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Acid-Base Properties of Salt Solutions

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					(8.3)
Acid-Base Properties of Salt
Solutions
pH Review
 Recall
      Acidic    [H3O+] > [OH-]

      Basic     [H3O+] < [OH-]

      Neutral   [H3O+] = [OH-]
Demo - pH Review
  a) NaCl   (s)   à Na+    (aq)   + Cl-(aq) Neutral

  b) NaOH      (s)   à Na+ (aq) + OH-(aq) Basic

  c)   NH4Cl   (s)   à NH4+ (aq) + Cl-(aq) Acidic


Why? This is because of salt hydrolysis
Salt Hydrolysis
 When added to water, salts dissociate
  into ions
     If these ions react with H2O by
  taking a proton or by giving a proton
  then the pH of the solution will be
  affected.

These ions are said to hydrolyze
Salt Hydrolysis

There are 7 different ways to classify/predict
  the affect this dissociation of ions will have
  on the pH of the solution.


We must consider both ions from the salt in
  order to determine the effect on pH of an
  aqueous solution.
1. Neutral Salt Solution
Ex. NaCl   (s)   à Na+ (aq) + Cl- (aq)
  Na+
    - cannot accept H+ from H2O
    - no H+ to donate to form H3O+
    -
      therefore won’t hydrolyze H2O
  Cl-
    - no H+ to donate
    - possibly could accept H+

  Cl-(aq) + H20    (l)   Û HCl   (aq)   + OH-(aq)
        BUT lets consider the conjugate pairs
Neutral Salt Solution
  Cl-(aq) + H20   (l)   Û HCl   (aq)   + OH-(aq)

Recall: The stronger an acid, the weaker its
  conjugate base

Cl- could accept a proton, but HCl is a strong
   acid, therefore the H+ and Cl- ions will not
   come together.

Therefore, the ions do not upset balance of
  [H3O+] or [OH-] in H2O so solution is
  neutral, pH=7
2. Basic Salt Solutions
Ex. Na2CO3(s) Û 2 Na+(aq) + CO32-(aq)

  Na+
  - no H+ to donate to H2O to form H3O+
  -
    cannot accept H+
  -
    therefore will not hydrolyze H2O
  CO32-
    - no H+ to donate to H2O to form H3O+
     -
         possibly could accept H+

CO32- (aq) + 2 H2O   (l)   Û H2CO3(aq) + 2 OH-(aq)
Basic Salt Solutions
CO32-(aq) +2H2O(l) Û H2CO3(aq)+ 2OH-
  (aq)


If we consider the conjugate pairs, this is a
   weak base.

Anions whose conjugate acids are weak will
  accept H+ from H2O creating OH-

Therefore it’s a BASIC solution.
3. Acidic Salt Solutions
Ex. NH4Cl(s) à NH4+ (aq) + Cl- (aq)
  Cl-
     - no H+ to donate
     - possibly could accept H+ (forming HCl),
     but HCl is a SA (dissociates completely) so
     won’t occur.
  NH4+
    - won’t accept H+ (positives repel)
    - possibly could donate H+ to H2O

NH4+ (aq) + H2O   (l)   Û NH3 (aq) + H3O+ (aq)
Acidic Salt Solutions
NH4+ (aq) + H2O   (l)   Û NH3 (aq) + H3O+ (aq)

NH3 is the conjugate base and is weak.
  Therefore it won’t readily accept H+

Cations whose conjugate bases are weak
  will donate H+ to H2O creating H3O+

Therefore it’s an ACIDIC solution.
  4. If Both Salt Ions Hydrolyze

Ex. NH4CH3COO      (s)   à NH4+ (aq) + CH3COO-(aq)

Acid
NH4+ (aq) + H2O   (l)   Û NH3 (aq) + H3O+ (aq)
   (Ka)


Basic
CH3COO-(aq) + H2O(l) Û CH3COOH          (aq)   + OH-(aq)
  (Kb)
If Both Salt Ions Hydrolyze
 pH of the solution will depend on the
  balance between the H3O+ and OH- created

 Therefore, we use our Ka and Kb appendix.

          Ka>Kb = acidic
          Ka<Kb = basic

  The higher value determines if the solution
  is acidic or basic
5. Metal Cations
Ex. Al3+(aq) (Pg. 582, table 3)
   - has a high charge density (large charge in
   small volume)
   - produce H+ ions indirectly
   - is hydrated in water

          3+
     Al        (aq)   + 6H20   (l)   Û Al(H2O)6 3+(aq)

  - high charge of Al 3+ increases polarity
  in –OH bond in the H2O molecules
  hydrating it
  Metal Cations
      3+
 Al        (aq)   + 6H20   (l)   Û Al(H2O)6 3+(aq)

  - therefore H in H2O hydrates becomes even
  more positive
  - so is more easily pulled off by solvent H2O
  molecules creating H3O+

Al(H2O)6 3+(aq)+ H2O(l) Û H3O+(aq)+Al(H2O)5OH
   2+
      (aq)

  - experiments show only one of the 6 waters will
  donate H+
Homework
 Read pg 581-594
 Do pg 588 #1-5

 Test Chapter 8 in 5 days (see review)
(8.3)
Acid-Base Properties of Salt
Solutions
6. Hydrolysis of Amphoteric Ions
Ex. NaHCO3 (s) à Na+ (aq) + HCO3-(aq)

Na+   - cannot donate or accept H+
HCO3- - possibly could donate or accept H+

HCO3-(aq) + H2O   (l)   Û CO32-(aq) + H3O+ (aq)
HCO3-(aq) + H2O   (l)   Û H2CO3 (aq) + OH-(aq)

Again, we refer to the Ka and Kb values!

Kb>Ka therefore solution will be basic
7. Metal and Nonmetal Oxides
Metal oxides react with water to produce basic
  solutions

Ex. CaO(s) + H2O(l) ® Ca2+       (aq)   + 2OH- (aq)


Nonmetal oxides react with water to produce
  acidic solutions

Ex.CO2(g) +2H2O   (l)   Û HCO3- (aq) + H3O+ (aq)
Calculating pH of Salt Solutions
Ex 1. Calculate the pH of a 0.20mol/L
  NH4Cl solution.

Ans. 4.97
Calculating pH of Salt Solutions
Ex 2. A 0.100mol/L NaCN solution has a
   pH of 11.15.

a) Calculate the Kb of CN-
Ans: 2.0x10-5

b) What is the Ka of its conjugate acid?
Ans: 5.0x10 -10

				
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