Acids and Bases: Ionization & Properties

⚗️ Lewis Acids and Bases

Lewis Acid: Accepts an electron pair (e.g., BF3, AlCl3)
Lewis Base: Donates an electron pair (e.g., NH3, OH–, H2O)

Example reaction: \[ \text{BF}_3 + \text{NH}_3 \rightarrow \text{BF}_3:\text{NH}_3 \] Here, BF3 (no proton!) acts as the acid.

💪 Strong vs. Weak Acids/Bases

Strong Acids: Fully dissociate in water (e.g., HCl, HNO3, H2SO4)
Strong Bases: Fully dissociate (e.g., NaOH, KOH)
Weak Acids: Partially dissociate (e.g., HF, CH3COOH)
Weak Bases: Partially dissociate (e.g., NH3, CH3NH2)

Key idea: Strong acid = Weak conjugate base (e.g., HCl → Cl– is weak base).

🔢 pH Scale & Water Ionization

Water self-ionizes: \[ \text{H}_2\text{O} + \text{H}_2\text{O} \rightleftharpoons \text{H}_3\text{O}^+ + \text{OH}^- \] Ionic product: \( K_w = [\text{H}^+][\text{OH}^-] = 1 \times 10^{-14} \) (at 298K)

pH = -log[H+]
pOH = -log[OH–]
pH + pOH = 14 (at 25°C)

Solutions:
– Acidic: pH < 7, [H+] > [OH–]
– Neutral: pH = 7, [H+] = [OH–]
– Basic: pH > 7, [H+] < [OH–]

📊 Ionization Constants (Ka & Kb)

For weak acid HA: \[ K_a = \frac{[\text{H}^+][\text{A}^-]}{[\text{HA}]} \] For weak base B: \[ K_b = \frac{[\text{BH}^+][\text{OH}^-]}{[\text{B}]} \] pKa = -log Ka, pKb = -log Kb

Conjugate pairs magic: \[ K_a \times K_b = K_w \quad \text{or} \quad \text{p}K_a + \text{p}K_b = 14 \]

🧪 Common Ion Effect & Hydrolysis

Common ion effect: Adding ions already present shifts equilibrium (Le Chatelier’s principle).
Example: Adding acetate (CH3COO–) to acetic acid reduces [H+].

Salt Hydrolysis:
– Salt of weak acid + strong base → Basic solution (e.g., CH3COONa, pH > 7)
– Salt of strong acid + weak base → Acidic solution (e.g., NH4Cl, pH < 7)
– Salt of weak acid + weak base → pH depends on Ka & Kb: \[ \text{pH} = 7 + \frac{1}{2}(\text{p}K_a – \text{p}K_b) \]

🚨 NEET Super Shorts!

  1. Conjugate Pairs: Strong acid → Weak conjugate base (HCl → Cl⁻).
  2. pH Calculations: pH = -log[H⁺]; for weak acids use \( K_a = \frac{(c\alpha)^2}{c(1-\alpha)} \).
  3. Kw & Temperature: Kw = 10⁻¹⁴ at 25°C → pH + pOH = 14.
  4. Salt Hydrolysis: NH₄Cl (acidic), CH₃COONa (basic), CH₃COONH₄ (pH ≈ 7).
  5. Lewis Acids: Electron-deficient species (BF₃, AlCl₃) accept e⁻ pairs.