Page 89 - Bruno Linder Elementary Physical Chemistry

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

74 Elementary Physical Chemistry

Note: In a unimolecular reaction, a single molecule breaks apart.
(Radioctive decay is a good example.) The order is first order. In
a bimolecular reaction, two molecules must combine but the order is
not necessarily second order. It may be first order, zero order ... in
fact it could be any order.

Question: Why is there no direct correspondence between molec-
ularity and order of reaction? The reason is that most reactions
are complex, not elementary. In general, reactions go through many
intermediate steps.

Reactions that tend to cause complications are
1) Opposing or reverse reactions A + B ↔ C (7.31)

B

2) Complex reactions A (7.32)

C
3) Consecutive reactions A → B → C (7.33)
Such reactions greatly complicate the differential equations and, in general,
exact solutions are not feasible. When there are many intermediate steps,
one generally resorts to approximations.
The most common approximations are

1) Steady-State Approximation,
2) Rate-Determining (or Rate-Limiting) Step or Equilibrium Approxima-
tions.

7.12. The Steady State Approximation

Complex reactions usually involve intermediate species that do not appear
in the overall reaction. If [A] represent a reactant, [P] a product and [I]
an intermediate, then it is generally true that [I] is much smaller than [A]
or [P]. The reason is that intermediates are reactive, and generally do not
accumulate to any extent. It is therefore assumed that d[I]/dt =0. This is
the steady-state assumption.
Example 7.7. Consider the reaction
2NO(g) + O 2 (g) → 2NO 2 (g) (7.34)