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)

Chemistry 1011 Slot 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)

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

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

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

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

Chemistry 1011 Slot 57 Anions as Weak Bases Base K b pK b PO 4 3  2.2 x 10  CO 3 2  2.1 x 10  HPO 4 2  1.6 x 10  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  ]

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 + ]

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

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   x 10  5 CH 3 COOH CH 3 COO  5.6 x 10  x 10  10 NH 4 + NH  x 10  5 H 2 O OH  very smallOH  (strong base)O 2  very large

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

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”

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”

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