Complex reactions

1
Complex reactions
a) Parallel reactions
hv CHBr3--gt CH ..
1. CH CH4--gt C2H4 H--gt
2. CH CHBr3 --gt products
CHltltCHBr3
AB--gtD k1
AC--gt E k2
-dA/dt k1AB k2AC Ak1Bk2C kA
(because B, C gtgtA)
2
de-NOx -- add NH3 to smoke stacks
NH2 NO --gt N2 H2O ?H0 -127 kcal/mol
--gt N2H(?) OH ?H0 - 3kcal/mol
--gt N2 H OH ?H0 -6 kcal/mol
Rate of disappearance known, but not branching
measured by LIF of NH2
3
1. Measure NH2 NO--photolyze NH3/NO
4
Pseudo 1st order
AA0e-kt
kk1Bk2C
Measure k for various B _at_ constant C
Slope k1 intercept k2C
Plot k vs B
dD/dt k1ABk1A0e-ktBk1 A0 B0 e-kt
D k1 A0B0(1-e-kt)
5
k2
Equilibrium all that
A B C
k-2
At equilibrium, RfRb
Detailed balance
But!
k2
M A B C M
k-2
danger here must knowmechanism near equil
6
Detailed balance rate of fwd rxrate of back rxn
Microscopic reversibility (follows from time
reversal)
At equilibrium each microscopic process and its
exact opposite occur at the same rate andby the
reverse mechanism.
7
Termolecular reactions?
Dissociation of Hydrogen--whats the mechanism?
Look at reverse reaction
H H ?H2
Third body to conserve energy
H H M ? H2 M
energy must be removed
Moving H atomske1/2mv2
Asymptote two H atoms at rest
8
Perturbations from equilibrium
H OH- H2O
AAe?
BBe ?
CCe- ?
E1/2CV2
C10µF, V20kV
E2kJ 1l of H2O 4kJ/deg
T N T
Equil
0
9
Consecutive Reactions
often very reactive intermediate
A--gtB--gtC
k1
k2
For B
RformRreaction
Steady state
Conservation of mass