Acid-Base Chemistry: Definitions, Reactions, and pH

Arrhenius Theory of Acids and Bases

According to the Arrhenius theory, acids and bases exist only in aqueous solution:

• Acid: Any substance that in aqueous solution dissociates with the formation of hydrogen ions, H⁺.
• Base: Any substance that in aqueous solution dissociates with the formation of hydroxyl ions, OH^-.

Neutralization Reaction

Neutralization is the complete reaction of an acid and a base with the formation of a salt and water. HA + BOH → AB + H₂O.

Brønsted-Lowry Theory

According to the Brønsted-Lowry theory, the concepts of acid and base were expanded:

• Acid: Any chemical species, molecular or ionic, capable of yielding an H⁺ ion to another substance.
• Base: Any chemical species, molecule or ion, capable of receiving an H⁺ ion from another substance.

Neutralization is the transfer of an H⁺ ion (a proton) from an acid to a base, giving rise to a conjugate base and a conjugate acid. HA + B → A^- + BH⁺

Degree of Ionization

The degree of ionization of an acid or a weak base is the relationship between the concentration of acid or base at equilibrium and its initial concentration.

Autoionization of Water

Water undergoes a process of autoionization in which one molecule acts as an acid and a base; a proton transfers from one to another, according to the equilibrium equation: H₂O + H₂O ↔ H₃O⁺ + OH^-

Ionic Product of Water

The constant K_w of this equilibrium is called the ionic product of water and is 1 · 10^-14 at 25 °C.

pH and pOH

The pH of a solution is the negative logarithm of the numerical expression of the concentration of H₃O⁺ ions.

The pOH is the negative logarithm of the numerical expression of the concentration of OH^- ions. pH + pOH = 14.

Buffer Solutions

Buffer solutions, also called regulatory or buffer solutions, are those able to maintain their pH virtually constant, though they are diluted or small amounts of acid or base are added.

Hydrolysis of a Salt

Hydrolysis of a salt is the acid-base reaction that can be performed by salt ions with water.

1. Salts from a strong acid and a strong base: hydrolysis reaction does not occur, and the solution is neutral (pH = 7).
2. Salts from a strong base and a weak acid: hydrolysis occurs, and the solution is alkaline (pH > 7).
3. Salts from a weak base and a strong acid: hydrolysis occurs, and the solution is acidic (pH < 7).
4. Salts from a weak acid and a weak base: both acidic and alkaline hydrolysis occur, and the pH depends on its constant. If Ka > Kb, it is acidic; if Kb > Ka, it is basic; if Ka = Kb, then it is neutral.

Acid-Base Indicator

An acid-base indicator is a substance of weak acidic or basic character that has the property of presenting different colors depending on the pH of the solution in which it is dissolved.

Acid-Base Titration

An acid-base titration is the determination of the unknown concentration of an acid in solution from the known concentration of a base or vice-versa, based on the neutralization reaction. If you assess an acid, it is called acidimetry, and if a base, alcalimetría.