Engineering
Chemistry
Reaction Mechanism and Molecularity
Question

For a two step reaction.
A ⇌ R + B       

 R +  C →k2 P

(Where, R is a reactive intermediate whose concentration is maintained at some low steady state throughout the reaction). If the concentration of C is very high then the order of reaction for formation of "P" is

0

1

2

3

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Solution

Rate law =dpdt = K2 [R] [C]
at steady state d[R]dt=0

d[R]dt=K1 [A] – K2 [R] [B]– K2[R] [C] = 0
K1 [A] = {K2[B] + K2[C]} [R]
[R]=K1[A]K2{[B]+ [C]}
Rate=K2×K1K2[A][B] +[C]×[C]

=K1[A][B][C]+1
where C is very high
Rate = K1 [A]
∴ order of reaction = 1