The last few lectures we dealt with kinetics, straight
lines and slopes …
Dilbert by Scott Adams
PS 2 is posted online. You can solve question 1. 1 2
The rate constant is given by k = p Z f Transition state model
Assuming a favorable collision occurs, what is
p = steric factor only a proper orientation is
. dewo l l a
Plot reaction profile or activation energy diagram
Z = # of collision per unit time per unit volume
f = fraction of molecules with energy E a Energy vs. rxn progress
So k = p Z e –E aRT
Bonds in the reactants stretch and become weaker.
New intermolecular bonds start to form (still weak
The larger E a the smaller k and hence a slower rxn.
at this stage).
Ea = 0 k = pZ e = pZ
The transition species is known as the transition
– RT/RT –1
Ea= RT k = p Z e = pZ e = 0.37 pZ state or the activated complex.
See fig. 16.21 The transition state (TS) or activated complex has
- high energy
- short–lived (unstable)
- exact structure is unknown (can’t be isolated)
- in equilibrium (low concentration) with reactants
- can proceed in either direction
When the potential energy reaches a maximum, we
call it the transition state.
When E reactantsEproductsexothermic reaction
E < E endothermic reaction
reactants products 5 6
Temperature effect on rxn rates See fig 16.16
All rxn’s go faster with temperature increase.
Increase in number of molecules with energy > E a
Rule of thumb
If Ea 50 kJ/mol 10 degrees increase (room T)