Solubility Equilibria Flashcards
Define the solubility of a substance
Max amount of solid that would dissolve in a given volume of solvent
how does the solubility of gases in water change with temp?
solubility decreases with temp
solubility of atmospheric gases
LOW water-solubility
less than 0.005g/100g
solubility of CO2
moderate - about 4g / 100g at 20˚C
like in fizzy drinks
give two examples of gases that are highly soluble
HCl and NH3 gas
both over 70g/100g
forms ammonia and hydrochloric acid
how is the mass of gas dissolved and the pressure related?
they are proportional
more dissolves at higher pressure
what does solubility depend on? particle level
the difference in strength of ion-ion interaction in the lattice and the ion-water interaction in hydrated cations
describe the particles in ionic solids that have lower solubility.
multiply charged a/o small ions
they have relatively larger lattice enthalpies
hydration < lattice
expression for the solubility of a solid + define
s(solid) a measure of the AMOUNT DISSOLVED in a saturated solution molL-1 NOT A CONSTANT given n and V of a saturated sln, s=n/V
eg. s(CaF2) = [Ca2+] = [F-]/2
for CaF2(s) == Ca2+(aq) + 2F-(aq)
**ASSUMING NO FURTHER REACTION OF IONS
use known solubility to calculate conc ions
what is the solubility constant?
Ks
equilibrium constant describing the extent of a dissolving reaction in a saturated sln
Constant for all aq soln of ionic solid at same temp
expression for Ks
related to ion concentrations, [P]/[R]
for solid == ions
note: solid is 1
K2 «_space;1 for sparingly soluble solids
what is the solubility quotient?
Qs
= [M+][X-] for ANY mixture
also known as the ‘ionic product’
Qs and Ks relationship at equilibrium
Qs = Ks = [M+][X-]
equilibrium = saturated
dissolving and precipitation reactions occur at EQUAL RATES
What happens when Qs > Ks?
precipitation accumulates.
the solution is ‘supersaturated’
reaction resulting in lower conc ions in sln (pption) is FASTER until Qs=Ks
what happens when Qs < Ks?
IF ppt present, Dissolving is faster until Qs=Ks
No ppt, no change observed.
not saturated
why does solubility of metal hydroxides depend on pH?
conc OH- depends on pH
for M(OH)n Ks = [M+][OH-]^n
deduce [OH-] before and after mixing
how to use ICE tables with solubility eq
molar solubility (s) is related to ∆[ions] eg. s(CaF2) = ∆[Ca2+]/1 = ∆[F-]/2
the ‘change’ part is expressed in terms of s
sub expressions [cation] or [anion] into Qs
how does solubility change if a solution already contains an ion present in the solid to be dissolved?
the solubility of the ionic solid is LOWER
because [M+] already high, [X-] is lower to equal Ks
rules for combining K’s and Q’s
reverse eqn, inverse K and Q
multiply eqn by a coefficient, raise K and Q to that power
sum two eqns, multiply the two K’s for the new K value
define dissolution and the requirements
solute dissolves in solvent to form a solution.
Occurs if the product salt is more soluble than the reactant salt
eg. CO3^2- –> HCO3-
stronger base –> weaker base
how to cause more solid to dissolve in a saturated sln
lowering ion concentration, causing more solid to dissolve until equilibrium is reestablished.
removing basic anions due to reaction with H+ or H2O
or formation of complex ions
what basic anion produced on dissolution reacts fully with H2O?
the sulfide (S-) anion reacts fully to exist as HS- in aq sln
hence for FeS in water, must construct two Q expressions:
FeS + H2O == Fe2+ + HS- + OH-
H2S(g) + H2O == HS- + H3O+
note that Fe2+ is the only species arising not further involved, so s=[Fe2+]
what is the reaction quotient for the formation of complex ions from metal ions and ligands?
ß
subscript is the number of ligands
ßn is high for stable complexes.
can combine with Ks when summing eqns
