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Acid-Base Equilibria and Solubility Equilibria

Chapter 16 1

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 Consider mixture of salt NaA and weak acid HA. NaA (s) Na + (aq) + A - (aq) [H + ][A - ] HA (aq) H + (aq) + A - (aq) K a = [HA] [H + ] = K a [HA] [A - ] -log [H + ] = -log K a - log [HA] [A - ] [A - ] pH = pK a + log [conjugate base] [acid] -log [H + ] = -log K a + log [HA] pH = pK a + log [HA] [A ] - pK a = -log K a 3 HCl H + + Cl - HCl + CH 3 COO - CH 3 COOH + Cl - 5

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Common ion effect:

The presence of a common ion of a weak acid or a weak base.

the ionization Consider mixture of CH 3 COONa (strong electrolyte) and
CH 3 COOH (weak acid).
CH 3 COONa (s)
Na + (aq) + CH 3 COO - (aq)
CH 3 COOH (aq)
H + (aq) + CH 3 COO - (aq)
2
 Buffer solution is a soln of: 1. 2. A buffer soln has the ability to resist changes in pH upon the addition of small amounts of either acid or base. Consider an equal molar mixture of CH 3 COOH and CH 3 COONa Add strong acid H + (aq) + CH 3 COO - (aq) CH 3 COOH (aq) Add strong base OH - (aq) + CH 3 COOH (aq) CH 3 COO - (aq) + H 2 O (l) 4 Titrations Titration: Equivalence point – Indicator – Slowly add base to unknown acid UNTIL The indicator changes color ( ) 6
 Alternative Method of Equivalence Point Detection monitor pH 7 Weak Acid-Strong Base Titrations CH 3 COOH (aq) + NaOH (aq) CH 3 COONa (aq) + H 2 O (l) CH 3 COOH (aq) + OH - (aq) CH 3 COO - (aq) + H 2 O (l) At equivalence point (pH > 7): CH 3 COO - (aq) + H 2 O (l) OH - (aq) + CH 3 COOH (aq) 9 Acid-Base Indicators HIn (aq) H + (aq) + In - (aq) [HIn] [In - ]  10 Color of acid (HIn) predominates [HIn] [In - ]  10 Color of conjugate base (In - ) predominates 11

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Strong Acid-Strong Base Titrations

NaOH (aq) + HCl (aq)

H 2 O (l) + NaCl (aq)

OH - (aq) + H + (aq)

H 2 O (l) 8

Strong Acid-Weak Base Titrations

 HCl (aq) + NH 3 (aq) H + (aq) + NH 3 (aq) NH 4 Cl (aq) NH 4 Cl (aq)

At equivalence point (pH < 7):

NH 4 + (aq) + H 2 O (l)

NH 3 (aq) + H + (aq) 10

Solutions of Red Cabbage Extract pH 12

The titration curve of a strong acid with a strong base. 13 Solubility Equilibria
AgCl (s)
Ag + (aq) + Cl - (aq)
K sp = [Ag + ][Cl - ]
K sp is the solubility product constant
MgF 2 (s)
Mg 2+ (aq) + 2F - (aq)
2Ag + (aq) + CO 3 2- (aq)
K sp = [Mg 2+ ][F - ] 2
Ag 2 CO 3 (s)
K sp = [Ag + ] 2 [CO 3 2- ]
Ca 3 (PO 4 ) 2 (s)
3Ca 2+ (aq) + 2PO 4 3- (aq)
K sp = [Ca 2+ ] 3 [PO 4 3- ] 2
Dissolution of an ionic solid in aqueous solution:
Q
< K sp
Q
= K sp
Q
> K sp
15

Molar solubility (mol/L):

Solubility (g/L): 17

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Which indicator(s) would you use for a titration of HNO 2 with KOH ? 14 16 18

The Common Ion Effect and Solubility

What is the molar solubility of AgBr in (a) pure water and (b) 0.0010 M NaBr? Complex Ion Equilibria and Solubility
A complex ion
2-
Co 2+ (aq) + 4Cl - (aq)
CoCl 4
(aq)
The formation constant or stability constant (K f ):
2-
[CoCl 4 ]
Co(H 2 O) 6 2+
CoCl
4
stability of
K f
complex
HCl
21

2-

K f =

[Co 2+ ][Cl - ] 4 23

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 pH and Solubility • The presence of a common ion decreases the solubility. • Insoluble bases dissolve in acidic solutions • Insoluble acids dissolve in basic solutions add remove Mg(OH) 2 (s) Mg 2+ (aq) + 2OH - (aq) At pH less than 10.45 K sp = [Mg 2+ ][OH - ] 2 = 1.2 x 10 -11 Lower [OH - ] K sp = (s)(2s) 2 = 4s 3 4s 3 = 1.2 x 10 -11 OH - (aq) + H + (aq) H 2 O (l) Increase solubility of Mg(OH) 2 s = 1.4 x 10 -4 M At pH greater than 10.45 [OH - ] = 2s = 2.8 x 10 -4 M pOH = 3.55 pH = 10.45 Raise [OH - ] Decrease solubility of Mg(OH) 2 20 Effect of Complexation on Solubility AgNO Add NH 3 3 + NaCl AgCl Ag(NH 3 ) 2 + 22 24

Qualitative

Analysis of

Cations 25

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Flame Test for Cations lithium sodium potassium copper

26
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