Chemistry, Third edition

244 14 · SPEED OF CHEMICAL REACTIONS

Products

Reactants

Reaction over

Time/s

Reaction starts

when reactants

are mixed

Concentration/mol dm

(a) –3 (b)

0 d

Reactants

Time/s

Concentration/mol dm

–3

c t

b

a

Fig. 14.2(a) How the concentrations of the reactants and products change with time.

(b) Calculating the instantaneous rate of a reaction M NMN at tseconds after mixing.

Calculating the rate of

reaction from graphs (1)

The following graph shows how the

concentration of M varies during the

reaction M Nproducts. Draw tangents

at 0 and 25 s. Calculate the rate of

reaction at these times.

[M] / mol dm

–3

Time/ s

0

0.1

0.2

0.4

0.3

0.5

0.6

10 20 30

Exercise 14B

Calculating the rate of

reaction from graphs (2)

The rate at which a solution of hydrogen

peroxide decomposes at 39 °C,

2H 2 O 2 (l)2H 2 O(l) O 2 (g)

was studied by monitoring the volume of

oxygen gas produced with time:

Vol of O 2 /cm^3 Time/s

00

366 1 000

895 2 500

1 723 5 000

3 199 10 000

4 462 15 000

5 545 20 000

7 265 30 000

8 527 40 000

9 453 50 000

10 132 60 000

(i)Plot the volume of oxygen against time.

(ii)Join the points with a smooth curve.

(iii)Estimate the rate (in units of cm^3 of O 2

per second) at which oxygen is produced at

(a)t0 s and (b) t50 000 s.

Exercise 14C

Figure 14.2(b) shows the tangent to the reactant curve at time t. The rate of reac-

tion at this instant is

change in concentration



ba

mol dm^3 s^1

change in time d c

This equation may be used with the concentration of reactants (Exercise 14B) or

products (Exercise 14C).