Chapter 13 - Chemical Kinetics

rate =

change in concentration of a reactant or product over an interval of time.

• rate of disappearance (reactions) is neg

• rate of apperance (products) is pos

• to make the rates equal to each other, divide each rate by their respective coeff in the chem eqn

reaction mechanism

• depends on reaction rate

• and concen dependence

rate law

k[A]^m[B]^n[C]^p

k is the constant

the exponents give

• the rxn order

• can be positive integers or fractions

• is det thru experiment so you often have to interpret a table

1st order

exp in rate law = 1

means that the rate goes up by a factor of 2 if the concentration goes up by a factor of 2

2nd order

• exponent in rate law = 2

• if the concen goes up by a factor of 2, the rate goes up by a fatcor of 4

• 3 → 9

0 order

expoennt on rate law = 0 so the whole thing = 1

to interpret the rate law

you must find the shit where all other concens are constant (initial and final are the same)

rate of rxn =

the rate law expression

so you can calc the k constant after figuring out the exponents for the compounds

units of k

L/mol*s

integrated rate law for 0 order

integrated rate law for first order rxn

half life

time taken for half of a reactant to be consumed

[A]t = 1/2*[A]0

reaction after 2 half lives → ¼ of the reactant remains

reaction after 3 half lives → 1/8 of the reactant remains

2^#ofhalflives

integrated rate law for 2nd order

reactions require (according to collision theory)

• activation en

• correct geo

• a reaction that requires low activation en happens earlier so the smallest hill features the fastest rxn

another way to find k

where Ea is the activation en and A is the freq factor

this makes a line of lnk vs 1/T

RATE DETERMINIG STEP

the slowest step in a rxn is rate determinng in the rxn mechanism

correlating mechanism with rate law

• elementary steps must add up to the overall balance eqn

• the mech must correlate with the OBSERVED rate law, aka the rate determing step

• the rate law also would not have a compound that was made and consusumed in the mechanism (does not end up in final eqn)

homo catalysrt

catalyst = same phase as the reactants

hetero catalyst

when the catalyst is in a diff phase than the reactants