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# Rate Law mechanism observed

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The rate law for the Br—- catalyzed reaction
Br-
C6H5NH2 + HNO2 + H+ C6H5N2+ + 2H2O

is observed to be rate = = k [H+][HNO2][Br -].

A proposed mechanism is
k1
(1) H+ + HNO2 H2NO2+ (rapid equilibrium)
k-1

k2
(2) H2NO2+ + Br- NOBr + H2O (slow)

k3
(3) NOBr + C6H5NH2 C6H5N2+ + H2O + Br- (fast)

Deduce the rate law for this mechanism and relate the observed rate constant k to the rate constants appearing in the assumed mechanism.

Notes:
1. Step (1) is a rapid equilibrium, meaning k1 and k-1 are much faster than k2.
2. The forward and reverse rates are equal for step (1) since it is an equilibrium; this fact can
be used to get an expression for the concentration of the intermediate H2NO2+.
3. Recall, the reaction orders for elementary reactions are determined by the reaction
stoichiometry.
4. Step (2) is sufficiently slow that it can be considered rate-determining.

##### Solution Summary

The expert deduces the rate law for the mechanism that relates to the rate constant.

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The rate law for the Br—- catalyzed reaction
Br-
C6H5NH2 + HNO2 + H+ C6H5N2+ + 2H2O

is observed to be rate = = k [H+][HNO2][Br -].

A proposed mechanism is
k1
(1) H+ + HNO2 H2NO2+ (rapid equilibrium)
k-1

k2
(2) H2NO2+ + Br- NOBr + H2O (slow)

k3
(3) NOBr + C6H5NH2 C6H5N2+ + H2O + Br- (fast)

Deduce the rate law for this mechanism and relate the observed rate ...

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