Chemistry, Third edition

FACTORS AFFECTING REACTION RATE

Why reaction rate falls with time

So far we have not explained why reaction rate should begin to fall as soon as the

reactants are mixed. Our explanation depends upon two assumptions. The first is

that in order for a reaction to take place, collisions must occur between reactant

molecules (say, M and N to give the product molecule MN). The second, is that the

number of collisions per second between M and N falls as the concentration of M

and N falls. There are no product molecules (MN) at the start of the reaction, and

the concentration of M and N is at a maximum. As soon as reaction starts, M and N

molecules start to combine. The concentrations of M and N begin to fall (and of MN

rises), so reducing the number of collisions per second between unreacted M and N

molecules. This reduces the reaction rate (Fig. 14.3).

245

M

Start of reaction

NMN

After considerable

reaction has taken

place

Fig. 14.3For the general reaction, M NMN,

reaction can only occur if molecules M and N

collide. The chance of collisions between M and N

molecules falls as more product (MN) molecules are

made.

Factors affecting reaction rate

1. Nature of the reaction

Some reactions are naturally faster than others. Compare the reaction of hydronium

and hydroxide ions,

H 3 O(aq) OH(aq)2H 2 O(l)

with the hydrolysis of bromoethane,

C 2 H 5 Br(l) OH(aq)C 2 H 5 OH(aq) Br(aq)

If in an experiment we kept the initial reactant concentrations in both reactions the

same, and carried out the experiment at room temperature, the initial rate of the first

reaction would be about 1 000 000 000 000 000 times faster than the second reaction.

In fact, the reaction between H 3 Oand OHis so fast that it appears to be instant-

aneous. Special equipment is needed to study such extremely fast reactions.

It is sometimes possible to predict whether reactions will be fast by simply look-

ing at the reactants. Reactions in which the reactants are oppositely charged ions are

usually very fast because the charges pull the ions towards each other. This is the case

with the reaction,

Ag+(aq) Cl(aq)AgCl(s)

and many other common precipitation reactions. Reactants which are atoms or

molecules with odd electrons (known as free radicals, such as H•and CH 3 • ) also react

14.2