(8.3)Acid-Base Properties of Salt Solutions

pH Review

RecallAcidic [H3O+] > [OH-]

Basic [H3O+] < [OH-]

Neutral [H3O+] = [OH-]

Demo - pH Review

a) NaCl (s) Na+ (aq) + Cl-(aq) Neutral

a) NaOH (s) Na+ (aq) + OH-

(aq) Basic

c) NH4Cl (s) NH4+

(aq) + Cl-(aq) Acidic

Why? This is because of salt hydrolysis

Salt Hydrolysis When added to water, salts dissociate

into ionsIf 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 SolutionEx. NaCl (s) 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) HCl (aq) + OH-(aq)

BUT lets consider the conjugate pairs

Neutral Salt Solution

Cl-(aq) + H20 (l) 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) 2 Na+(aq) + CO3

2-(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) H2CO3(aq) + 2 OH-(aq)

Basic Salt Solutions

CO32-

(aq) +2H2O(l) 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) 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) NH3 (aq) + H3O+ (aq)

Acidic Salt Solutions

NH4+

(aq) + H2O (l) 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) NH4+

(aq) + CH3COO-(aq)

Acid NH4

+ (aq) + H2O (l) NH3 (aq) + H3O+

(aq) (Ka)

Basic CH3COO-

(aq) + H2O(l) 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 = acidicKa<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

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

(aq)

- high charge of Al 3+ increases polarity in –OH bond in the H2O molecules hydrating it

Metal Cations

Al 3+ (aq) + 6H20 (l) 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) 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) Na+ (aq) + HCO3

-(aq)

Na+ - cannot donate or accept H+

HCO3- - possibly could donate or accept H+

HCO3-(aq) + H2O (l) CO3

2-(aq) + H3O+

(aq)

HCO3-(aq) + H2O (l) 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) 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