Common Ion Effect

CH3COOH H+(aq) + CH3COO(aq)

pH of 0.1 M soln =

Add 0.1 M CH3COONa: CH3COONa Na+ + CH3COO(aq)

pH = What happened to[CH3COO]? [CH3COOH]?

Buffer Solutions

A buffer is a solution that “resists” a change in pH

E.g. blood contains substances that keep its pH fixed at 7.3 - important for life functions

Buffer solutions consist of either:

A weak acid + salt of its conjugate baseor

A weak base + salt of its conjugate acid

Buffers

HX(aq) H+(aq) + X(aq)

[ ][ ][ ] [ ][ ]

[ ]HXX

HHX

XHKa

−+

−+

==

[ ][ ]⎟⎟⎠

⎞⎜⎜⎝

⎛−=

−

HX

XpHpKa log

[ ][ ]⎟⎟⎠

⎞⎜⎜⎝

⎛+=

−

HXX

pKpH a log

Note: If [HX] = [X], pH = pKa

Buffers work best near pH = pKa

Henderson-Hasselbalch equation

Buffer Capacity

CH3COOH H+(aq) + CH3COO(aq)

[CH3COOH] = [CH3COO] = 1.0 M

1 L solution, pH = pKa = 4.74

1. Add a dropper (~20ml) of 1M HCl

2. Add a dropper (~20ml) of NaOH

Repeat calculation starting from a 1.8 x 10-5 M HCl solution (pH = 4.74)

Buffer Capacity

A CH3COOH + CH3COONa (both 1 M)

pH =

B dilute solution A 10x

pH =

Repeat with 1 M HCl + 1 M NaCl solution

SOLUBILITY

Solubility: quantity of a substance that dissolves to form a saturated solution

Solubility: g/LMolar solubility: mole/L

Some salts are very soluble (> 0.1 M). Recall solubility rules.

Some salts are sparingly soluble (< 0.1 M) sometimes referred to as ‘insoluble’.

Precipitation and Solubility of ionic salts and their equilibrium in water

MX(s) M+(aq) + X(aq)

SOLUBILITY EQUILIBRIA• Precipitation

Ag+(aq) + Cl(aq) AgCl(s) • Dissolution

AgCl(s) Ag+(aq) + Cl(aq) At equilibrium when forward rate = backward rate

AgCl(s) Ag+(aq) + Cl(aq)

Keq = [Ag+][Cl]/ [AgCl(s)]

Keq [AgCl(s)] = [Ag+][Cl]Ksp = [Ag+][Cl]

The concentration of solid does not change at equilibrium

Solubility Product: Ksp

AgCl 1.8 x 10-10

AgBr 5.0 x 10-13

AgI 8.3 x 10-17

CdS 8.0 x 10-27

ZnS 1.1 x 10-21

Mg(OH)2 1.8 x 10-11

Ca(OH)2 5.5 x 10-6

CaF2 3.9 x 10-11

BaCO3 5.1 x 10-9

BaSO4 1.1 x 10-10

SOLUBILITY

Ksp is constant for a given solid at a given temp.

SOLUBILITY CALCULATION

Calculate [Ca2+] and [F-] for a saturated CaF2 solution.

CaF2 (s) Ca+2 (aq) +2F (aq)

Ksp = [Ca2+][F]2 =3.9 x 1011 at 25oC

What is the solubility? solubility = amount of CaF2 dissociated

COMMON ION EFFECT

If Q = [Ag+][Cl-] > Ksp, AgCl precipitates (Ion product > solubility product)

If [Ag+][Cl-] < Ksp; AgCl dissolves

When [Ag+][Cl-] = Ksp, the solution is saturatedAdding either [Ag+] or [Cl-] will precipitate AgCl(s)

[Cl-]

[Ag+]

precipitation

dissolutionsaturation

What about CaF2 in 0.01 M NaF solution?

[F] = 0.01 M Ksp = 3.9 x 1011

CaF2(s) Ca2+(aq) + 2F(aq)

SOLUBILITY CALCULATIONCommon Ion Effect

Water Chemistry (Ch. 18.5-6)

Water in State College/UP Campus

Predominantly well water

23 wells + 1 open reservoir

[Ca2+ ] 165-185 ppm 10-15 ppm

Mixed to reduce water hardness

• Cl2 injected to kill bacteria

• F- added

• Alum (K2SO4.Al2(SO4)3

.24H2O) added to improve clarity

• NaOH added to neutralize pH

Remediation of Water

Ca2+ (hard water), Pb2+ (toxic) are precipitated by CO32-

Ksp

CaCO3 8.7 x 10-9

PbCO3 3.3 x 10-14

Which compound(s) could we use to supply CO32-?

How much do we need to add ?

If we add a stoichiometric amount of Na2CO3,

[Ca2+] = [CO32-] = Ksp

1/2

[Ca2+] = 3 x 10-3 M

2.9 x 10-3 mol/L x 40 g/mol x 1000 mg/g = 116 mg/L

(116 ppm)

If [CO32-] = 3 mM, what is [Pb2+]?

[Pb2+] = Ksp/[CO32-] = 3.3 x 10-14/3 x 10-3 = 1 x 10-11 M

= 2 parts per trillion

Effect of pH

What is the solubility of Mg(OH)2 in pure water?

Ksp = 1.8 x 10-11

What is the solubility of Mg(OH)2 in a solution with a pH of 9?

Effect of pH on common ions

If either the anion or the cation is involved in an acid base equilibrium, then it is a common ion problem.

•Basic metal hydroxides Low pH increases solubility

e.g., Mg(OH)2

•Salts of weakly basic anions

Low pH increases solubility

examples (write out equilibria for practice)

Mg(HCO3)2

ZnCO3

Ca3(PO4)2

NaF

Contrast with NaCl, Ca(NO3)2

There are amphoteric hydroxides of Al3+ Cr3+ Zn2+ Sn2+ and many transition metal ions

Al(OH)3 , Cr(OH)3 , Zn(OH)2 , Sn(OH)2 , …

Dissolution involves formation of complex ions:

Al(OH)3(OH2)3 (s)

+ H+ Al(OH)2(OH2)4+

+ OH Al(OH)4(OH2)2

Amphoteric hydroxides:

Both low and high pH increases solubility

AMPHOTERIC METAL HYDROXIDESAMPHOTERIC METAL HYDROXIDES

Hydration of metal ions

Cu2+(aq) + 4 H2O(l) [Cu(OH2)4]2+(aq)

Lewis + Lewis Lewis Acid/Base Acid Base Adduct

= Metal Complex

Other Lewis bases react with metal ions to form complexes

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

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

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

FORMATION OF COMPLEX IONS

METAL COMPLEX STABILITY

Cu(NH3)42+ + 4H2O Cu(OH2)4

2+ + 4NH3

Cu2+(aq)

Cu(OH2)42+ + 4NH3 Cu(NH3)4

2+ + 4H2O

[H2O] = constant

€

K = Cu(OH2 )4

2+[ ] NH3[ ]

4

Cu (NH3 )42+

[ ] H2O[ ]4

€

KF = Cu(NH3 )4

2+[ ]

Cu 2+[ ] NH3[ ]

4

€

KD = Cu2+[ ] NH3[ ]

4

Cu (NH 3 )42+

[ ]Dissociation constant

Formation constant

€

KF = 1

KD

Kf VALUES OF SOME COMPLEXES

Ag(NH3)2+ 2 x 107

Cu(NH3)42+ 5 x 1012

Cu(CN)42- 1 x 1025

Ag(CN)2- 1 x 1021

Ag(S2O3)23- 3 x 1013

What is the conc of free Cu2+ ions in a 1 L solution that contains 1 x10-3 moles total Cu2+ and is 0.1 M in NH3?

Cu2+(aq) + 4 NH3(aq) Cu(NH3)42+(aq)

Kf = 5 x 1012

Complex Ion Formation

Complex Ion Formation

What is the equilibrium constant for the following reaction?

CuCO3(s) + 4CN(aq) CO32(aq) + Cu(CN)4

2(aq)

Ksp CuCO3 = 2.3 x 1010

Kf Cu(CN)42 = 1 x 1025

CuCO3 is a sparingly soluble salt?

Ksp CuCO3 = 2.3 x 1010

How can I get it to dissolve?