Acid-base chemistry

Acid-base Chemistry

Acid-base chemistry is a fundamental area of chemistry that deals with the properties and reactions of acids and bases. This field is crucial for understanding a wide range of chemical processes in both inorganic and organic chemistry, as well as in biological systems.

Definitions

Acids

An acid is a substance that can donate a proton (H⁺ ion) to another substance. According to the Brønsted–Lowry acid–base theory, an acid is a proton donor. In the Lewis acid–base theory, an acid is an electron pair acceptor.

Bases

A base is a substance that can accept a proton. In the Brønsted–Lowry theory, a base is a proton acceptor. According to the Lewis theory, a base is an electron pair donor.

Acid-Base Theories

Arrhenius Theory

The Arrhenius acid–base theory defines acids as substances that increase the concentration of hydrogen ions (H⁺) in aqueous solution, while bases increase the concentration of hydroxide ions (OH⁻).

Brønsted–Lowry Theory

The Brønsted–Lowry theory expands the definition of acids and bases to include non-aqueous systems. An acid is a proton donor, and a base is a proton acceptor.

Lewis Theory

The Lewis acid–base theory defines acids as electron pair acceptors and bases as electron pair donors. This theory broadens the scope of acid-base reactions beyond those involving protons.

pH and pKa

pH

The pH is a measure of the acidity or basicity of an aqueous solution. It is defined as the negative logarithm of the hydrogen ion concentration:

\[ pH = -\log[H⁺] \]

A pH less than 7 indicates an acidic solution, while a pH greater than 7 indicates a basic solution.

pKa

The pKa is the negative logarithm of the acid dissociation constant (Ka) of a solution. It provides a measure of the strength of an acid:

\[ pK_a = -\log K_a \]

A lower pKa value indicates a stronger acid.

Acid-Base Reactions

Acid-base reactions involve the transfer of protons from acids to bases. These reactions can be represented by the general equation:

\[ \text{Acid} + \text{Base} \rightarrow \text{Conjugate Base} + \text{Conjugate Acid} \]

Buffers

A buffer solution is a solution that resists changes in pH upon the addition of small amounts of acid or base. Buffers typically consist of a weak acid and its conjugate base or a weak base and its conjugate acid.

Applications

Acid-base chemistry is essential in various fields, including:

• Biochemistry: Enzyme activity and metabolic pathways are often pH-dependent.
• Environmental science: Acid rain and ocean acidification are significant environmental issues.
• Medicine: The human body maintains a tightly regulated pH balance, and deviations can lead to medical conditions.

Also see

• pH scale
• Titration
• Acid dissociation constant
• Conjugate acid-base pair