U8.6 · Molecular Structure of Acids and BasesPractice · 分子结构与酸碱性

Down a group, the binary acids get stronger: atoms grow, the H–X bond gets longer and weaker, and H⁺ leaves more easily (this outweighs electronegativity). In fact Se is slightly less electronegative than S, and both molecules have 2 H atoms.

With the same central atom, more O atoms on the Cl pull more electron density away from the O–H bond, making H⁺ easier to release, and the conjugate base can spread its negative charge over more O atoms, making it more stable. So HClO₄ > HClO₃ > HClO₂ > HClO.

Cl is very electronegative and pulls electron density through the bonds away from the carboxyl group. That spreads out and stabilizes the negative charge of –COO⁻ and makes the O–H give up H⁺ more easily. The more Cl atoms, the stronger the pull, so Cl₃C–COOH is strongest.

The weaker the conjugate acid, the stronger the conjugate base. HF is a weak acid, so F⁻ is a (weak) base; HCl, HNO₃, and HClO₄ are strong acids, so Cl⁻, NO₃⁻, and ClO₄⁻ are essentially not basic.

An amine is basic because the lone pair on N bonds to H⁺. The highly electronegative F atoms draw electron density away through the bonds, lowering the electron density on N and making the lone pair less available, so the base is weaker. Ethylamine is the stronger base.