Page 208 - Instant notes

Physical chemistry 194

rate of formation of HBr=k 2a[Br][H 2]+k 2b[H][Br 2]−k 3[H][HBr]

Expressions for the concentrations of the Br and H intermediates are obtained by using
the steady state assumption:
rate of formation of Br
2
=2k 1[Br 2]−k 2a[Br][H 2]+k 2b[H][Br 2]+k 3[H][HBr]−2k 4[Br] =0
and,
rate of formation of H=k 2a[Br][H 2]−k 2b[H][Br 2]−k 3[H][HBr]=0

Solving the two equations gives the steady state concentrations:

and

These are substituted into the rate expression for formation of HBr to obtain, after
rearrangement:

This equation is the same as the empirical rate law with the empirical rate coefficients
identified as:

and
The agreement of this rate law with the empirical rate law does not prove that the
proposed mechanism is correct but provides consistent evidence that it is correct. Further
evidence could be obtained by laboratory measurement of values for the elementary rate
coefficients (see Topic F1) and showing that the appropriate combinations correctly
matched the values of the observed composite rate coefficients.