Chemistry, Third edition

14 · SPEED OF CHEMICAL REACTIONS

that 4.5 billion years ago (when the earth was formed), there was double the amount

of uranium-238 present than is the case now.

2. Degradation of organic compounds in the environment

Organic compounds (including solvents and pesticides) break up in the environment

as a result of hydrolysis (reaction with water), photolysis (breakdown by light) or by

the action of bacteria. These reactions are usually first order (or pseudo first order).

Ideally, toxic organic compounds in waste should have a low half-life so that they

break down quickly. The insecticide parathionslowly hydrolyses at room temperature:

260

SP

OC 2 H 5

OC 2 H 5

O NO 2 +H 2 O SP +

OC 2 H 5

OC 2 H 5

OH HO NO 2

Thet (^1) ⁄ 2 of parathion in water due to hydrolysis is about 25 days – sufficiently long for
it to pose a hazard.
Reaction mechanisms
Chlorination of methane
Chloromethane (CH 3 Cl) is made by reacting chlorine and methane. The overall
equation for the reaction is
Cl 2 (g) CH 4 (g)CH 3 Cl(g) HCl(g)
A spark or a flash of light is needed to start the reaction. In either case the reaction
can be explosively violent.
The overall equation suggests that chloromethane is produced on a molecular
scale by the collision of chlorine and methane molecules. Experimental evidence
shows that this is not the case: the production of chloromethane occurs in several
(simple) steps known as elementary reactions. The sequence of steps is called the
reaction mechanism. For the chlorination of methane the steps include
Cl 2 (g) heat/light2Cl•(g) (14.3)
Cl•(g) CH 4 (g)CH 3 • (g)HCl(g) (14.4)
CH 3 • (g) Cl 2 (g)CH 3 Cl(g)Cl•(g) (14.5)
The spark (or flash of light) supplies the energy for reaction (14.3). Reactions
(14.4) and (14.5) involve chlorine and methyl free radicals(Cl•and CH 3 • ) as the
reaction intermediates.Free radicals are chemically very reactive because they are
eager to use their odd electrons (symbolized •) to covalently bond with other atoms.
Note that chlorine radicals are regeneratedin reaction (14.5) and that these then
react with more methane (reaction (14.4)). In this way the reaction gets faster and
faster (a chain reaction) which may cause the reaction mixture to explode. But the
reaction cannot go on indefinitely and eventually termination reactions,such as
CH 3 • (g) CH 3 • (g)C 2 H 6 (g)
or
Cl•(g) Cl•(g)Cl 2 (g)
use up the free radicals and stop the overall reaction.
14.7