Acids, Bases, and Salts

Acids, bases, and salts are everywhere in nature! 🍋 For example:

  • Hydrochloric acid (HCl) in your stomach helps digestion.
  • Vinegar contains acetic acid.
  • Lemon juice has citric acid, and tamarind has tartaric acid.

Properties of Acids & Bases

  • Acids taste sour, turn blue litmus → red, and release H₂ gas with metals.
  • Bases taste bitter, turn red litmus → blue, and feel soapy (e.g., washing soda).
  • Mixing acids and bases gives salts (e.g., NaCl from HCl + NaOH).

Salts Dissolving in Water

Salts like NaCl split into ions (Na⁺ and Cl⁻) in water. Water’s high dielectric constant (80) weakens the electrostatic forces between ions, letting them move freely 💧→⚡.

Three Ways to Define Acids & Bases

1. Arrhenius Concept (Water-Based)

  • Acid: Releases H⁺(aq) ions in water.
    Example: HCl → H⁺ + Cl⁻ or HCl + H₂O → H₃O⁺ + Cl⁻
  • Base: Releases OH⁻(aq) ions.
    Example: NaOH → Na⁺ + OH⁻
  • ⚠️ Limitation: Only works in water. Can’t explain bases like ammonia (NH₃) that lack OH⁻ groups.

2. Brønsted-Lowry Concept (Proton Transfer)

  • Acid: Donates a proton (H⁺).
    Base: Accepts a proton (H⁺).
  • Example: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
    Here, H₂O (acid) gives H⁺ to NH₃ (base).
  • Conjugate Pairs:
    • After donating H⁺, an acid becomes its conjugate base.
    • After accepting H⁺, a base becomes its conjugate acid.
    • Example: In HCl + H₂O → H₃O⁺ + Cl⁻, Cl⁻ is conjugate base of HCl, H₃O⁺ is conjugate acid of H₂O.

3. Lewis Concept (Electron Pairs)

  • Acid: Accepts an electron pair (e.g., BF₃).
  • Base: Donates an electron pair (e.g., NH₃).
  • Example: BF₃ + :NH₃ → BF₃NH₃ (BF₃ accepts : from NH₃).

Strong vs. Weak Acids/Bases

  • Strong acids (HCl, HNO₃, H₂SO₄) ionize completely in water → good H⁺ donors.
  • Strong bases (NaOH, KOH) ionize completely → good OH⁻ donors.
  • Weak acids (e.g., acetic acid) partially ionize (<5%). Their dissociation is an equilibrium:
    HA(aq) + H₂O(l) ⇌ H₃O⁺(aq) + A⁻(aq)
  • In equilibrium, the reaction favors the side with the weaker acid.

NEET Important Concepts 🔥

  1. Conjugate Acid-Base Pairs (e.g., H₂O/OH⁻, NH₄⁺/NH₃).
  2. Amphoteric Species (substances that act as both acid AND base, like H₂O, HCO₃⁻).
  3. Lewis Acids/Bases (electron pair acceptors/donors, e.g., BF₃, NH₃).
  4. Strength of Acids/Bases: Strong acids completely dissociate; weak acids establish equilibrium.

Quick Practice Problems

Q: Conjugate base of HF? A: F⁻
Q: Conjugate acid of NH₃? A: NH₄⁺
Q: Why is BCl₃ a Lewis acid? A: It accepts electron pairs (e.g., from NH₃).