Chemistry, Third edition

FACTORS AFFECTING REACTION RATE 249

BOX 14.1

As the bonds in reacting molecules are

rearranging during a collision, the energy of

the molecules rises to a maximum above the

starting energy of the reactants (Fig. 14.7).

This maximum is the activation energyEA(in

kJ mol–1) of the (forward) reaction. The

arrangement (or configuration) of atoms at

this energy maximum is called the transition

state. The transition state is a ‘half-way house’

between reactants and products. It can never

be isolated and exists for only a vanishingly

short period of time. Once formed, the

transition state immediately ‘falls back’ to the

reactants or else ‘turns’ into products.

Figure 14.7 also shows the activation energy

for the reversereaction (MN MN),

symbolisedEA. When the forward reaction is

exothermic, the activation energy of the

forward reaction is always smaller than the

activation energy for the reverse reaction, i.e.

EAEA. (The reverse is true for an

endothermic reaction.) If a chemical reaction

occurs in several steps, each step possesses

its own transition state, with its own forward

and back activation energies.

The reaction between 1–bromopropane and

hydroxide ion:

RCH 2 Br(l)OH(aq)RCH 2 OH(aq)Br(aq)

(RCH 3 CH 2 ) occurs in one stage. The

probable structure of the transition state is

shown in Fig. 14.8 with the negative charge of

the OH–being spread out over the structure.

Transition states; activation energy

Fig. 14.7Connection between reaction

activation energy EAand the reaction

enthalpy change Hfor the exothermic

single-step reaction M NMN.

Fig. 14.8Reactants, transition state and products for the hydrolysis of 1–bromopropane.

4. Catalysts

Catalysts are used to speed up reactions. Their general characteristics are:

●Catalysts take part in chemical reactions but are regenerated during the

reaction.

●Catalysts do not affect the enthalpy change or equilibrium constant of the reac-

tion.

●Catalysts are often specific, i.e. they will only affect the rate of a particular reac-

tion. This is particularly true of some biological catalysts (enzymes). Enzymes are

proteins. They are often deactivated (or even destroyed) if the temperature rises

much above 40 °C (Fig. 14.9). Most enzymes operate in a narrow band of pH, and

many require the presence of a metal ion or another complex organic molecule as

cofactor. For more about enzymes, see Case Study 1 on the website.