We covered several models and theories …
Bizarro by Dan Piraro
PS #2 You can solve Q 1 1 2
Rxn mechanisms Theoretical concept Elementary rxn = rxn that occurs in a single step.
Chemical equation summary of overall process It has special properties:
no information how it happens 1. rate law follows stoichiometry
aA + bB cC + dD
Rxn mechanism = sequence of steps by which a rxn
Rate = k [reactants]toich coeff= k [A] [B]b
s r ucco
For example, if the following is an elementary rxn
It MUST reproduce the observed overall NO + CO CO + NO
3 2 2
stoichiometry AND reproduce the observed rate law. Then Rate = k [NO ][CO]
Mechanism can involve:
2. The molecularity for an elementary rxn is
Intermediate species = species that appear as a
product in one step and being consumed in a
different step and that do not appear in the overall
NO 4 or higher ! no fractions ! no zeros, no –ve
What is molecularity? theoretical concept Most rxn’s are not elementary
Molecularity = number of molecules that are
but they can be broken into several elementary rxn’s.
involved/collide in an elementary reaction having
sufficient energy Ea and the proper orientation to A + B C overall rxnnot elementary (from
) sno i t av r e sbo
form the transition state.
A D elementar(theoretical)
Obviously we can’t have fraction or negative D + B elementar(theoretical)
molecules colliding. Also the probability of 4
molecules fulfilling all these requirements is too
small. First we check that the net reaction of the proposed
mechanism matches the observed reaction equation.
So 2 A + 2 B C + D can’t be elementary
b/c 4 molecules are involved in the collision. A + D + B C + D
Also watch out for the reverse step in a mechanism. is equivalent to the net reactionA + B C
Then we can derive a theoretical rate law based on
A + B 3 C + D can’t be elementary b/c of the
this proposed mechanism.
reverse step. 5