Salt Solubility
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Transcript Salt Solubility
Salt Solubility
Chapter 18
1
Solubility product constant
• Ksp
• Unitless
• CaF2(s) Ca2+(aq) + 2F-(aq)
• Ksp = [Ca2+][F-]2
• [Ca2+] = molar solubility (in M)
• Get Ksp values from Appendix J, pages A24-25
2
We have problems
• Ksp for BaSO4 = 1.1 x 10-10 @ 25°C
– Calculate this salt’s solubility in water in g/L.
3
Solution
BaSO4( s ) Ba 2 ( aq ) SO4 2 ( aq )
I
C
E
0
+x
x
K sp 1.1 10
10
0
+x
x
2
2
[ Ba ][ SO4 ] x
2
x 1.0 105 M
g
g
m
ol
1.0 10
233
0.0024
L
mol
L
5
4
Problem
• If [Ca2+] = 2.4 x 10-4 M, what is the Ksp
value of CaF2?
5
Solution
CaF2(s) Ca
I
C
E
----
2+
-
(aq)
+ 2F (aq)
0
+x
x
0
+2x
2x
(2.4 10 )
-4
(2 x 2.4 10 )
-4
K sp = [Ca 2+ ][F- ]2 [2.4 10-4 ][4.8 10-4 ]2 5.5 1011
6
Solubility and the Common Ion
Effect
• Problem: How much AgCl (g/L) would
dissolve in water given Ksp= 1.8 x 10-10?
7
AgCl( s ) Ag
I
C
E
( aq )
Cl
0
+x
x
K sp =1.8 10
-10
( aq )
0
+x
x
[ Ag ][Cl ] x
2
x 1.3 10 M
-5
g
1.3 10 M 143
-5
mol
g
0.0019
L
8
Solubility and the Common Ion
Effect
• If solid AgCl is placed in 1.00L of 0.55M
NaCl, what mass of AgCl (g/L) would
dissolve?
• Before solving this, would you expect it to
be lesser or greater than in pure water?
– Think back to Le Châtelier’s Principle and the
pH of buffers
9
AgCl( s ) Ag
I
C
E
( aq )
Cl
0
+x
x
K sp =1.8 10
-10
( aq )
0.55
+x
x+0.55
[ Ag ][Cl ] [ x][ x 0.55] [ x][0.55]
(We can approximate since K is very small.)
x = 3.3 10 M [ Ag ]
g
-10
-8 g
3.3 10 M 143
4.7 10
mol
L
-10
10
Basic anions and salt solubility
• Do we recall what a conjugate acid/conjugate
•
base is?
If salt has conjugate base of weak acid, then
salt more soluble than value given by Ksp
– Why?
PbS(s) Pb2+(aq) + S2-(aq)
S2-(aq) + H2O(l) HS-(aq) + OH-(aq);
Where net = Ksp Kb (>Ksp)
11
Effect of pH on solubility
• Mg(OH)2 (s) Mg2+(aq) + 2OH-(aq)
• If one adds base, OH-, which way will the
equilibrium shift?
– Will more solid form or dissolve?
• If one adds acid, H3O+, which way will the
equilibrium shift?
– Will more solid form or dissolve?
12
Effect of pH on solubility
• Mg3(PO4)2 (s) 3Mg2+(aq) + 2PO4-3(aq)
• If one adds base, OH-, which way will the
equilibrium shift?
– Will more solid form or dissolve?
• If one adds acid, H3O+, which way will the
equilibrium shift?
– Will more solid form or dissolve?
13
Effect of pH on solubility
• MgCl2 (s) Mg2+(aq) + 2Cl-(aq)
• If one adds base, OH-, which way will the
equilibrium shift?
– Will more solid form or dissolve?
• If one adds acid, H3O+, which way will the
equilibrium shift?
– Will more solid form or dissolve?
14
So what can we say about the
conjugate base and solubility?
15
The reaction quotient, Q
• Q tells us:
– Whether it’s at equilibrium
• If not, which way it’ll shift
• CaF2(s) Ca2+(aq)+ 2F-(aq)
• Ksp = [Ca2+][F-]2
• Q = [Ca2+][F-]2
• If Q = Ksp, then @ eq. &
•
•
soln is saturated
If Q < Ksp, then will shift
to right (dissolve
more) & soln is
unsaturated
If Q > Ksp, then will shift
to left (not dissolve
anymore, ppt out) &
soln is supersaturated
(will ppt out)
16
Check out the heating pads & video
• http://genchem.chem.wisc.edu/demonstra
tions/Gen_Chem_Pages/11solutionspage/c
rystallization_from_super.htm
17
Problem
• AgCl placed in water. After a certain
amount of time, concentration= 1.2 x 10-5
M.
• Has the system reached eq.?
• If not, will more solid dissolve?
• How much more?
• (Ksp = 1.8 x 10-10)
18
Q [ Ag ][Cl ] [1.2 105 ][1.2 105 ] 1.4 1010
1.8 1010 1.4 1010
K sp Q
Thus, the system is not yet at equilibrium and will proceed to the right,
dissolving more solid.
At equilibrium, 1.3 105 mol of each ion are present.
Therefore, 1.3 105 mol 1.2 105 mol 0.110 5 mol moles required to dissolve to reach eq.
0.1105 mol 143 g
1104 g of additional AgCl will dissolve
mol
19
Selective precipitation
• Will a precipitate form, given the ion
concentrations?
• What concentrations of ions are needed to
bring about precipitation?
• Basically, compare Q to Ksp
20
Problem
• If [Mg2+] = 1.5x10-6 M, and enough
hydroxide ions are added to make the
solution 1.0 x 10-4 M of hydroxide ions, will
Mg(OH)2 precipitate?
• If not, will it occur if the concentration of
hydroxide ions is increased to 1.0 x 10-2
M?
• Ksp = 5.6 x 10-12
21
Mg(OH) 2(s) Mg
2+
-
(aq)
+2OH (aq)
Q [1.5 106 ][1.0 104 ]2 1.5 1014
Q(1.5 1014 ) K sp (5.6 1012 )
Unsaturated solution and precipitation doesn't occur yet.
When [OH - ]=1.0 10-2 M :
Q [1.5 106 ][1.0 10-2 ]2 1.5 1010
Q(1.5 1010 ) K sp (5.6 1012 )
Precipitation will occur.
22
Complex ion equilibria
• Complex ion = a central
•
•
metal ion (e- acceptor =
Lewis acid) bound to 1 or
more ligands
AgCl(s) + 2NH3(aq) [Ag(NH3 )2 ]+(aq) Cl-(aq)
Ligand = neutral
molecule or ion that acts
as Lewis base (e- donor)
with the central metal ion
Kf values on page A-26,
Appendix K
23
Example
• What is Knet for the reaction below?
AgCl(s) + 2NH3(aq) [Ag(NH3 )2 ]
+
Cl (aq)
-
(aq)
24
AgCl(s) + 2NH 3(aq) [Ag(NH 3 ) 2 ]+ (aq) Cl -(aq)
AgCl(s) Ag + (aq) Cl-(aq) ; K sp 1.8 10-10
Ag + (aq) 2NH 3(aq) [Ag(NH 3 ) 2 ]+ (aq) ; K f = 1.6 107
K net
[Ag(NH 3 ) 2 ]+ [Cl- ]
[Ag(NH 3 ) 2 ]+
+
[Ag ][Cl ]
K sp K f (1.8 10-10 ) (1.6 107 ) 2.9 10 3
2
+
2
[NH 3 ]
[Ag ][NH 3 ]
25
Problem
• What is the net rxn and value of Knet for
dissolving AgBr in a solution containing
S2O32-?
• Ksp = 5.4 x 10-13
• Kf = 2.0 x 1013
26
AgBr(s) Ag + (aq) Br - (aq) ; K sp 5.4 10-13
Ag
+
(aq)
2S2O3
2-
[Ag(S2O3 ) 2 ]
3-
(aq)
; K f = 2.0 10
13
(aq)
net rxn: AgBr(s) + 2S2O32-(aq) [Ag(S2O3 ) 2 ]3-(aq) Br -(aq)
K net K sp K f (5.4 10-13 ) (2.0 1013 ) 10.
27