Practical course in general and inorganic chemistry III. Common acid-base theories The Arrhenius definition - Dissociation of strong/weak acids and bases - Preparation of salts - The limitations of the Arrhenius acid-base theory The Brønsted-Lowry definition - Relative strengths of Brønsted-Lowry acids and bases - Amphiprotic species (the amphoterism) - Types of acid-base reactions - Structures of different boric acids and borax - Preparation of normal salts - Acid salts The Arrhenius acid-base theory (1884) • Acid : dissociate in aqueous solution form hydrogen + or the later-termed oxonium (H 3O ) ions univalent, strong acid: univalent, weak acid: diprotic, strong acid: diprotic, weak acid: triprotic, weak acid: • Stepwise acid dissociation : 1 Relative strength of inorganic oxoacids (Pauling) − + + Om X(OH) n Om+1X(OH) n−1 H acid anhydride m = 3 very strong acid m = 2 strong acid m = 1 weak acid m = 0 very weak acid Arrhenius-bases and anhydrides • Base : in aqueous solution form hydroxide (OH −) ion(s) and cation(s) univalent, strong base: divalent, strong base: univalent, weak base: divalent, weak base: trivalent, weak base: • Base anhydrides = metal-oxides : derived from the base by subtracting the molecules of water • base anhydrides of strong bases: 2 Preparation of salts (Arrhenius) base + acid = salt + water • Preparation of salt from a strong base: • Preparation of salt from a weak base : • If the acid is partially neutralized: acid salt The limitations of the Arrhenius acid-base theory • Although ammonia is well-known as a base, it has no OH − ion in its formula: ⇒ Brönsted: • in case of bases, not always OH − reacts: + • The dissociation of an acid leads to H3O , the water is amphoteric ⇒ Brönsted: • Why is the solution of a salt acidic/basic/neutral? ⇒ Brönsted: „hydrolysis” • Acid-base reactions in water free solvents (glacial acetic acid, liquid ammonia): 3 The Brønsted-Lowry acid-base theory (1923) Acid : any species which has the tendency to lose, or "donate" a hydrogen ion Base : a substance with a tendency to gain or "accept" a hydrogen ion (proton) corresponding acid-base pairs + + e.g.: NH 4 NH 3 + H acid base • Acid-base reaction (ingeneral ) every acid has its conjugate base, and every base has its conjugate acid: The role of the water in relative strengths of B-L acids and bases + + + − • self ionization of water: H2O H2O H3O OH base 1 acid 2 acid 1 base 2 equlibrium constant: ion product of water /ionization constant: • Relative strengths of conjugated acid-base pairs 4 Direction of B-L acid-base reactions, the amphoteric nature of water HA + B A − + BH + stronger stronger weaker weaker acid 1 base 2 base 1 acid 2 + Stronger acids than H3O : weaker base than H2O: + → − + + + − ← − + HCl H2O Cl H3O H2O Cl OH HCl stronger a1 b2 b1 a2 a1 weaker b2 b1 a2 weak bases , weaker than OH −, stronger than weak acids , weaker than H O+, stronger than H O: 3 2 H2O: + − + + − + + − H2CO 3 H2O HCO 3 H3O HCO 3 H2O H2CO 3 OH a1 b2 b1 a2 b1 a2 a1 b2 + + + + + + + − NH 4 H2O NH 3(aq) H3O NH 3(aq) H2O NH 4 OH a1 b2 b1 a2 b1 a2 a1 b2 − + 2− + + 2− + − + − HCO 3 H2O CO 3 H3O CO 3 H2O HCO 3 OH a1 b2 b1 a2 b1 a2 a1 b2 −: weaker acids than H2O: Stronger bases than OH − + ← 2− + + 2− + → − + − OH H2O O H3O O H2O OH OH weaker a1 b2 b1 a2 b1 a2 a1 b2 Types of acid-base reactions according to Brönsted-Lowry definition I. 1. Dissociation of strong/weak acids/bases, the amphoteric nature of water 2. Neutralization reactions • Strong acid + strong base: • Weak acid + strong base: • Strong acid + weak base: • Weak acid + weak base: 3. Reaction between acids 5 Types of acid-base reactions according to Brönsted-Lowry definition II. 4. Reaction of normal salts (Arrhenius-salt) with stronger acids or bases • NH 4Cl + NaOH: • KCN + H 2CO 3: • NaHCO 3 + CH 3COOH: • Na 2B4O7 + H 2SO 4: Boric acid anhydride, boric acid and borax • relationship between boric acid anhydride and the different boric acids: B2O3 + 3 H2O = 2 H3BO 3 +2 H2O B2O3 + H2O = 2 HBO 2 2 B2O3 + H2O = H2B4O7 Na 2B4O7 • dissociation of orthoboric acid (Lewis theory): + − + + H3BO 3 H2O B(OH) 4 H • preparation of orthoboric acid from borax (Brönsted): Na 2B4O7 + H2SO 4 = H2B4O7 + Na 2SO 4 spontaneous degradation of the az unstable tetraboric acid in water: H2B4O7 + 5 H2O = 4 H3BO 3 6 Orthoboric acid Metaboric acid H H3BO 3 O (HBO 2)n B H H OO O H O H O H O B H H OO H O B B O H O B O O B O H O B B O H O O H O H O H B H H O O H O O B O H B H B H OO O B O OO H B O H H H H O H O O O O H B H B O O H H Tetraborate anion Metaboric acid 2− 2− [B 4O5(OH) 4] = [B 4O7] (HBO 2)n OH O H B OH B O H O O O H O B B O B O O B O O B O HO B O B B O H O OH H O H O O H there are two four-coordinate B O boron atoms and two O B O three-coordinate boron atoms. B O H H O Borax: ⋅ Na 2[B 4O5(OH) 4] 8 H 2O ⋅ Na 2B4O7 10 H 2O 7 ⋅ Borax: Na 2B4O7 10 H 2O Death Valley, California Use of borax: - detergents, soaps - water-softening - enamel ware, ceramics, - fire retardent Preparation of normal salts • Arrhenius: base + acid = salt + water + = + Cu(OH) 2 H2SO 4 CuSO 4 2 H2O • Brönsted: base anhydride + acid = salt + water + = + CuO H2SO 4 CuSO 4 H2O • Lewis (Lux): base + acid anhydride = salt + water + = + Cu(OH) 2 SO 3 CuSO 4 H2O • Lewis (Lux): base anhydride + acid anhydride = salt + = CuO SO 3 CuSO 4 • Brönsted: salt of weak acid + strong acid = salt of strong acid + weak acid + = + + CuCO 3 H2SO 4 CuSO 4 CO 2 H2O + = + + • Redox reaction: Cu 2 H2SO 4 CuSO 4 SO 2 H2O 8 Types of acid-base reactions according to Brönsted-Lowry definition III. 5. Amphiprotic behavior of acid salts • Dissociation of Na 2HPO 4: • hidrogenphosphate acts as base: • hidrogenphosphate can act as an acid : Types of acid-base reactions according to Brönsted-Lowry definition IV. 6. Acid-base properties of some common ions in water solution The acid-base properties of salts are determined by the behavior of their ions. • neutral ions • basic anions • acidic ions 9.