Reaction Mechanism: Reactions take place in series of steps called reaction mechanism.
– Collision unlikely between more than 2 species in one step.
– Also stoichiometry in rate equation may not match overall equation.
Rate- Determining Step: The steps in reactions at different rates. Slowest step is ratedetermining step.
Creating a Reaction Mechanism: Overall equation (CH3)3CBr + OH-
-> (CH3)3COH + Br-
. Know
overall equation by cancelling common terms.
What doesn’t appear in rate equation is zero order, has no effect on reaction rate so must be in
fast step, OH-
.
Create the two steps. Step 1- (CH3)3CBr -> Br-
+ (CH3)3C
+
Slow
Step 2- (CH3)3C
+ + OH-
-> (CH3)3CBr Fast
Rate equation gives reactants and stoichiometry/ orders of slow step.
Step 1 forms two species- one of them is other product in overall reaction. Other is an
intermediate which also appears in step 2.
If equation quite doesn’t work out, check don’t have to balance equation.
Avoid putting in charges, unless asks for it.
If 2NO in overall equation, one NO in step 1 and another NO in step 2.
H+ is not a catalyst because it is used up and not regenerated. H+ appears in overall equation.
If have to put charges make sure charges balance. For example both sides must be zero, so can
have + charge and – charge together on side as cancel to 0. OR both sides can have -1 charge.
What can be deduced from a rate equation and an overall equation: Power of concentration of
reactant shows — moles in rate determining step. If this doesn’t match overall it’s a multistep
reaction. If something doesn’t appear in rate equation it’s zero order.
Sometimes have to be flexible if equations do not match rate equation and include a few fast
step species. Question will direct me to it though.
– Step 1 and 2 give rate equation. Even though step 1 is fast step.