Physical Chemistry 3: — Chemical Kinetics — - Christian-Albrechts ...

2.1 Definitions and conventions 11
I Example: Considering a generic rate law, like
− [A] = | | [A] [B] (2.16)
the total order of the reaction is = + . We also say, the reaction is -th order in
[A] and -th order in B, ...
The order of a reaction may be determined by the tangent method (Fig. 2.1): Its effects
on the reaction rate are observable by increasing (e.g., doubling) the concentrations of
the reaction partners one by one. Better methods will be explored later (⇒ homework
assignments).
I
Caveats:
• The order of a reaction is, in general, an empirical quantity; it has to be determined
by an experimental measurement.
• The order of a complex reaction cannot be determined by simply looking at the
overall reaction.
• There are reactions with integer order, like the first-order or second-order reactions
above (Eqs. 2.14 and 2.15). However, the order can also be fractional, as for the
following reactions:
(1) CH 3 CHO → CH 4 +CO:
[CH 4 ]
= [CH 3 CHO] 32 (2.17)
(2) H 2 +Br 2 → 2HBr:
[HBr]
= 0 [H 2 ][Br 2 ] 12
1+ 00 [HBr] [Br 2 ]
(2.18)
• A negative order in a concentration means that the respective species acts as an
inhibitor.
• A positive order in a product concentration means that the reaction rate is enhanced
by autocatalysis.
2.1.4 The molecularity of a reaction
I
Definition 2.3: The molecularity of a reaction is the number of molecules in an elementary
reaction step at the microscopic (molecular) level.