solubility equilibria Flashcards
what is the solubility of a salt?
the solubility of a salt is the number of moles of solute that can be dissolved in 1 dm3 of a given solvent to form a saturated solution at a given temperature
what nitrates are soluble?
all nitrates are soluble
what halides (Cl-, Br-, I-) are soluble?
all halides are soluble except Ag+ & Pb2+
slbc
what sulfates are soluble?
all sulfates are soluble except Pb2+, Ba2+ & Ca2+
what carbonates are soluble?
Na+, K+ & NH4+ are soluble
the rest are insoluble
what oxides are soluble?
Na+ & K+ are soluble
the rest are insoluble
what hydroxides are soluble?
Na+ & K+ are soluble
the rest are insoluble
what is solubility product (Ksp)?
solubility product, Ksp, of a sparingly soluble salt, is the product of the molar concentrations of the constituent ions in a saturated solution, raised to the appropriate powers at a given temperature
Ksp only changes with temperature, and is only applicable for SPARINGLY SOLUBLE SALTS
example: write the solubility product of SrF2
SrF2 ⇌ Sr2+ + 2F-
Ksp = [Sr²⁺] [F⁻]²
what is ionic product?
ionic product (IP) of a sparingly soluble salt is the product of the molar concentrations of the constituent ions in the solution, raised to appropriate powers
applicable to all solutions, including unsaturated solutions (unlike Ksp)
when IP = Ksp, solution is saturated
IP & Ksp have the same mathematical expression
how can Ksp & IP be used to predict the occurrence of precipitation?
consider AgCl ⇌ Ag+ + Cl-, IP = [Ag+] [Cl-]
- when IP < Ksp, the forward reaction occurs. the solution is not saturated -> more AgCl can dissolve in the solution until IP = Ksp -> no precipitation occurs
- when IP = Ksp, the solution is saturated -> no more AgCl can dissolve -> the solution contains the maximum amount of Ag+ & Cl- (of the salt/solute) -> upon further addition of AgCl, IP remains equal to Ksp (undissolved salt is present at equilibrium with the saturated soultion) -> the first trace of precipitate just appears/just about to appear
- when IP > Ksp, the backward reaction (precipitation) occurs -> position of equilibrium shifts to the left and precipitate is formed until IP = Ksp
note that IP & Ksp have the same mathematical expression, but IP can take on any value while Ksp has a constant value at constant temperature
what are the 3 factors affecting solubility?
- common ion effect
- formation of complex ion
- effects of pH
what is the common ion effect and how does it affect solubility?
consider CaF2 (s) ⇌ Ca2+ (aq) + 2F- (aq)
if a water soluble solute containing the common ion Ca2+/F- is added, by LCP, POE shifts left to decrease [Ca2+]/[F-] -> less CaF2 dissolves -> solubility of CaF2 is reduced
i.e. in the presence of a second solute that contains a common ion, the solubility of a sparingly soluble salt decreases
how does the formation of a complex ion affect solubility?
some metal ions can accept electron pairs from neutral molecules/anions to form complex ions. eg [Al(OH4)]- and [Ag(NH3)2]+ are complex ions
consider [Al(OH)4]-
Al(OH)3 (s) ⇌ Al3+ (aq) + 3OH- (aq)
Al3+ (aq) + 4OH- (aq) ⇌ [Al(OH)4]- (aq)
the formation of a complex ion, [Al(OH)4]-, removes Al3+ from the solution -> [Al3+] decreases -> by LCP, POE in 1st eqn shifts right to produce more Al3+ -> white ppt of Al(OH3) dissolves -> solubility of Al(OH3) increases
how does pH affect solubility?
consider Mg(OH)2 (s) ⇌ Mg2+ (aq) + OH- (aq)
adding a bronsted acid eg HCl decreases pH & neutralises OH- -> [OH-] decreases -> by LCP, POE shifts right to produce more [OH-] -> more Mg(OH)2 disolves -> solubility of Mg(OH)2 increases
