Question

Equlibrium constant of some reaction are given as under
K= 10_1
K= 2×10_2
K= 3×10_4
K= 2×10_3
Intial concentration of the reactants for each reaction was taken be equal .

Answer 1

Rate of appearance of P

Increase in concentration of P P = Time taken t (4.2)

Since, Δ[R] is a negative quantity (as concentration of reactants is

decreasing), it is multiplied with –1 to make the rate of the reaction a

positive quantity.

Equations (4.1) and (4.2) given above represent the average rate of

a reaction, rav.

Average rate depends upon the change in concentration of reactants

or products and the time taken for that change to occur (Fig. 4.1).

Fig. 4.1: Instantaneous and average rate of a reaction

Units of rate of a reaction

From equations (4.1) and (4.2), it is clear that units of rate are

concentration time–1. For example, if concentration is in mol L–1 and

time is in seconds then the units will be mol L-1s–1. However, in gaseous

reactions, when the concentration of gases is expressed in terms of their

partial pressures, then the units of the rate equation will be atm s–1.

From the concentrations of C4H9Cl (butyl chloride) at different times given

below, calculate the average rate of the reaction:

C4H9Cl + H2O → C4H9OH + HCl

during different intervals of time.

t/s 0 50 100 150 200 300 400 700 800

[C4H9Cl]/mol L–1 0.100 0.0905 0.0820 0.0741 0.0671 0.0549 0.0439 0.0210 0.017

We can determine the difference in concentration over different intervals

of time and thus determine the average rate by dividing Δ[R] by Δt

(Table 4.1).

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Example 4.1 Example 4.1

Solution Solution

Chemistry 96

It can be seen (Table 4.1) that the average rate falls from 1.90 × 0-4 mol L-1s-1

to 0.4 × 10-4 mol L-1s-1. However, average rate cannot be used to predict

the rate of a reaction at a particular instant as it would be constant for the

time interval for which it is calculated. So, to express the rate at a particular

moment of time we determine the instantaneous rate. It is obtained when

we consider the average rate at the smallest time interval say dt ( i.e. when

Δt approaches zero). Hence, mathematically for an infinitesimally small

dt instantaneous rate is given by

av

R P

r

t t (4.3)

As Δt → 0 or     inst

d d R P

d d

r

t t