 'n' are not the stoichiometric numbers obtained from the balanced equation; unless; the equation is deemed to be a one-step reaction, but more on this latter. The only valid way to obtain the values of m and n is to use experimental data.

The exponents, m and n may be zero, fractions or integers.

The sum of the exponents is called the reaction order.

eg. H2(g) + I2(g) -----> 2 HI(g)

r = k[H2][I2]

This is a second order reaction. The sum of the components is 2.

In this case the values of 'm' and 'n' just happen to be the same as the stoichiometric numbers in the balanced equation. Therefore it must be a one-step reaction. i.e., there is only the one reaction step needed to convert reactants into products.

This is not always the case and a simple reaction may proceed through a number of intermediate steps.