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Chemical Kinetics 11 131

The Steric Factor, p
For a reaction to occur, the relative orientation of the molecules at the
point of collision (Figure 9.2) must be favourable.

No reaction ...... relative orientation incorrect!

-d
No reaction ...... relative orientation incorrect!

Reaction ...... correct relative orientation of A:! and B?!

Figure 9.2 Collisions leading to probable reaction: AZ(g) -+ BZ(g) + 2AB

The steric factor, p, is the fraction of collisions in which the
molecules have a favourable relative orientation for the reaction to
occur. p is sometimes called the probability factor, the orientation
factor or the fudge factor.

Activation Energy of a Reaction, EPCt
In any group of reactant molecules, only a fraction of molecules have
energies at least equal to Eact, the activation energy of the reaction. It
has been shown that this fraction of molecules is given by the
expression:
e- EdRT
where E,, = activation energy of the reaction, measured in J mol-';