Practical course in general and inorganic chemistry III. Common - theories The Arrhenius definition - Dissociation of strong/weak 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 + or the later-termed oxonium (H 3O )

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 (OH −) (s) and cation(s) univalent, strong base:

divalent, strong base: univalent, :

divalent, weak base: trivalent, weak base:

• Base anhydrides = metal- : derived from the base by subtracting the molecules of

• 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 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 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 :

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

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