Lecture 28 : Elementary Reactions and Reaction Mechanisms
28.2
Elementary Reactions
Elementary reactions are those where in the molecularity of the reaction is the same as the order of the reaction. Note that the converse is not true! We will now give a few examples of elementary reactions.
NO +
O NO2
(28.1a)
O + O O2
(28.1b)
O2 O
+ O
(28.1c)
SO2 + O
SO3
(28.1d)
BR22 BR
(28.1e)
CH3 + H
CH4
(28.1f)
CH3 + CH3 C2H6
(28.1g)
In all these reactions, no further steps or intermediates are involved in going from left to right.
In 28.1(a) a single molecule of NO "collides" with an oxygen atom to give NO2. The reaction is bimolecular and of second order.
In 28.1(c) an oxygen molecule breaks up into two oxygen atoms when excited photochemically (by the absorption of light). The rate of decomposition of O2, d[O2]/dt is proportional to the concentration of oxygen molecules [O2].
If more O2 molecules are present, more of them are likely to absorb light and form the products. The fact that a reaction is elementary has also to be established by experiment alone. It can not be decided by the simplicity of the appearance of the equation.
NO2 
+ O 
SO3 
CH4 
