Reaction Mechanisms

Reaction Mechanisms

reactions progress in a series of elementary steps that involve intermediate molecules that only exist briefly.

• 1 reactant = a unimolecular reaction.

• Two reactants = bimolecular

• Three reactants = trimolecular

Reaction mechanisms — More molecules = ??

More molecules = slower rate of reaction

elementary reaction; definition & properties(?)

• single-step reaction

• orders of reaction ARE equal to the stoichiometric coefficients of the reactants

• order of reaction for a reactant is equal to the coefficient of the reactant of the rate limiting elementary step

Why do elementary steps make sense?

If all reactions were one step, it would be insanely rare to get so many particles to collide at the right speed, energy, orientation, etc… as opposed to several smaller steps

slowest step in this reaction is called…

rate limiting step

cuz they can only go as fast as the slowest reaction

in the image which is the intermediate

NO2 and NO3

elementary steps MUST ADD UP to the final reaction

in a reaction mech graph where r intermediates and elementary steps

• Intermediates exist in the ‘valleys’

• step with the highest peak is the rate limiting step

• elementary steps are between intermediates

A valid mechanism must meet three criteria:

1. The elementary steps must add up to the overall balanced equation.

2. The elementary steps must be reasonable. They should generally involve one reactant particle (unimolecular) or two (bimolecular), no more.

3. The mechanism must correlate with the rate law, not the other way around.