COMPOSITE REACTIONS AND THE STEADY STATE APPROXIMATION

Review

If the rate law for the reaction 2NO₂ + F₂ → 2NO₂F, was experimentally determined as r = k[NO₂][F₂], the reaction would not be elementary.

The Steady State Approximation

The Steady State Approximation (SSA) is the setting of the rate of change of concentration of a species equal to zero. It is usually applied to the intermediate species, provided that the intermediate is highly reactive and present in low concentrations.

Example:

The proposed mechanism for the reaction N₂O₅ + NO → 3NO₂, is

N₂O₅ → NO₂ + NO₃

NO + NO₃ → 2NO₂

From experimental data the following rates for N₂O₅, NO and NO₃ have been obtained: From the plot it can be seen that the rate of change in the concentration of NO₃ rapidly falls to a very small value. In fact it approaches zero, i.e. d[NO₃]/dt ~ 0.

The Steady State Approximation is applied by setting this rate equal to zero, i.e. d[NO₃]/dt = 0.

Review Question

For the concentration profile given below

Time > 0 Mins

Time < 7 Mins

Time > 7 Mins

Time < 20 Mins