Solubility Equilibria

Question 1

Define the solubility of a substance

Answer 1

Max amount of solid that would dissolve in a given volume of solvent

Question 2

how does the solubility of gases in water change with temp?

Answer 2

solubility decreases with temp

Question 3

solubility of atmospheric gases

Answer 3

LOW water-solubility

less than 0.005g/100g

Question 4

solubility of CO2

Answer 4

moderate - about 4g / 100g at 20˚C

like in fizzy drinks

Question 5

give two examples of gases that are highly soluble

Answer 5

HCl and NH3 gas
both over 70g/100g

forms ammonia and hydrochloric acid

Question 6

how is the mass of gas dissolved and the pressure related?

Answer 6

they are proportional

more dissolves at higher pressure

Question 7

what does solubility depend on? particle level

Answer 7

the difference in strength of ion-ion interaction in the lattice and the ion-water interaction in hydrated cations

Question 8

describe the particles in ionic solids that have lower solubility.

Answer 8

multiply charged a/o small ions
they have relatively larger lattice enthalpies

hydration < lattice

Question 9

expression for the solubility of a solid + define

Answer 9

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

Question 10

what is the solubility constant?

Answer 10

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

Question 11

expression for Ks

Answer 11

related to ion concentrations, [P]/[R]
for solid == ions
note: solid is 1

K2 &laquo_space;1 for sparingly soluble solids

Question 12

what is the solubility quotient?

Answer 12

Qs
= [M+][X-] for ANY mixture

also known as the ‘ionic product’

Question 13

Qs and Ks relationship at equilibrium

Answer 13

Qs = Ks = [M+][X-]

equilibrium = saturated
dissolving and precipitation reactions occur at EQUAL RATES

Question 14

What happens when Qs > Ks?

Answer 14

precipitation accumulates.

the solution is ‘supersaturated’
reaction resulting in lower conc ions in sln (pption) is FASTER until Qs=Ks

Question 15

what happens when Qs < Ks?

Answer 15

IF ppt present, Dissolving is faster until Qs=Ks
No ppt, no change observed.
not saturated

Question 16

why does solubility of metal hydroxides depend on pH?

Answer 16

conc OH- depends on pH

for M(OH)n 
Ks = [M+][OH-]^n 

deduce [OH-] before and after mixing

Question 17

how to use ICE tables with solubility eq

Answer 17

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

Question 18

how does solubility change if a solution already contains an ion present in the solid to be dissolved?

Answer 18

the solubility of the ionic solid is LOWER

because [M+] already high, [X-] is lower to equal Ks

Question 19

rules for combining K’s and Q’s

Answer 19

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

Question 20

define dissolution and the requirements

Answer 20

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

Question 21

how to cause more solid to dissolve in a saturated sln

Answer 21

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

Question 22

what basic anion produced on dissolution reacts fully with H2O?

Answer 22

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+]

Question 23

what is the reaction quotient for the formation of complex ions from metal ions and ligands?

Answer 23

ß
subscript is the number of ligands

ßn is high for stable complexes.

can combine with Ks when summing eqns