Chemistry, Third edition

REACTION RATE

Experimental measurements of the rate of a chemical

reaction

In order to measure reaction rates experimentally, we need to monitor the concen-

tration of reactants or products with time. Such concentrations are measured using

standard analytical techniques such as titration, those depending upon the degree of

light absorption of a substance, or by changes in the electrical conductivity of the

reaction mixture.

Occasionally we are able to follow the rate of a reaction because one of the

products or reactants is coloured. An example is the reaction between bromine

solution and methanoic acid:

H+(aq)

Br 2 (aq) HCOOH(aq) —2Br(aq) 2H+(aq) CO 2 (g)

dark red

(Note that His used as a catalyst.) It happens that bromine is the only coloured

species involved in the reaction, and as the reaction proceeds its colour fades

(Fig. 14.1). The intensity of the colour is proportional to the concentration of bromine.

Change of rate of reaction with time

When two reactants are mixed together (for example, in a beaker) the rate of

reaction is greatest at the start of the reaction. As the reaction continues, the rate con-

tinues to fall. If the concentration of reactant (such as bromine, Fig. 14.1) or product

is plotted against time, a graph like Fig. 14.2(a) is produced. (If the reaction is an

equilibrium reaction, the concentration of reactants in Fig. 14.2(a) will level out

when equilibrium has been achieved, but will not fall to zero.)

The rate of reaction at an instant tseconds after the reactants are mixed, is calcu-

lated by working out the slope of the tangentof the concentration–time graph at that

time. As time increases, the slope of the tangents at each instant of time decreases,

confirming that the reaction rate is continuously falling.

243

Fig. 14.1The reaction

between bromine and

methanoic acid. As the

reaction continues, the red

colour of bromine fades.

attwicethe rate at which N 2 O 5 decomposes, and also that O 2 is produced at halfthe

rate at which N 2 O 5 is destroyed.

Suppose that under a particular temperature and set of concentrations, the rate at

which N 2 O 5 decomposes at an instant was found to be 1 10 ^4 mol dm^3 s^1. The

rate of NO 2 and O 2 production at the same instant is 2  10 ^4 and 0.5

10 ^4 mol dm^3 s^1 , respectively. To avoid ambiguity in such cases, we should state to

which substance a measured rate refers.

Rate of reaction

The reaction between nitric oxide (NO) and hydrogen to make nitrous oxide (N 2 O) and water

obeys the equation:

H 2 (g)2NO(g)N 2 O(g) H 2 O(g)

If the rate (at a particular instant) at which hydrogen disappears was found to be

0.01 mol dm^3 s^1 , what is the simultaneous rate at which (i)nitric oxide disappears, (ii)

nitrous oxide appears, (iii)water appears?

Exercise 14A