5 11 1 33 Reaction Mechanism

k${}_{observed}$ k - [reactants] relationship k - [products] relationship
 -> come up with appropriate mechanism to fit the experimental relationships
[NO]${}^2$ [O${}^2$]

step 1. NO + NO ⇔ N${}_2$O${}_2$ -(k1)-> <-(k-1)-
 rate${}_{forw}$ = k${}_1$[NO]${}^2$ bimolecular
 rate${}_{rev}$ = k${}_{-1}$[N${}^2$O${}^2$] monomolecular
step 2.  O${}_2$ + N${}^2$O${}^2$ -(k2)-> NO${}_2$ + NO${}_2$
 rate = k${}_2$[O${}_2$][N${}^2$O${}^2$]
 (*) overall rate of NO${}_2$ formation = 2 k${}_2$[O${}_2$][N${}^2$O${}^2$]
you need to get rid of intermediate [O${}_2$][N${}^2$O${}^2$]
net formation of [O${}_2$][N${}^2$O${}^2$] = k${}_1$[NO]${}^2$ (formed) - k${}_{-1}$[N${}^2$O${}^2$] (decomposed) - k${}_2$[O${}_2$][N${}^2$O${}^2$] (consumed) = 0 @ equilibrium
[N${}^2$O${}^2$] (k${}_{-1}$ + k${}_2$[O${}_2$]) = k${}_1$[NO]${}^2$
 (*) = 2k${}_1$k${}_2$[NO]${}^2$[O${}_2$] / (k${}_{-1}$ + k${}_2$[O${}_2$]) !!! not consistent
fast first step(also reversible) & slow second step. 'rate determining step'
first step @ quasi-equilibrium during the reaction
[NO][Br${}_2$]

Lec 33 | MIT 5.111 Principles of Chemical Science, Fall 2005