Page 180 - Instant notes

Physical chemistry 166

or
−1
−1
2 A→P conc. time

−1
−1
2 A+B→P See text conc. time

Integrated rate law: zeroth order reactions

A reaction which is zeroth order, or pseudo-zeroth order in removal of A, has a rate
law of the form:

where k is the zeroth order rate constant (or pseudo-zeroth order rate constant if the
method of isolation has been used). Separation of the variables and integrating:

with the condition that [A]=[A] 0 at t=0 gives:
[A] 0−[A]=kt

Thus a zeroth order reaction is identified by linearity in a plot of [A] against t and the
gradient of the plot equals −k (Table 1). The rate of removal of A is independent of [A]
as long as some A remains present. An example of such a reaction is the catalytic
decomposition of ammonia, NH 3, at high concentrations, on hot tungsten. The observed
zeroth order behavior is a consequence of decomposition of molecules of NH 3 adsorbed
to the tungsten surface according to the Langmuir adsorption isotherm (see Topic F5).

Integrated rate law: first order reactions

The rate of removal of reactant A in a first order (or pseudo-first order) reaction is
given by:

Separating the variables and integrating: