concentrations/partial pressures of all species remain constant
forward and reverse processes continue to occur at equal rates
graphs of concentration, partial pressure, or rate of reaction versus time can be used to understand the establishment of chemical equilibrium
direction of reversible reactions
rate of forward reaction is greater than the reverse reaction: net conversion of reactants to products
rate of reverse reaction is greater than the forward reaction: net conversion of products to reactants
equilibrium: rates are equal
reaction quotient (Q): describes the relative concentrations of reaction species at any time
gas phase: may be written in terms of partial pressures (Q_p)
the reaction quotient tends towards the equilibrium constant such that at equilibrium:
the reaction quotient does not include substances whose concentrations/partial pressures are independent of the amount (e.g. solids, pure liquids)
equilibrium constant (K)
can be calculated from experimental measurements of the concentrations or partial pressures of the reactants and products at equilibrium
magnitude
very large: proceed essentially to completion
very small: barely proceed at all
properties (also applicable to Q)
reaction is reversed: reciprocal of K
reaction is multiplied: K is raised to that power
reactions are added: K is the product of the Ks for the reactions summed
calculating equilibrium concentrations
the concentrations/partial pressures of species at equilibrium can be predicted given:
balanced reaction
initial concentrations
appropriate K
Q < K: net consumption of reactants and generation of products
Q > K: net consumption of products and generation of reactants
Q = K: dynamic equilibrium
forward and reverse reactions proceed at the same rate
proportion of reactants and products remains constant
particulate representations can be used to describe:
relative numbers of reactant and product particles present prior to and at equilibrium
value of the equilibrium constant
le Châtelier’s principle
can be used to predict the response of a system to stresses
addition or removal of chemical species
change in temperature
change in volume/pressure of a gas-phase system
dilution of a reaction system
can be used to predict the effect that a stress will have on experimentally measurable properties
pH
temperature
color of a solution
a disturbance to a system at equilibrium causes concentrations/partial pressures to redistribute to establish a new equilibrium state
some stresses (e.g. changes in concentration) cause a change in Q only
some stresses (e.g. changes in temperature) cause a change in K only
solubility-product constant (K_sp): describes the extent of the dissolution of a salt (reversible)
solubility can be calculated from the K_sp for the dissolution process
the relationship can be used to predict the relative solubility of different substances
if K_sp is greater than one, the salt is soluble
the molar solubility of species in a saturated solution can be used to calculate the K_sp of a substance
common-ion effect: the solubility of a salt is reduced when it is dissolved into a solution that already contains one of the presions present in the salt
understood qualitatively using le Châtelier’s principle
calculated from the K_sp for the dissolution process
