MCAT Chemistry: Kinetics and Equilibrium

reaction rates

change in concentration over time

rate = (-1/a)(delta [A]/time)

factors that affect the rate of a reaction

Temperature, surface area, concentration, catalyst, pressure

rate constant

the probability that a collision will result in a successful reaction. inversely proportional to activation energy, directly proportional to temperature

k = Ae^(activation energy/RT)

catalyst

substance that speeds up the rate of a chemical reaction; used then regenerated, constant in overall reaction

intermediate

produced than used, detectable (unlike transition states), high-energy

transition state

high-energy, unstable state; not shown in mechanisms, undetectable

rate law

rate in terms of the initial concentrations of reactants and the rate constant only for elementary steps WITHOUT solids/solvents

rate = k [A]^m

method of initial rates

given rates with changing concentrations of reactants

- write out generalized rate law

- make sure all powers of 10 are the same

- compare rates where only one reactant changes: use this to determine the power in the rate law (0th, 1st, 2nd)

- you are able to find rate constant from concentrations and rate

dynamic equilibrium

when the forward and reverse rates are equal

equilibrium constant

describes the position of the equilibrium, ratio of product to reactant concentrations/partial pressures at equilibrium

Kc=[products]^[coefficients] / [reactants]^[coefficients]

reaction quotient

The ratio of the concentrations of the products to the concentrations of the reactants at any point during the reaction aside from equilibrium,

Qc=[C][D]/[A][B]

Q vs K

Q<K: shift right

Q=K: at equilibrium

Q>K: shift left

free energy with the reaction quotient

Grxn= Gstandard + RTlnQ

free energy at equilibrium

Gstandard = -RTlnKeq

Le Chatelier's Principle

States that if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress. stress can include changes in: concentration, volume, pressure and temperature

volume stresses an equilibrium

only if the number of moles of gas changes over the reaction. increasing pressure by decreasing volume shifts towards fewer mols of gas, and vice versa

temperature stress at equilibrium

if the reaction is endothermic: heat is a reactant, if the reaction is exothermic: heat is a product

forward vs reverse rate constants

If the reaction occurs in a single step, at equilibrium, the forward and reverse rates are equal. Keq=kf/kr

equilibrium of a coordination complex

is a formation constant; combining multiple equilibria results in multiplication of the equilibrium constants

solubility product

the constant for the equilibrium expression representing the dissolving of an ionic solid in water. as Ksp increases, solubility increases

molar solubility

(S) the solubility of a compound in units of moles per liter

comparing Qsp to Ksp

when a precipitate will form:

Qsp > Ksp: percipitates will form

Qsp < ksp: no precipitate will form

Qsp=Ksp: saturated solution

common ion effect

a decrease in the solubility of an ionic compound caused by the addition of a common ion

solubility effects in acid/base

salt's solubility will increase if it is added to a solution containing something that removes a common ion

affinity constant

Reciprocal of the dissociation constant.

Kd= 1/Ka; affinity constant (Ka)