Page 131 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

Chemical Kinetics I 115

ORDER OF A REACTION
The order of a reaction is the sum of the exponents on the concentra-
tion terms in the rate equation, i.e. [ I", where n = the order of the
reactant in question. In this text the abbreviations I", 2", 3" etc. will
occasionally be used for simplicity.
Consider a reaction, such that: Rate = k[AI3[B]'
This reaction is: (a) third-order in A; (b) first-order in [B] and (c) (3
+ 1) = fourth-order overall.
Orders do not necessarily have to be whole numbers (e.g. -1, -2,
+3, + 1 etc.), but can also take fractional values e.g. f, f etc.), i.e.
orders can be intermediate, or even zero.

Zero-Order Reactions
Consider a reaction such that A + Products
Rate = --- d[A1 = k[A]O = k
1 dt
since xo = 1, from the rules of indices
* -- d[A] =k
dt
* -d[A] =kdt
=3 -Sd[A] =kJdt
* -[A] = kt+c

At t = 0, [A] = [Ao] = the initial concentration + c = -[A01
=+ -[A] = kt + (-[&I)
[A] = -kt + [&I
y = rnx + c (zero-order reaction)

This is a linear equation, which is the equation of a line, where rn =
the slope or gradient = 02 - yl)/(x2 - XI) = Ay/Ax, i.e. (difference
of the y's)/(difference of the x's), and c = the intercept, i.e. the point
where the graph cuts the y-axis, at x = 0 (Figure 8.1).

Units of k for a zero-order reaction:
-kt = [A] - [Ao] + kt = [&I - [A]
+ k = [&I - [A]/t + units of k are M s-'.

It should be noted that zero-order reactions are rare, usually found in
the case of surface reactions. First-order reactions are much more
common, found for species in the gaseous or aqueous phase.