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 .
Answers
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