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Chemistry 1011

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). 13.6 Acid-Base Properties of Salt Solutions. YOU ARE EXPECTED TO BE ABLE TO:

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Chemistry 1011

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  1. 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 5

  2. 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 Ka and Kb (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 5

  3. 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) CH3COONa(s) CH3COO-(aq) + Na+(aq) NH4Cl(s) NH4+(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 • CH3COONa is the salt of a weak acid and a strong base • NH4Cl is the salt of a strong acid and a weak base Chemistry 1011 Slot 5

  4. Hydrolysis of Salts • CH3COONa is the salt of a weak acid (acetic acid) and a strong base (sodium hydroxide). • The CH3COO- ions will react with water: CH3COO-(aq) + H2O(aq)CH3COOH(aq) + OH-(aq) • The CH3COO- ion is a Bronsted-Lowry base. The solution will be basic. Kb = [CH3COOH] x[OH-] = 5.6 x 10-10 [CH3COO-] • Any anion derived from a weak acid will be a weak base Chemistry 1011 Slot 5

  5. Hydrolysis of Salts • NH4Cl is the salt of a strong acid (hydrochloric acid) and a weak base (ammonia) • The NH4+ions will react with water: NH4+(aq)+ H2O(aq)NH3(aq) + H3O+(aq) • The NH4+ion is a Bronsted-Lowry acid. The solution will be acidic. Ka = [NH3] x[H3O+] = 5.6 x 10-10 [NH4+] • Any cation derived from a weak base will be a weak acid Chemistry 1011 Slot 5

  6. 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: • Al3+ is [Al(H2O)6]3+ • Zn2+ is [Zn (H2O)4]2+ • The hydrated ions act as Bronsted-Lowry acids: Al(H2O)63+(aq)+ H2O(aq)H3O+(aq) + Al(H2O)5(OH)2+( aq) ca cb ca cb Chemistry 1011 Slot 5

  7. Anions as Weak Bases Base Kb pKb PO43- 2.2 x 10-2 1.66 CO32- 2.1 x 10-4 3.68 HPO42- 1.6 x 10-7 6.80 The larger the value of Kb, the stronger the acid PO43-(aq) + H2O(aq)HPO42-(aq) + OH-(aq) • The PO43-ion is a Bronsted-Lowry base. The solution will be basic. Kb = [HPO42-] x[OH-] = 2.2 x 10-2 [PO43-] Chemistry 1011 Slot 5

  8. Relation between Ka and Kb • Ammonia in aqueous solution is basic: NH3(aq) + H2O(aq)NH4+(aq) + OH-(aq) Kb = [NH4+] x[OH-] = 1.8 x 10-5 [NH3] • The ammonium ion is its conjugate acid • We can write an equation for NH4+acting as an acid as: NH4+(aq)+ H2O(aq)NH3(aq) + H3O+(aq) Ka = [NH3] x[H3O+] = 5.6 x 10-10 [NH4+] Chemistry 1011 Slot 5

  9. Relation between Ka and Kb • Add the two equations: H2O(aq) + H2O(aq)H3O+(aq) + OH-(aq) • The equilibrium constant of the resulting equation is the product of the two equilibrium constants: Kw = [H3O+] x [OH-] = Ka xKb = (5.6 x 10-10) x (1.8 x 10-5) = 1.0 x 10-14 • This relationship can be used to determine Kb for a weak acid from a measurement of Ka for its conjugate acid, and vice versa Chemistry 1011 Slot 5

  10. Relative Strengths of Bronsted-Lowry Acids and Bases KaConjugate Acid Conjugate BaseKb very large HCl (strong acid) Cl- very small H3O+ H2O 6.9 x 10-4 HF F- 1.4 x 10-11 1.8 x10-5 CH3COOH CH3COO - 5.6 x 10-10 5.6 x 10-10 NH4+ NH3 1.8 x10-5 H2O OH- very small OH- (strong base) O2- very large Chemistry 1011 Slot 5

  11. Relative Strengths of Acids and Bases • Bronsted-Lowry acids: • Strong acids - stronger proton donors than H3O+ • Weak acids - weaker proton donors than H3O+ but stronger than H2O • 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 H2O • Species that do not form basic aqueous solutions Chemistry 1011 Slot 5

  12. 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 5

  13. 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 5

  14. 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: NH4F • KaNH4+ = 5.6 x 10-10 • KaF- = 1.4 x 10 -11 • Solution has pH 6.2 Chemistry 1011 Slot 5

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