Acids and Bases

Brønsted-Lowry acids and bases are defined by proton transfer in a chemical reaction. In this model, a Brønsted acid donates a proton and becomes its conjugate base. A Brønsted base accepts a proton and becomes its conjugate acid.

This theory was proposed in 1923 by Johannes Brønsted and Thomas Lowry. It helps explain acid-base reactions by tracking how protons move between species. The electron shifts that drive these transfers are shown with curved arrows.

A reaction between acetic acid and...

Lewis theory explains acid-base reactions through electron pairs, especially for electron-deficient compounds that do not fit the Brønsted definition. It is often used to describe group 13 elements such as boron and aluminium. These elements have three valence electrons, so their trivalent compounds can have a sextet of electrons and an empty orbital at the central atom.

Because of that electron shortage, these compounds can accept electrons from another species to complete their octet.

Acid and base strength can be compared using Ka and pKa. These values help show how much an acid dissociates in water and how strong a base is through its conjugate acid.

Acid strength depends on dissociation into a conjugate base and hydronium ion in water. A strong acid dissociates almost completely, so the hydronium ion concentration is very high. A weak acid dissociates only partly, so more acid remains undissociated and the equilibrium stays farther back.

For dilute weak acids, the...

Acid-base equilibrium direction depends on acid strength and pKa values. A lower pKa means a stronger acid, while a higher pKa means a weaker acid. A negative pKa shows a stronger acid, and a positive pKa shows a weaker acid.

In the reaction between ammonia and an ethoxide ion, ethanol has a pKa of 15.9 and ammonia has a pKa of 38. That means ethanol is the stronger acid in this pair. A strong acid forms a weak conjugate base, and a weak acid forms a strong conjugate base, so the ethoxide ion...

Acid strength depends on how stable the conjugate base is after the acid loses a proton. When deprotonation forms a more stable anion, the acid is stronger. If the anion is unstable, the acid is weaker. Chemists compare several structure-based factors to judge that stability.

One factor is size. When acids come from elements in the same column of the periodic table, atomic size is compared. A larger atom can spread out a negative charge over a bigger space. That makes the conjugate base more...

Solvent effects can change how strong acids and bases react in a chemical system. The leveling effect is one important example. It shows that a solvent can take part in the reaction before the acid and base react with each other.

For a generic acid-base reaction, a generic acid (HA) reacts with a generic base (B−) to form the conjugate base (A−) and the conjugate acid (HB). In a solvent called HX, the solvent may also react if it is stronger than one of the reactants. If HA is weaker than the...

Solvation changes acidity by affecting how stable a conjugate base is. The strength of an acid is commonly compared with its pKa value. When an ion dissolves, solvent molecules surround it and help stabilize it through solvation.

Solvation of ions can happen in three main ways. In donor interaction, the solvent donates unshared electron pairs to the ion. The solvent acts as a Lewis base, and the ion acts as a Lewis acid. In charge-dipole interaction, polar solvent molecules align their partial...