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Transcript
323
3.
(f) The overall reaction can never occur faster than its rate determining step.
(g) The rate determining step can occur anywhere in the reaction mechanism.
(h) The rate law of a rate determining step can be obtained from its stoichiometric equation.
Example :
For example, the reaction
→ 2NO 2 (g) + Cl 2 (g)
2NO 2 Cl(g) ⎯⎯
The reaction takes place in two steps :
k1
NO 2Cl(g) ⎯⎯
→ NO 2 (g) + Cl(g)
k2
NO 2 (g) + Cl 2 (g)
(ii) NO 2Cl(g) + Cl(g) ⎯⎯→
(i)
2NO 2Cl(g) ⎯⎯
→ 2NO 2 (g) + Cl 2 (g)
(slow, unimolecular)
(fast, bimolecular)
(overall reaction)
The first step being slower than the second is the rate determining step. The rate law
predicted by slow step is rate = k1[NO2Cl]
It can be seen that Cl is formed in the first step and consumed in the second. Hence,
it is the reaction intermediate.
(h) Reaction intermediate :
1. Definition : “The additional species other than the reactants or products formed in
the reaction mechanism is called reaction intermediate.”
2. Explanation :
(a) The reaction intermediate appears in the mechanism but does not appear in the
overall reaction.
(b) It is always formed in one step of the mechanism and is consumed in the
subsequent step.
(c) The concentration of reaction intermediate does not appear in the rate law, since its
concentration is very small and undetermined.
(d) It is not present at the start of the reaction neither at the end of the reaction.
3. For example, the reaction
2NO 2 Cl(g)
⎯⎯
→ 2NO 2 (g) + Cl 2 (g)
The reaction takes place in two steps :
(i)
k1
NO 2Cl(g) ⎯⎯
→ NO 2 (g) + Cl(g)
k2
NO 2 (g) + Cl 2 (g)
(ii) NO 2Cl(g) + Cl(g) ⎯⎯→
2NO 2Cl(g) ⎯⎯
→ 2NO 2 (g) + Cl 2 (g)
(slow, unimolecular)
(fast, bimolecular)
(overall reaction)
The first step being slower than the second is the rate determining step. The rate law
predicted by slow step is rate = k1[NO2Cl].
Chapter - 5 Chemical Kinetics