Mechanism - The sequence of steps of a multi-step chemical reaction.

Two-Step Mechanism

Consider the following reaction:

2 NO + O₂ 2 NO₂

This reaction actually is the sum of two steps:

  Step 1:  2 NO N₂O₂         (fast step)
  Step 2:  N₂O₂ + O₂  2 NO₂    (slow step)

When the two equations are added together, the resulting equation is the original equation:

2 NO N2O2

+ N2O2 + O2  2 NO2

2 NO + O2 2 NO2

Since the rate of the entire reaction is dependent upon the rate limiting step, the rate law can be written as:

rate = k[N₂O₂][O₂]

However, there is one problem.  The rate law of a reaction has to be in terms of the concentrations of the reactants.

• N₂O₂ is an intermediate, not an original reactant.
• Therefore, we must solve for [N₂O₂] in terms of [NO].

First, find the rate law of the other equilibrium reaction (fast step):

• NOTE: Since the reaction is an equilibrium reaction, it can be assumed that at equilibrium, the rates of the forward and reverse reactions are equal.

We end up with the rate law in terms of the original reactants.

• Even though the coefficients of the reactants in the balanced equation happened to be the exponents of the concentrations of the reactants in the rate law, this is not always true.
  • The rate law for a reaction cannot be determined by the coefficients of the balanced reaction; it must be determined experimentally.

Three-Step Mechanism

2 A + 2 B C + D

  Step 1:  A + A X           (fast)
  Step 2:  X + B  C + Y      (slow)
  Step 3:  Y + B D           (fast)

Is the mechanism consistent with the rate law?

Next:  "Factors Affecting Reaction Rates"