Binary Acids, Oxoacids, and Polyprotic Acids

Introduction

Definition: Composed of two elements (Hydrogen + one other)
- Common examples: Hydrohalic acids (HF, HCl, HBr, HI)
- Water (H₂O) can also be seen as a binary acid
Trend in Acid Strength:
- Down a group: Acid strength increases due to larger size
  - Longer bond length → weaker bond → greater dissociation
- Across a period: Acid strength increases with increasing electronegativity
  - Stability of conjugate base; more electronegative atoms stabilize negative charge better

Definition: Contains hydrogen, oxygen, and one other element (heteroatom)
Trend in Acid Strength:
- More oxygens: More acidic due to resonance stabilization of the conjugate base
  - Example: HClO₄ > HClO₃ (more resonance structures in HClO₄)
- More electronegative heteroatom: More acidic due to inductive stabilization of the conjugate base
  - Example: HClO₄ (chlorine more electronegative) > HBrO₄
Key Points:
- Bond length to acidic hydrogen does not vary, hence size of the heteroatom is irrelevant
- Stability & reactivity inversely related

Definition: Acids that can donate more than one proton
- Example: H₃PO₄ (phosphoric acid), H₂SO₄ (sulfuric acid)
Trend in Acid Strength:
- Successive proton donations become harder; each has its own decreasing equilibrium constant (Kₐ)
- Strong Polyprotic Acid (H₂SO₄): First proton donation is complete (strong), second less so (weak)
- Weak Polyprotic Acid (H₃PO₄): Successive dissociations less than 5%, decreasing significantly in strength
- Amphoteric species: Found in middle of polyprotic series (e.g., H₂PO₄⁻, HPO₄²⁻, HSO₄⁻)