1. Chemistry 1011 Slot 51 Chemistry 1011 TOPIC Acids and Bases TEXT REFERENCE Masterton and Hurley Chapter 4.2 (Review), 13, 14.1, 15.1 (page 427), 21.2 (page589)

2. Chemistry 1011 Slot 52 13.6 Acid-Base Properties of Salt Solutions YOU ARE EXPECTED TO BE ABLE TO: Account for the acidity, basicity or neutrality of salts in terms of the hydrolysis of cations and anions Identify and apply the relationship between K a and K b (This section will include material contained in 13.4 and 13.5 related to the acid-base properties of cations and anions)

3. Chemistry 1011 Slot 53 Hydrolysis of Salts All salts are strong electrolytes – this means that they are fully ionized in dilute aqueous solution NaCl (s) Na + (aq) + Cl  (aq) CH 3 COONa (s) CH 3 COO  (aq) + Na + (aq) NH 4 Cl (s) NH 4 + (aq) + Cl  (aq) If the salts are formed from either weak acids or bases, then their ions may react with water (hydrolysis) –NaCl is the salt of a strong acid and strong base –CH 3 COONa is the salt of a weak acid and a strong base –NH 4 Cl is the salt of a strong acid and a weak base

4. Chemistry 1011 Slot 54 Hydrolysis of Salts CH 3 COONa is the salt of a weak acid (acetic acid) and a strong base (sodium hydroxide). The CH 3 COO  ions will react with water: CH 3 COO  (aq) + H 2 O (aq) CH 3 COOH (aq) + OH  (aq) The CH 3 COO  ion is a Bronsted-Lowry base. The solution will be basic. K b = [CH 3 COOH] x[OH  ] = 5.6 x 10  10 [CH 3 COO  ] Any anion derived from a weak acid will be a weak base

5. Chemistry 1011 Slot 55 Hydrolysis of Salts NH 4 Cl is the salt of a strong acid (hydrochloric acid) and a weak base (ammonia) The NH 4 + ions will react with water: NH 4 + (aq) + H 2 O (aq) NH 3(aq) + H 3 O  (aq) The NH 4 + ion is a Bronsted-Lowry acid. The solution will be acidic. K a = [NH 3 ] x[H 3 O  ] = 5.6 x 10  10 [NH 4 + ] Any cation derived from a weak base will be a weak acid

6. Chemistry 1011 Slot 56 Metal Cations as Weak Acids Excluding Groups 1 and 2 (alkali metals and alkaline earths), many metal cations are weak acids –A 0.10mol/L solution of aluminum sulfate has a pH of  3 –A 0.10mol/L solution of zinc nitrate has a pH of  5 These metal ions in solution are hydrated: –Al 3+ is [Al(H 2 O) 6 ] 3+ –Zn 2+ is [Zn (H 2 O) 4 ] 2+ The hydrated ions act as Bronsted-Lowry acids: Al(H 2 O) 6 3+ (aq) + H 2 O (aq) H 3 O + (aq) + Al(H 2 O) 5 (OH) 2+ ( aq) ca cb ca cb

7. Chemistry 1011 Slot 57 Anions as Weak Bases Base K b pK b PO 4 3  2.2 x 10  2 1.66 CO 3 2  2.1 x 10  4 3.68 HPO 4 2  1.6 x 10  7 6.80 The larger the value of K b, the stronger the acid PO 4 3  (aq) + H 2 O (aq) HPO 4 2  (aq) + OH  (aq) The PO 4 3  ion is a Bronsted-Lowry base. The solution will be basic. K b = [HPO 4 2  ] x[OH  ] = 2.2 x 10  2 [PO 4 3  ]

8. Chemistry 1011 Slot 58 Relation between K a and K b Ammonia in aqueous solution is basic: NH 3(aq) + H 2 O (aq) NH 4 + (aq) + OH  (aq) K b = [NH 4 + ] x[OH  ] = 1.8 x 10  5 [NH 3 ] The ammonium ion is its conjugate acid We can write an equation for NH 4 + acting as an acid as: NH 4 + (aq) + H 2 O (aq) NH 3(aq) + H 3 O  (aq) K a = [NH 3 ] x[H 3 O  ] = 5.6 x 10  10 [NH 4 + ]

9. Chemistry 1011 Slot 59 Relation between K a and K b Add the two equations: H 2 O (aq) + H 2 O (aq) H 3 O  (aq) + OH  (aq) The equilibrium constant of the resulting equation is the product of the two equilibrium constants: K w = [H 3 O  ] x [OH  ] = K a x K b = (5.6 x 10  10 ) x (1.8 x 10  5 ) = 1.0 x 10  14 This relationship can be used to determine K b for a weak acid from a measurement of K a for its conjugate acid, and vice versa

10. Chemistry 1011 Slot 510 Relative Strengths of Bronsted- Lowry Acids and Bases K a Conjugate AcidConjugate BaseK b very largeHCl (strong acid)Cl  very small H 3 O + H 2 O 6.9 x 10  4 HF F  1.4 x 10  11 1.8  x 10  5 CH 3 COOH CH 3 COO  5.6 x 10  10 5.6 x 10  10 NH 4 + NH 3 1.8  x 10  5 H 2 O OH  very smallOH  (strong base)O 2  very large

11. Chemistry 1011 Slot 511 Relative Strengths of Acids and Bases Bronsted-Lowry acids: –Strong acids - stronger proton donors than H 3 O + –Weak acids - weaker proton donors than H 3 O + but stronger than H 2 O –Species that do not form acidic aqueous solutions Bronsted-Lowry acids: –Strong bases - stronger proton acceptors than OH  –Weak bases - weaker proton acceptors than OH  but stronger than H 2 O –Species that do not form basic aqueous solutions

12. Chemistry 1011 Slot 512 Predicting Acid-Base Properties of Salts What will be the effect of the cation on the pH of water? Will the cation act as a weak acid or will it be a “spectator ion” –If this is a salt of a weak base, the cation will be a weak acid –If this is a salt of a strong base, the cation will be a “spectator ion” –If the cation is a hydrated metal ion, the solution will be a weak acid –If the cation is a group 1 or 2 metal ion, the cation will be a “spectator ion”

13. Chemistry 1011 Slot 513 Predicting Acid-Base Properties of Salts What will be the effect of the anion on the pH of water? Will the anion act as a weak base or will it be a “spectator ion” –If this is a salt of a weak acid, the anion will be a weak base –If this is a salt of a strong acid, the anion will be a “spectator ion”

14. Chemistry 1011 Slot 514 Salts Formed by Reacting a Weak Acid with a Weak Base To determine whether a salt of a weak acid and weak base will be basic or acidic, it is necessary to compare the values of Ka and Kb to determine relative strength –If Ka > Kb salt will be acidic –If Ka < Kb salt will be basic eg: NH 4 F –K a NH 4 + = 5.6 x 10  10 –K a F  = 1.4 x 10  11 –Solution has pH 6.2