Chapter 19: Rates, Equilibrium, Spontaneity

Collision Theory

• A + B (reactants) must collide

• must have minimum kinetic energy (activation energy)

• proper molecular orientation

dimerization

when a compound reacts with itself

acid/base formula writing

for compounds with nonmetals, formula must be written from most electropositive to electronegative

activation energy

energy of activated complex - energy of reactants

enthalpy (delta H)

energy_products - energy_reactants

energy barrier

a graph depicting the energy changes in a reaction

endothermic reaction

energy of products is higher than energy of reactants

exothermic reaction

energy of products is less than energy of reactants

factors affecting reaction rate

• Temperature: increasing temperature increases frequency of collisions and # of particles that have enough energy

  • Maxwell-Boltzmann diagram

• Concentration: increased concentration allows more particles to collide with each other

• Particle Size: smaller particle size means more surface area

• Catalyst: lowers the activation energy

the concentration of a reactant ___________ as the rxn moves forward

decreases

relative rates

• determined from stoichiometry

• take the rxn rate and multiply by coefficient

rate

• formula: -(delta concentration/delta time)

• units: mol/(L*s)

two types of rate laws

• differential (rate and concentration)

• integrated (rate, concentration, time)

notes about rate laws

• powers for units of k (rate constant) are one less than the overall rxn order

• add orders to find the overall order

reaction mechanism

• multiple steps (mechanism) by which a reaction occurs

• # of intermediates is one less than number of elementary steps

a given rate law is

experimentally determined

elementary step

• individual reactions in a mechanism

• a rxn for which you can write rate law based on molecularity

valid mechanism?

• elementary steps must add up to overall reaction

• rate from mechanism must be same as experimental rate law

  • a rxn cannot go faster than the rate of the slowest step

  • slow step AKA rate-determining step

equilibrium

the point in a reaction where there is enough products to make it go the other way

dynamic equilibrium

• rate forward = rate reverse

• reaction is still occurring, equilibrium is the rate of exchange

• must have a closed system

law of mass action

• equilibrium constant expression (K_eq = products to the power of their coeff/reactants to the power of their coeff)

• only at equilibrium

• only include concentrations of gases and aqueous solutions

  • not solids or liquids because their concentrations don’t change

K_eq is

• unitless

• temperature-dependent

if K_eq > 1

• products are favored

• equilibrium shifts far to the right

if K_eq < 1

• reactants are favored

• equilibrium shifts far to the left

if K_eq = 1

no shift

K_P

• for gases, write the equilibrium constant using their pressures

• label K_eq as K_P

• K_P = K_eq(RT)^delta_n

  • where delta_n is the difference in # moles of only gaseous (products - reactants) taken from reaction equation

5% rule

what you are neglecting is < 5% of what you are neglecting it from

use the ICE table when

calculating equilibrium concentrations from given values (initial concentrations, etc…)

le chatelier’s principle

when a stress is applied on a system at equilibrium, the system will change to relieve that stress

• changes in pressure or volume primarily affect gases

notes about K_eq

• at a given temperature, there is only one value for K_eq

• you can have infinite # of equilibrium positions (concentrations) at a given temperature

spontaneous

favors formation of the products

what determines spontaneity

• driving forces

  • enthalpy (delta H): favorable if negative

  • entropy (delta S): favorable if positive (increase)

• positive enthalpy and negative entropy is never spontaneous

entropy (delta S)

• the degree of disorder of a system

• the specie with more atoms has more entropy

Law of Disorder

• the entropy of the universe is always increasing

• there is a higher probability of disorder than order

gibbs free energy (delta G)

• energy available to do work

• delta G = delta H - (T)(delta S)

• if delta G < 0: spontaneous

• if delta G > 0: nonspontaneous