Solubility & Solubility & SOLUBILITY SOLUBILITY PRODUCT CONSTANTSPRODUCT CONSTANTS

Solubility Rules

All Group 1 (alkali metals) and NH4+

compounds are water soluble.All nitrate, acetate, and chlorate compounds are water soluble.All Br-, Cl-, I- (except mercury (I), Pb+2, and Ag+) compounds are water soluble.All sulfate (except Pb+2, Ba+2, Sr+2; Ag+ and Ca+2 are sparingly soluble) compounds are water soluble.

Solubility Rules

All oxides, hydroxides, phosphates, carbonates, and sulfides form water INSOLUBLE compounds except Group 1 (alkali metals) and ammonium compounds.

Solubility Products

Copper (I) bromide has a solubility of 0.02869 g/L at 25°C. What is the equilibrium constant for this reaction?The equation for the solubility of this compound is written:

CuBr(s) Cu + Br(aq)

(aq)

Solubility Products

The equilibrium constant is written:

Ksp = ][Br ][Cu -

Solubility Products

Convert mass to moles:

0.02869 g ( ) = 2 x 10-4 moles

[Cu+] = 2 x 10-4 moles/L

[Br-] = 2 x 10-4 moles/L

g 143.45mole 1

Solubility Products

Calculate the Ksp

Ksp = (2 x 10-4 moles/L) (2 x 10-4 moles/L)

Ksp = 4 x 10-8

SOLUBILITY PRODUCT CONSTANTSSOLUBILITY PRODUCT CONSTANTS

Consider the following reaction

The equilibrium constant expression is

Ksp = [Pb2+][Cl-]2

Ksp is called the solubility product constant or

simply solubility product

For a compound of general formula, MyXz

PbCl2(s) Pb2+(aq) + 2Cl- (aq)

Ksp = [Mz+]y[Xy-]z

Ksp = [Mg2+][NH4+][PO4

3-]

Ksp = [Zn2+][OH-]2

Ksp = [Ca2+]3[PO43-]2

MyXz(s) yMz+(aq) + zXy- (aq)

Ca3(PO4)2(s) 3Ca2+(aq) + 2PO43- (aq)

Zn(OH)2(s) Zn2+(aq) + 2OH- (aq)

MgNH4PO4(s) Mg2+(aq) + NH4+(aq) + PO4

3- (aq)

Molar solubility: the number of moles that

dissolve to give 1 liter of saturated solution

As with any equilibrium constant the numerical

value must be determined from experiment

The Ksp expression is useful because it applies

to all saturated solutions

- the origins of the ions are not relevant

Consider that Ksp BaSO4 = 1.1 x 10-10

FACTORS AFFECTING SOLUBILITY

The common ion effect

This effect affects solubility equilibria as it

does other ionic equilibria

The solubility of a compound is less in a solution

that contains a common ion than in pure water

Consider again the reaction

Adding Pb2+ or Cl- precipitates PbCl2

PbCl2(s) Pb2+(aq) + 2Cl-(aq)

Can be explained using LeChatelier’s principle

Likewise adding PbCl2 to aqueous NaCl results

in [Pb2+][Cl-]2 > Ksp

So for instance MgF2 is less soluble in 0.10 M

NaF than in pure water by a factor of 35000

pH of the solution

Solubility of some solids in water may be

increased by changing the pH

e.g. Al(OH)3 is more soluble in acidic solutions

than in pure water

Salts that contain basic anions such as CN-, F-,

and PO43- will react in this fashion

Salts that contains anions of strong acids do

not show this reactivity

Al(OH)3(s) Al3+(aq) + 3OH- (aq)

3H+(aq) + 3OH- (aq) 3H2O(l)

Al(OH)3(s) + 3H+(aq) Al3+(aq) + 3H2O(l)

Complex ion formation

Solubility of an ionic compound increases in a

solution that contains a coordinating Lewis

base

Typical examples are NH3, CN-, OH- and Cl-

Cu(OH)2(s) Cu2+(aq) + 2OH- (aq)

Cu2+(aq) + 4NH3(aq) [Cu(NH3)4]2+(aq)

Cu(OH)2(s) + 4NH3(aq)

[Cu(NH3)4]2+(aq) + 2OH- (aq)

Similarly,

The reaction decrease the metal ion

concentration so that [M2+][OH-]2 < Ksp and

the M(OH)2 dissolves

Zn(OH)2(s) + 4NH3(aq)

[Zn(NH3)4]2- (aq) + 2OH- (aq)

THE REACTION QUOTIENTTHE REACTION QUOTIENT

The reaction quotient (called ion product) may

be applied to solubility equilibria - determines

if a substance will precipitate from solution

Qsp < Ksp Forward process occurs

No precipitation occurs

Qsp = Ksp Solution is just saturated

Qsp > Ksp Reverse process occurs

Precipitation occurs

SELECTIVE PRECIPITATION

This is a method which allows separation of

metal ions or anions based on solubility

e.g. Cl-, Br-, and I- can be separated by

selective precipitation with silver ions

Ksp AgCl = 1.8 x 10-10

AgBr = 3.3 x 10-13

AgI = 1.5 x 10-16