8.6 Molecular Structure of Acids and Bases

Binary acid strength trends

For binary acids (H-X), acid strength increases with increasing bond polarity and increasing atomic size/decreasing bond strength of X. Moving across a period, electronegativity of X increases, strengthening the acid (CH4 < NH3 < H2O < HF). Moving down a group, bond strength decreases despite lower electronegativity, so acid strength increases (HF < HCl < HBr < HI).

Oxyacid strength trends

For oxyacids, more electronegative central atoms or more oxygen atoms attached pull electron density away from the O-H bond, weakening it and stabilizing the resulting anion — increasing acid strength. Comparing HOCl vs HOI, chlorine's higher electronegativity makes HOCl stronger. Comparing HClO vs HClO4, more oxygens make HClO4 far stronger.

Carboxylic acids and structure–strength links

Carboxylic acids (R-COOH) are acidic because the conjugate base (carboxylate ion) is stabilized by resonance, delocalizing negative charge over two oxygens. Electron-withdrawing groups near the acidic proton (like additional electronegative atoms) further stabilize the conjugate base, increasing acidity by lowering electron density and weakening the O-H or X-H bond.

Key terms

Binary acid strength trend across a period

Increases with increasing electronegativity of X (e.g., CH4 < NH3 < H2O < HF).

Binary acid strength trend down a group

Increases with decreasing bond strength/increasing atomic size (e.g., HF < HCl < HBr < HI).

Oxyacid strength factors

Increases with more electronegative central atom or more oxygen atoms attached, which stabilize the conjugate base.

Why carboxylic acids are acidic

Their conjugate base (carboxylate) is stabilized by resonance, delocalizing negative charge over two oxygens.

Effect of electron-withdrawing groups on acid strength

They stabilize the conjugate base by pulling electron density away, increasing acid strength.

Try an AP-style question

Why is HClO₄ a stronger acid than HClO?

1. HClO₄ has a much stronger O–H bond than HClO
2. HClO₄ is simply a larger molecule than HClO
3. HClO has more oxygen atoms than HClO₄
4. More O atoms stabilize the conjugate base