Chemistry 111 Exam 2 - Chemical Kinetics & Equilibrium

Main idea of Chemical Kinetics

the speed of chemical reactions

k (rate constant)

• large value = fast rxn

• small value = slow rxn

• if k=0 no rxn

factors of speed

1. energy

2. catalyst

3. surface area

4. concentration

Energy effect on speed

energy (temperature) ↑, speed ↑

Catalyst effect on speed

catalyst added, speed ↑

• provides an alternative pathway with lower E_a

• can be recovered post rxn

• non reactive metals trap gasses on surface → more collisions → faster

Surface area effect on speed

SA ↑, speed ↑

• surface area correlates to physical state

Concentration affect on speed

conc ↑, speed ↑

avg speed equation

total distance

total time

rate equation

Δ[A]

Δt

• [A] is concentration of A

• t is time

• coefficients in chemical formula become fractions (negative for reactants)

• changes with time

rate law equation

rate = k [A]^m [B]ⁿ

• k is rate constant

• m & n signify rate order

• only includes reactants

• experimental values are needed to find rate order & k

half-life of rxn

t_1/2 = 0.693

k

• time required for concentration to drop to half of initial value

• does not depend on conc

• only for large amounts

reaction rate depends on

1. state of reactants

2. concentration of reactants

3. temperature ↑, rate ↑

4. catalyst added, rate ↑

first order reaction

• linear relationship

• x¹

second order reaction

• 2x relationship

• x²

zero order reaction

• does not impact the rate

• still needed for rxn to happen

• x⁰

overall order

add up all the exponents in the rate law

first order integrated rate law

ln[A]_t = ln[A]₀ -kt

second order integrated rate law

1 = kt + 1

[A]_t [A]₀

zero order integrated rate law

[A]_t = -kt + [A]₀

Collision Theory

• most collisions don’t cause a rxn (futile)

• orientation is important for rxns happening

• connects to Arrhenius equation

Arrhenius equation

k = A e^-E_a/RT

• k is rate constant

• A is collision factor

• E_a is activation energy

• R is gas constant

• T is temp

effective collisions ↑, k↑

E_a ↑, k↓

T↑, k↑

Activation Energy (E_a)

minimum energy needed to start a reaction

intermediates (potential energy diagram)

• troughs

• in between transition states

• most stable

transition states (potential energy diagram)

• peaks

• where E_a goes to

elementary reaction

actual molecular events happening, from start to transition state

rate determining step

• slowest step of reaction

• rate can only go as fast as the slowest step

Chemical Equilibrium

• forward rate = reverse rate

• balance and equality of reaction rates

signs of equilibrium

1. concentration of reactants and products dont change with time

2. rate_forward = rate_reverse

3. same equilibrium constant (K) no matter how the reaction is started

4. if temp changes → new eq established

K (equilibrium constant) equation

[products]

[reactants]

• only aqueous and gases in equation

• K_c for concentration

• K_p for pressure

• depends on balanced formula only

K_c for aA + bB ↔ cC + dD

K_c = [C]^c[D]^d

[A]^a[B]^b

K > 1

reaction favors products

K < 1

reaction favors reactants

K = 1

reaction at equilibrium

Converting K_c into K_p equation

K_p = K_c (RT)^(Δn)

• Δn = gas moles of product - gas moles of reactant

Q (reaction quotient)

• used when you don’t know if equilibrium has been reached

• calculated the same way as K (products/reactants)

Q < K

before equilibrium

Q = K

at equilibrium

Q > K

after equilibrium

Le Chatelier Principle

if you stress a system at eq → eq responds and counteracts the change

Equilibrium Stressors

1. pressure

2. temperature

3. concentration

4. pH

concentration stressed equilibrium

substance is added → reaction consumes substance to get back to equilibrium

temperature stressed equilibrium

high temp (heat) → reduce energy, favors products

low temp (cool) → increase energy, favors reactants

exothermic reaction

releases energy

what temperature are exothermic reactions favored at?

high temps

endothermic

absorbs energy

what temperature are endothermic reactions favored at?

high temperatures

pressure stressed equilibrium

• depends on the moles on each side of product/reactant

• shifts towards the side with less moles of gas (less molecules in volume)

does adding a catalyst affect equilibrium?

no, it just gets there faster