AP CHEM REVIEW U5-9

rate of reaction

change in concentration per unit time

rate expression

change in concentration of a substance / change in time

reactant = -

product = +

relative rate

write rate expression for each substance in reaction and set them equal to each other

put inverse of stoich coefficients in front of it

add - sign for reactants, + for products

instantaneous rate

rate at any specific time

take slope of tangent line of the point = rate

inital rate

rate at start of reaction

rate law

rate = k[A]^m[B]^n

for REACTANTS only

exponents are NOT the stoich coefficients

rate law exponents

tell us the reaction order

overall reaction order

sum of exponents in rate law

how to find reaction order for the reactants

compare rate data for the reactants

select 2 trials where the concentration of 1 reactant changes but other remains the same

how to find order of a reactant if it doesn’t increase by a whole number

rate 2 / rate 1 and solve for the exponent

the rate that is constant cancels out

how to find rate constant (k) after finding rate law

plug in data from one of the trials and solve for k

integrated rate laws

concentration of reactant at any time

zero order rate law

not dependent on concentration

R = k

m = -k

first order rate law

for half life problems

R = k[A]

m = -k

second order rate law

R = k[A]²

m = k

rate constant

0 order = M / s

1 order = 1/s

2 order = 1/Ms

must cancel out with concentration (M) to get R = M/s

half life

only for 1st order

time it takes for ½ of reactant to be used up

t_1/2 = 0.693 / k

Rate is independent of initial concentration

find k

collision theory

rate of reaction is proportionial to collisions

molecules collide with a particular orientation in order to react

collisions must have enough energy to overcome activation energy

temperature affect on reaction rate

average kinetic energy of particles increases

more particles collide with enough energy to overcome activation energy

reaction mechanisms

reactions have steps (elementary steps)

intermediate

product earlier

reactant later

not in overall equation

catalyst

reactant earlier

product later

not in overall equation

molecularity

number of reactants

unimolecular

one reactant

rate is proportional to concentration

bimolecular

2 reactant molecules

first order for 2 reactants or 2nd order for 1 reactant

rate determining step

the slowest step in a reaction

reaction rate law for a slow initial step

just the rate law of the slow initial step

reaction rate law for a fast initial step

for first step: set rate of forward equal to rate of reverse

set that rate equal to the intermediate in the reaction

write rate of slow step

plug what intermediate equals into the slow step- that is the rate law

distance btw reactants and products on energy diagram

change in enthalpy

distance between reactants and transition state

activation energy

catalysts

lowers the activation energy of a reaction

more collisions can get over a lower activation energy

offers alternative reaction pathway

homogenous vs heterogeneous catalyst

homo: catalyst is in same phase as reactants

hetero: catalyst is in diff phase as reactants

concentration effect on rate

if concentration increases, collisions increase, rate increases

state with highest rate

gas

molecules collide more frequently

surface area affect on rate

powder or breaking solid

more particles can react with reactants

increased rate