Chem 1310: Introduction to physical chemistry Part 2d: rate laws and mechanisms
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Transcript of Chem 1310: Introduction to physical chemistry Part 2d: rate laws and mechanisms
Chem 1310: Introduction to physical chemistry
Part 2d: rate laws and mechanisms
Unimolecular vs bimolecular reactions
Unimolecular reactions happen within one molecule.
The molecule needs to acquire enough energy to overcome the barrier to its reaction.
Molecules collide all the time and so exchange energy.
At any time only a small fraction of them may have enough energy, most will not.
Even if they have enough energy, the reaction does not always happen.
To react or not?A game of chance
Lower barrier higher probability higher rate
Higher temperature higher average energy higher rate
Note: this figure is not to scale !!!
Dependence of rate on temperature
RT
EAk
eAk
a
RT
Ea
lnln
Arrhenius equation:
Interpretation:e-Ea/RT = "energy factor":
fraction of molecules havingenough energy for the reaction.
A = "frequency factor":collision frequency * probability that,given enough energy, the reaction happens.
Arrhenius equationat low temperature
0
10
20
30
40
50
60
70
0 200 400 600 800 1000
T (K)
k (L
mo
l-1
s-1)
T small-Ea/RT large and negativee-Ea/RT close to 0
Number of moleculeswith sufficient energygrows nearly exponentially.
Arrhenius equationat high temperature
0
500
1000
1500
2000
2500
3000
3500
4000
0 5000 10000 15000 20000
T (K)
k (L
mo
l-1
s-1)
Region ofexponentialgrowth.
Every molecule hasenough energy.Frequency factordetermines rate.
T large-Ea/RT close to 0e-Ea/RT close to 1k A
Dependence of rate on temperature
Plot ln k vs 1/T: straight line with slope -Ea/R and intercept ln A ("Arrhenius plot").
RT
EAk
eAk
a
RT
Ea
lnln
Doing an Arrhenius plot
Energy diagrams("reaction profiles")
Av molecular kinetic energy at RT
Ea
Erxn
Ea
Ea
Erxn
N CH CN H
Cyclobutene to butadiene
Ea
Erxn
Cyclobutene to butadiene
Hexadiene to hexadiene
Ea
Hexadiene to hexadiene
HNC to HCN
N CH
CN H
Ea
Erxn
Ea(reverse)
HNC to HCN
Unimolecular vs bimolecular reactions
Bimolecular reactions happen when two molecules meet.
The "encounter complex" also needs to have enough energy to overcome the barrier to its reaction.
At any time only a small fraction of the colliding pairs will have enough energy, most will not.
Even if they have enough energy, the reaction does not always happen.
Termolecular and higher reactions?
"They don't happen."
It is just too improbable for three molecules to meet each other simultaneously, with enough energy and the correct alignment.Even bimolecular reactions tend to be slower than
unimolecular ones with the same Ea.
But two molecules might meet each other, "stick", then meet a third one and finally undergo the "real" reaction.
Not every collisionresults in reaction
?
?