ap chem kinetics

thermodynamics vs kinetics

thermo: tells us if a reaction can occur

kinetics; tells us how quickly the reaction occurs

*some reactions that are thermodynamically feastible are kinetically so slow as to be impossible

collision theory

• explains why different reactions occur at different rates, and suggests ways to change the rate of a reaction

• for a chemical reaction to occur, the reacting particles must collide with one another

• 2 factors

2 factors for whether reaction occurs when molec collide

1. is there sufficient energy

2. is there proper orientation

only a small portion of collisions produce a reaction

increasing temp

increases reaction speed, inc kinetic energy

(heat them up, speed them up)

activation energy E_A

energy that must be overcome to produce a chemical reaction

endo/exothermic reaction

endo

• reaction in which a system ABSORBS heat from its surroundings, positive delta H

exo

• reaction in which a system RELEASES heat to its surroundings, negative delta H

rate of reaction depends on

rate of reaction depends on E_A, delta h has no effect on rate of reaction

*higher the E_A, the slower the reaction at a given temperature

maxwell boltzman distribution curve

probale speeds that molecule possess

y-axis: no of molecules / proportion of molec

x-axis: velocity v in m/s

• as temperature increases, the fraction of molecules with enough energy to surmount the activation energy barrier also increases

molecular orientation

atoms that are forming new bonds between them — MUST come in contact (and with sufficient energy) in order for a bond to form

Catalysts

substance that speeds up a reaction without being consumed (ex. enzymes)

• produces a new reaction pathway with a lower activation energy for both the forward and reverse reaction

*catalysts will not be shown in balanced equation but may show up in the rate law

homogenous/heterogenous catalysts

homo

• present in the same phase as the reacting molecules (usually liquid phase)

hetero

• exists in a different phase

• usually involves gaseous reactants being absorbed, penetration on the surface of a solid catalyst (such as a car’s catalytic convertor) — called surface catalyst

• either new reaction intermediate is formed or probability of successful collisions is increased

• 
  

the big 5 affecting reaction rates

1. Temperature

2. Adding a catalyst/inhibitor

3. Concentration: increase moles inc particles that can collide

4. Surface area: inc by crushing a substance, more contact → more collisions

5. pressure (only gas state): inc moles of substance, dec the volume, closer together

reaction rate of reactant

Rate = -delta[A]/ delta t

negative bc [ ] decreases

Rate Law

standard form: Rate = k[A]^m[B]ⁿ

*Rate laws must be experimentally determined - you CANNOT use the balanced equation to determine a rate law

k, rate constant

faster the reaction, the larger the k value

shortcut for determining units of k is as follows

k = L^x/(mol^x*time)

value of x will be one less than the order of reaction

reaction 3rd order: k = L²/(mol²*time)

reaction 2nd order: k = L¹/(mol¹*time)

reaction 1st order: k = 1/time

integrated rate law

• expresses how the [ ] of the reactant depends on time

• instead of changing initial [ ] and using multiple experiments, one experiment is done and [ ] changes over time are measured

• looking for the linear relationship of how a single reactant changes over time

• can only determine the exponent for one reactant at a time

zero order integrated rate law

[A]_t - [A]₀ = -kt

graph of [A] versus time produces a straight line with slope -k

first order integrated rate law

ln[A]_t - ln[A]₀ = -kt

ln(A_t/A₀) = -kt

graph of ln[A] vs time produces a straight line with slope -k

half life constant only in

only in 1st order reactions is half life independent of initial concentration (so it is constant for that reaction)

• half life for 0 and 2nd order reaction depends on the initial concentration (so you would just use the full equation)

half life (t_1/2)

radioactive decay is first order. Half-life (t_1/2) is the length of time required for the [ ] of a reactant to decrease to half of its initial value

half life equation

t_1/2 = 0.693/k

fast reaction, short t_1/2
short reaction, long t_1/2

1. LARGE k

2. SMALL k

second order integrated rate law

 1     -      1  = kt

[A]_t       [A]₀

graph of 1/[A] vs time produces a straight line with slope k

HALF LIFE FORMULAS