(8.3) Acid-Base Properties of Salt Solutions

pH Review  Recall Acidic [H 3 O + ] > [OH - ] Basic [H 3 O + ] < [OH - ] Neutral [H 3 O + ] = [OH - ]

Demo - pH Review a)NaCl (s)  Na + (aq) + Cl - (aq) Neutral a)NaOH (s)  Na + (aq) + OH - (aq) Basic c) NH 4 Cl (s)  NH 4 + (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 H 2 O 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)  Na + (aq) + Cl - (aq) Na + - cannot accept H + from H 2 O - no H + to donate to form H 3 O + - therefore won’t hydrolyze H 2 O Cl - - no H + to donate - possibly could accept H + Cl - (aq) + H 2 0 (l)  HCl (aq) + OH - (aq) BUT lets consider the conjugate pairs

Neutral Salt Solution Cl - (aq) + H 2 0 (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 [H 3 O + ] or [OH - ] in H 2 O so solution is neutral, pH=7

2. Basic Salt Solutions Ex. Na 2 CO 3(s)  2 Na + (aq) + CO 3 2- (aq) Na + - no H + to donate to H 2 O to form H 3 O + - cannot accept H + - therefore will not hydrolyze H 2 O CO no H + to donate to H 2 O to form H 3 O + - possibly could accept H + CO 3 2- (aq) + 2 H 2 O (l)  H 2 CO 3(aq) + 2 OH - (aq)

Basic Salt Solutions CO 3 2- (aq) +2H 2 O (l)  H 2 CO 3(aq) + 2OH - (aq) If we consider the conjugate pairs, this is a weak base. Anions whose conjugate acids are weak will accept H + from H 2 O creating OH - Therefore it’s a BASIC solution.

3. Acidic Salt Solutions Ex. NH 4 Cl (s)  NH 4 + (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. NH won’t accept H + (positives repel) - possibly could donate H + to H 2 O NH 4 + (aq) + H 2 O (l)  NH 3 (aq) + H 3 O + (aq)

Acidic Salt Solutions NH 4 + (aq) + H 2 O (l)  NH 3 (aq) + H 3 O + (aq) NH 3 is the conjugate base and is weak. Therefore it won’t readily accept H + Cations whose conjugate bases are weak will donate H + to H 2 O creating H 3 O + Therefore it’s an ACIDIC solution.

4. If Both Salt Ions Hydrolyze Ex. NH 4 CH 3 COO (s)  NH 4 + (aq) + CH 3 COO - (aq) Acid NH 4 + (aq) + H 2 O (l)  NH 3 (aq) + H 3 O + (aq) (Ka) Basic CH 3 COO - (aq) + H 2 O (l)  CH 3 COOH (aq) + OH - (aq) (Kb)

If Both Salt Ions Hydrolyze  pH of the solution will depend on the balance between the H 3 O + 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. Al 3+ (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) + 6H 2 0 (l)  Al(H 2 O) 6 3+ (aq) - high charge of Al 3+ increases polarity in –OH bond in the H 2 O molecules hydrating it

Metal Cations Al 3+ (aq) + 6H 2 0 (l)  Al(H 2 O) 6 3+ (aq) - therefore H in H 2 O hydrates becomes even more positive - so is more easily pulled off by solvent H 2 O molecules creating H 3 O + Al(H 2 O) 6 3+ (aq) + H 2 O (l)  H 3 O + (aq) +Al(H 2 O) 5 OH 2+ (aq) - experiments show only one of the 6 waters will donate H +

Homework  Read pg  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. NaHCO 3 (s)  Na + (aq) + HCO 3 - (aq) Na + - cannot donate or accept H + HCO possibly could donate or accept H + HCO 3 - (aq) + H 2 O (l)  CO 3 2- (aq) + H 3 O + (aq) HCO 3 - (aq) + H 2 O (l)  H 2 CO 3 (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) + H 2 O(l)  Ca 2+ (aq) + 2OH - (aq) Nonmetal oxides react with water to produce acidic solutions Ex.CO 2(g) +2H 2 O (l)  HCO 3 - (aq) + H 3 O + (aq)

Calculating pH of Salt Solutions Ex 1. Calculate the pH of a 0.20mol/L NH 4 Cl solution. Ans. 4.97

Calculating pH of Salt Solutions Ex 2. A 0.100mol/L NaCN solution has a pH of a)Calculate the Kb of CN - Ans: 2.0x10 -5 b) What is the Ka of its conjugate acid? Ans: 5.0x10 -10