Solubility Equilibria : Equilibrium involving the dissolving of Ionic Compounds
Solution:
Solute:
Solvent:
homogeneous combination of solute and solvent
the substance being dispersed
the substance doing the dispersion
Dissociation: the process whereby polar water molecules pulls apart the anions and cations
of an ionic compound. ____________________________________________________
________________________
H
O
H
H
Na
1+
Cl
O
O
Cl1-
Na1+
H
Na1+ O
H
H
H
O
H
H
H
1-
H
H
H
O
O
H
1-
Cl
H
Solubility:
Solubility Terms
Soluble:
Insoluble:
Miscible:
Immiscible:
solute dissolves
solute does not dissolve
two liquids mix to form a solution
two liquids that do not mix (heterogeneous combination )
Solubility Curve: Graph showing the changes in solubility with change in temperature
supersaturated
saturated
Solubility
g/100 g H2O
unsaturated
temperature
Molar Solubility:
Solubility Equilibria
solute(s)
 cation(aq) + anion(aq)
AgCl(s)
 Ag1+
Ion Product/ Ion Product Constant
Ksp = [Ag1+ ] [Cl1- ]
+ Cl1-
O
H
Ksp Problems
A) Solving For Ksp
1. (ex17.4) Silver bromide , AgBr , is a light sensitive compound in nearly all photographic
film. At 25oC, 1.0 L of water can dissolve 7.1x10-7 mol of AgBr. Calculate the Ksp.
molar solubility = n/V = 7.1x10-7mol/1L =7.1x10-7mol/L
AgBr
Ag1+
Br1[I]
0
0
-7
[C] molar
7.1x10
7.1x10-7
solubility
[E]
Ksp=[Ag1+ ] [Br1- ] = (7.1x10-7 ) 2 = 5.0x10-13
2. (ex17.5)
The molar solubility of silver chromate, Ag2CrO4 , in pure water is 6.7x10-5 mol/L
at 25oC . What is Ksp for silver chromate?
Ag2CrO4
2Ag1+
CrO42[I]
0
0
-4
[C] molar solubility 1.34x10
6.7x10-5
[E]
3. (17.6) Common Ion Effect
At 25oC, the molar solubility of PbCl2 in 0.10 M NaCl solution is
1.7x10-3 mol/L. Calculate the Ksp.
PbCl2
Pb2+
2Cl1[I]
0
0.10
[C] molar solubility
[E]
Practice Questions p744 5 , 6 , 7
B) Calculating Molar Solubility Given Ksp
1. (17.7)
What is the molar solubility of AgCl in pure water at 25oC ?
AgCl
Ag1+
Cl1[I]
0
0
[C] molar solubility
[E]
Ksp=[Ag1+ ] [Cl1- ]
2. (17.8)
[I]
[C]
[E]
Calculate the molar solubility of lead iodide, PbI2 , in water at 25oC .
PbI2
Pb2+
2I10
0
molar solubility
Practice Page 745 #8 and 9
Solubility Rules For Ionic Compounds
Soluble Compounds
1.
All compounds of the alkali metals ( Group 1A ) are soluble
2.
All salts containing NH4+ , NO3-, ClO4- , ClO3- and C2H3O2- are soluble . (except AgC2H3O2)
3.
All chlorides, bromides , and iodides (Cl- , Br- , I- ) are soluble. except with Ag+ , Pb2+ , Hg22+ )
4.
All sulphates (SO42- ) are soluble except those of Pb2+ , Ca2+ , Sr2+ , Hg22+ , and Ba2+ .
Insoluble Compounds
5.
All hydroxides ( OH- ) and all metal oxides ( O2- ) are insoluble except those of Group IA and of Ca 2+ ,
Sr2+ , Ba2+
Note: When metal oxides do dissolve, they react with water to form hydroxides. The oxide ion O 2- )
does not exist in water. Na2O + H2O ---> 2NaOH(aq)
6.
All compounds that contain PO43- , CO32- , SO32- , and S2- are insoluble, Except those of Group 1A and
NH4+ .
Additional Rule
All common hydrogen compounds are soluble
I = insoluble
Anions
/
SS = slightly soluble
Solubility Rules
S = soluble
D = decomposes N = not isolated
C2H3O2
-
Br-
CO32-
Cl-
CrO42
-
OH-
I-
NO3-
PO43-
SO42-
S2-
Al3+
SS
S
N
S
N
I
S
S
I
S
D
NH4+
S
S
S
S
S
S
S
S
S
S
S
Ba2+
S
S
I
S
I
S
S
S
S
I
D
Ca2+
S
S
I
S
S
SS
S
S
I
SS
D
Cu2+
S
S
I
S
I
I
D
S
I
S
I
Fe2+
S
S
I
S
N
I
S
S
I
S
I
Fe3+
S
S
N
S
I
I
N
S
I
SS
D
Pb2+
S
SS
I
SS
I
I
SS
S
I
I
I
Mg2+
S
S
I
S
S
I
S
S
I
S
D
Hg+
SS
I
I
I
SS
N
I
S
I
SS
I
Hg2+
S
S
I
S
SS
I
I
S
I
D
I
K+
S
S
S
S
S
S
S
S
S
S
S
Ag+
SS
I
I
I
SS
N
I
S
I
SS
I
Na+
S
S
S
S
S
S
S
S
S
S
S
Zn2+
S
I
I
S
S
I
S
S
I
S
I
Cation
Dissolving: Energy Concerns
Energy must be added to overcome the attractive forces holding the ions together in a crystal
( solute ). The magnitude of this Lattice Energy is reflected in properties such as melting
point , boiling point and heat of formation.
Energy needs to be added to separate the particles of the liquid ( solvent ) from one another.
The liquids are either polar covalent molecular ( dipole-dipole attraction ) or non-polar
covalent molecular (London Dispersion Forces )
Energy is released as the ions and the solvent molecules interact ( bond ) . This “Solvation
Energy” is greater for smaller more highly charged ions and more polar solvents.
Consider the following Data
Substance
M.P. oC
B.P. oC
NaF
NaCl
NaBr
NaI
980
801
755
651
1700
1413
1390
1300
H formation
Heat of Solution
-136.3
-98.4
-86.3
-69.5
-0.48 (12 oC)
-1.28 (18 oC)
-0.19 (18 oC)
+1.41 (18 oC)
Solubility
g/100 g H2O
4.22 (18 oC )
35.7 (0 oC )
79.5
Questions
1.
From considerations of ionic radii and Coulomb’s Law, which would you expect to
have the greater Lattice Energy, NaF or NaCl? Is your answer supported by the
values for their melting points?
2.
Given the melting point of CaO as 2580 oC, arrange the compounds CaO , SrO , MgO
and BaO in the expected order of decreasing melting point.
3.
Would you expect NaCl or NaF to have the greater Solvation energy when interacting
with water? Explain your answer.
4.
For the equation:
Lattice energy
Hsolution =
(energy put in to pull
ions apart)
+ Energy put in to
separate solvent
molecules
+Solvation Energy
(energy released when
ions and solvent
molecules interact
Consider your answers to questions 1 and 3 for NaCl and NaF. Which factor appears
to predominate in explaining their relative heats of solution?
5.
A) Define enthalpy and entropy
B) Considering the tendencies toward minimum energy and maximum randomness,
explain the fact that, NaCl is more soluble than NaF.
6.
What would be the effect on the solubility of NaCl if CCl4 were used as the solvent
rather than water? Explain your answer.
7.
Predict a value for the solubility of NaI in water.