Page 77 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch07 Elementary Physical Chemistry

62 Elementary Physical Chemistry

where k is the reaction rate constant. The exponents m,n,... are not
necessarily the coefficients a, b, etc. that appear in the reaction, but are
determined experimentally.
The order of reaction is given by the exponents of the concentrations
in theratelaw.Thus, in the above example, the order of the reaction with
respect to A is m, with respect to B is n,etc. The overall order of reaction
is the sum of all the exponents. When a reaction order is mentioned, it
generally refers to the overall order unless otherwise stated.

7.3. Units of the Reaction Rate Constant, k

Since the rate r =d[c]/dt has the dimention of concentration divided
n
m
by time, it follows that the product k[A] [B] ... must have the same
dimention. Thus, in a first-order reaction, [c]t −1 ↔ k[c]and so k has the
2
dimension of t −1 . In a second-order reaction, [c]t −1 ↔ k[c] and thus k has
the dimension [c] −1 −1 .
t
Relation of order of reaction to stoichiometric coefficients
Consider the two reactions:

1) 2N 2 O 5 → 4NO 2 +O 2 (7.8a)
2) 2NO 2 → 2NO+O 2 (7.8b)
The rates of these two reactions are respectively
1
1) r = − d[N 2 O 5]/dt = k[N 2O 5 ] (7.9a)
2
1 2
2) r = − d[NO 2]/dt = k[NO 2 ] (7.9b)
2
Obviously, reaction (1) is first order; reaction (2) is second order. This
clearly shows that there is no connection between the order of reaction and
the stoichiometric coefficients in these reactions.
Sometimes the order of a reaction can be zero or fractional. For
example, the rate law for

CH 3 CHO → CH 4 + CO (7.10a)
is

r = k[CH 3 CHO] 3/2 (7.10b)
In a zero-order reaction, the rate is independent of the concentration.