Chemistry, Third edition

FACTORS AFFECTING REACTION RATE

the reactant molecules. However, as the molecules of M and N approach each other

their outer electrons repel, so preventing the reactants from getting close enough for

the bonding electrons to be rearranged. The reactant molecules then simply ‘bounce

off’ each other, and the collision does not result in chemical change.

In order to overcome the repulsive forces between the molecules so that the

molecules are forced together, the colliding molecules must between them possess a

minimum value of kinetic energy, termed the activation energy(EA). Unless the

sum of the energies possessed by the reactant molecules involved in the collision

equals (or exceeds) this minimum energy, no reaction can take place (Fig. 14.5).

The activation energy for a reaction does not itselfvary with temperature, but

increasing the temperature of a material does increase the average energy of its

molecules.It follows that the higher the temperature, the greater is the fraction of colliding

molecules which possess at least the activation energy(Fig. 14.6). We then expect more

collisions to result in chemical reaction at higher temperatures, and this explains the

experimental observation of an increased reaction rate.

The rate of some reactions changes more rapidly with temperature than others

(see Fig. 14.4), and we are now in a position to explain this observation by

assuming that reactions whose rates are very sensitive to temperature possess relatively

high activation energies:

●The speed of a reaction which possesses a large activation energy changes rapidly

with temperature because few of the reacting molecules will possess an energy

equal to (or greater than) EAat room temperature. Therefore, an increase in tem-

perature will drastically increase the number of collisions which result in chemical

change.

247

Fig. 14.5(a) Unsuccessful collision – the kinetic energy of M

and N is less than EAand so the reactants bounce apart.

(b) Successful collision – the kinetic energy of M and N is greater

or equal to EAand a bond is formed between M and N.

Fig. 14.6The graph shows the spread of molecular energy for a mixture of gases at low
temperature and at high temperature. The higher the temperature, the greater the fraction of
molecules possessing energy equal to (or greater than) the activation energy EA.