Lecture 28 : Elementary Reactions and Reaction Mechanisms
d[A] / dt = - ka [A]
(28.4)
We assume that the rate of formation of A from B is too small to be of any significance. For the rate of change B we have,
d[B] /dt = ka [A] - k b [B]
(28.5)
This accounts for the formation of B as well as the consumption of B in formation C. Finally,
d[C] / dt = k b [B]
(28.6)
Let us assume that at the start of the reaction, only A is present with the concentration given [A]0. Then the concentration of A at any later time is given by the integrated expression
(28.7)
Substituting this in the equation for the rate of change of B and solving for [B]t we get
(28.8)
And since at all times, [A] + [B] + [C] = [A] 0,
we have,
(28.9)
For more complicated reactions, such explicit expressions for [A]t, [B]t and [C]t are hardly available. Therefore, the steady state approximation (SSA) as described below is invoked.
