Acids, Bases and Salts

Arrhenius Concept (1787):

• Acid: Substance that gives H⁺ ions in aqueous solution. e.g., HCl, HNO₃.
• Base: Substance that gives OH^- ions in aqueous solution. e.g., NaOH, KOH.
• Limitation: Applicable only in aqueous medium. Cannot explain acidic nature of CO₂ or basic nature of NH₃.

Bronsted-Lowry Concept (1923):

• Acid: Proton (H⁺) donor.
• Base: Proton (H⁺) acceptor.
• Conjugate Acid-Base Pair: Differ by a single proton. Acid donates proton to form Conjugate Base. Base accepts proton to form Conjugate Acid.
• Amphoteric: Substance that can act as both acid and base (e.g., Water).

Lewis Concept (1923):

• Acid: Electron pair acceptor. (e.g., BF₃, AlCl₃, H⁺).
• Base: Electron pair donor. (e.g., NH₃, OH^-).
• Adduct: Product of Lewis acid-base reaction formed by coordinate covalent bond.

Chemical Properties of Acids:

• With Metals: React with reactive metals (Zn, Mg, Fe) to form Salt + H₂ gas. (e.g., Zn + H₂SO₄ -> ZnSO₄ + H₂).
• With Carbonates/Bicarbonates: Form Salt + CO₂ + H₂O. (e.g., Na₂CO₃ + 2HCl -> 2NaCl + CO₂ + H₂O).
• With Bases (Neutralization): Form Salt + Water. (e.g., HCl + NaOH -> NaCl + H₂O).
• With Sulphites/Bisulphites: Form Salt + SO₂ + H₂O.
• With Sulphides: Form Salt + H₂S gas.

Chemical Properties of Bases:

• With Acids: Neutralization.
• With Ammonium Salts: Liberate Ammonia gas (NH₃).
• Precipitation of Hydroxides: Precipitate insoluble hydroxides of heavy metals (Cu, Fe, Zn, Pb) from their salt solutions.

Auto-Ionization of Water: Water is a weak electrolyte. H₂O <-> H⁺ + OH^-. Kw = [H⁺][OH^-] = 1.0 x 10^-14 at 25°C.

pH: Negative logarithm of molar concentration of H⁺ ions. pH = -log[H⁺].

pOH: Negative logarithm of molar concentration of OH^- ions. pOH = -log[OH^-].

Relationship: pH + pOH = 14.

Scale:

• pH = 7: Neutral.
• pH < 7: Acidic (Higher [H⁺]).
• pH > 7: Basic (Higher [OH^-]).

Indicators: Organic compounds with different colors in acidic/basic solutions. (e.g., Litmus: Red in acid, Blue in base. Phenolphthalein: Colorless in acid, Red in base).

Acids: H₂SO₄ (Fertilizers, Batteries), HNO₃ (Explosives, Fertilizers), HCl (Cleaning metals), Benzoic Acid (Food preservation), Acetic Acid (Pickles).

Bases: NaOH (Soap), Ca(OH)₂ (Bleaching powder, Softening water), KOH (Alkaline batteries), Mg(OH)₂ (Antacid), NH₄OH (Grease stain remover).

Salts: Ionic compounds formed by neutralization of acid with base. Contain Cation (Basic Radical) and Anion (Acidic Radical).

Preparation:

• Soluble Salts: Acid + Metal, Acid + Base, Acid + Carbonate, Acid + Metallic Oxide. Recovered by evaporation/crystallization.
• Insoluble Salts: Mixing solutions of soluble salts (Double displacement).

Types of Salts:

• Normal/Neutral Salts: Total replacement of H⁺ ions by metal ion (e.g., NaCl, KNO₃).
• Acidic Salts: Partial replacement of H⁺ ions. Contain replace-able H⁺. (e.g., NaHSO₄). Turn blue litmus red. React with base to form Normal salt.
• Basic Salts: Incomplete neutralization of polyhydroxy base. Contain OH group. (e.g., Al(OH)₂Cl, Pb(OH)NO₃). React with acid to form Normal salt.
• Double Salts: Crystallize from mixture of equimolar saturated solutions. Keep individual properties. (e.g., Mohr's Salt, Potash Alum).
• Mixed Salts: Contain more than one basic or acidic radical. (e.g., Bleaching powder CaOCl₂).
• Complex Salts: Provide simple cation and complex anion or vice versa. (e.g., Potassium Ferrocyanide K₄[Fe(CN)₆]).

NaCl: Table salt, De-icing roads, Manufacture of Caustic soda/Washing soda.

Na2CO3 (Soda Ash): Glass, Detergents, Paper.

Na2CO3.10H2O (Washing Soda): Cleaning agent, Softening water.

Na2SO4: Glass, Paper, Detergents.

CaO (Quick Lime): Drying agent, Steel making, Water treatment.

CaSO4.2H2O (Gypsum): Fertilizer, Plaster of Paris.