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320
2.
3.
Characteristics of rate constant :
(a) Greater the value of rate constant, faster is the reaction.
(b) Each reaction has a definite value of rate constant at particular temperature.
(c) Value of rate constant increases with increase in temperature.
(d) Value of rate constant does not depend upon the concentration of the reactants.
(e) For a given reaction, rate constant has a higher value in the presence of a catalyst
than in the absence of the catalyst.
(f) Units of rate constant depend upon order of reaction.
Units of rate constant :
The units of rate constant : (Mole Lit–1)1 – n sec–1, where n = order of reaction
Zero order
First order
Second order
Mole Lit–1sec–1
sec–1
Mole–1 Lit sec–1
(c) Order of a reaction :
1. Definition : “The order of a chemical reaction with respect to each reactant is defined
as the exponent to which the concentration term of that reactant, in the rate law
is raised.”
“The overall order of a reaction is defined as the sum of the exponents (powers)
to which the concentration terms in the rate law are raised.
2. Explanation :
Consider a chemical reaction,
aA + bB ⎯⎯
→ products
3.
According to rate law expression, Rates of Reaction = k [A]p[B]q.
Therefore, the order of reaction (n) is written as, n = p + q.
The order of reaction with respect to A is ‘p’ and that of with respect to B is ‘q’.
The overall order of reaction is p + q.
Examples :
(a) 2H 2 O 2 (g) ⎯⎯
→ 2H 2 O(l ) + O 2 (g) : For the above reaction, rate of the reaction is
experimentally found to be proportional to the concentration of H2O2. Hence, the rate
law for the reaction is Rate = k[H2O2].
The reaction is first order in terms of H2O2 and its overall order is also first.
(b) NO 2 (g) + CO (g) ⎯⎯
→ NO (g) + CO 2 (g) : For the above reaction, rate of the
reaction is experimentally found to be proportional to [NO2]2 and independent of
[CO]. Hence, the rate law for the reaction is Rate = k[NO2]2[CO]0.
The reaction is second order in terms of [NO2] and zero order in terms of [CO].
Overall order of reaction = 2 + 0 = 2.
Unique Solutions ®
S.Y.J.C. Science - Chemistry - Part I
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