E12 Changes in rate of reaction upon disturbing a chemical equilibrium

Aim: To describe the kinetic implications, the striving to a new equilibrium at which other but equal rates of reaction apply.

Rates of reaction relating to the dimerization of NO2 into the synthesis of N2O4 are indicated by the suffix s and those relating to the dissociation of N2O4 into NO2 are indicated by the suffix d.

In illustration E12, the changes in rates of reaction for the N2O4-synthesis and N2O4-dissociation reactions are shown as a function of time after adding NO2 to the closed system at time t1. The changes in NO2- and N2O4-concentration upon doubling the NO2-concentration have been already shown in illustration E09a and are reproduced in the background of illustration E12, the response being a slight increase in N2O4-concentration and a considerable reduction in NO2-concentration.


  The rate of N2O4-synthesis, vs, upon doubling the NO2- concentration, almost immediately increased by a factor of 4, due to the doubling of NO2-concentration and there being 2 molecules of NO2 involved in the synthesis of a single molecule of N2O4.

The rate of N2O4-dissociation, vd, upon doubling the NO2-concentration, changed much less. The temporarily higher rate of synthesis compared with the rate of dissociation, ensured the production of N2O4 at the expense of NO2. Between times t1 and t2, the rate of dissociation slowly increased while that of the synthesis decreased.
At t2 the rates of synthesis and dissociation were again equal, but these rates, v’se = v’de, were higher than those for the original equilibrium, vse = vde


where ks and kd are the rate constant of the synthesis reaction and dissociation respectively.