Acid-Base Equilibria: Definitions, Strength, and Quantification

Introduction to Acid-Base Equilibria

Definitions of Acids and Bases

Acids and bases are fundamental chemical species that participate in a wide range of chemical reactions. Their definitions and properties are essential for understanding chemical equilibria and solution chemistry.

• Mineral acids (inorganic acids) are typically oxyanions with dissociable H+ ions.

• Carboxylic acids are organic molecules containing a –CO2H group. The acidic hydrogen atom is bonded to oxygen; other hydrogens in the molecule are not acidic.

Examples of acids:

• Nitric acid: HNO3

• Sulfuric acid: H2SO4

• Phosphoric acid: H3PO4

• Acetic acid: HC2H3O2 or CH3CO2H

• Citric acid: H3C6H5O7

Common Bases

Types of Bases

Bases are substances that can accept protons or donate electron pairs. They are classified as inorganic or organic based on their composition.

• Inorganic bases are ionic compounds containing hydroxide (OH−) or carbonate (CO32−) anions.

• Organic bases are often alkaloids or amines (molecules with a basic nitrogen atom).

Examples of common bases:

Base	Formula
Sodium hydroxide	NaOH
Potassium hydroxide	KOH
Sodium bicarbonate	NaHCO3
Sodium carbonate	Na2CO3
Ammonia	NH3

Arrhenius Definition of Acids

Substances That Raise the Acidity ([H+]) of a Solution

The Arrhenius definition states that an acid is a substance that increases the concentration of hydrogen ions (H+) in aqueous solution.

• An acid contains an acidic bond to hydrogen.

• This bond ionizes or dissociates to form H+ ions:

$\text{HA (aq)} \rightarrow \text{H}^+ (aq) + \text{:A}^- (aq)$

• Acid strength refers to its ability to dissociate.

• Solution acidity is measured by the concentration of H+ ions.

Strong and Weak Acids

Classification and Dissociation

Acids are classified as strong or weak based on their degree of ionization in water.

• Strong acids fully ionize in solution and are strong electrolytes.

• Weak acids only partially ionize and are weak electrolytes.

List of common strong acids:

Strong Acid
Hydrochloric acid (HCl)
Hydrobromic acid (HBr)
Hydroiodic acid (HI)
Chloric acid (HClO3)
Perchloric acid (HClO4)
Nitric acid (HNO3)
Sulfuric acid (H2SO4)

If an acid is not on this list, it is generally considered a weak acid.

Examples of dissociation:

• Strong acid: $\text{HCl (g)} \rightarrow \text{H}^+ (aq) + \text{Cl}^- (aq)$

• Strong acid: $\text{HNO}_3 (l) \rightarrow \text{H}^+ (aq) + \text{NO}_3^- (aq)$

• Weak acid: $\text{CH}_3\text{CO}_2\text{H} (l) \rightleftharpoons \text{H}^+ (aq) + \text{CH}_3\text{CO}_2^- (aq)$

Strong and Weak Acids: Equilibrium

Irreversible vs. Reversible Dissociation

Strong acids dissociate irreversibly, while weak acids establish a reversible equilibrium in solution.

• Strong acid: $\text{HA} \rightarrow \text{H}^+ + \text{A}^-$, $K_a \gg 1$

• Weak acid: $\text{HA} \rightleftharpoons \text{H}^+ + \text{A}^-$, $K_a \ll 1$

Acid dissociation constant:

$K_a = \frac{[\text{H}^+][\text{A}^-]}{[\text{HA}]}$

Quantifying Acid Strength: Ka and pKa

Measurement and Logarithmic Scale

The strength of a weak acid is measured by its acid dissociation constant, Ka. The value of Ka typically ranges from $1 \times 10^{-2}$ to $1 \times 10^{-12}$.

• For weak acids: $\text{HA} \rightleftharpoons \text{H}^+ + \text{A}^-$

• $K_a = \frac{[\text{H}^+][\text{A}^-]}{[\text{HA}]} \ll 1$

• Acid strength is often reported as pKa:

$\mathrm{p}K_a = -\log K_a$

• The smaller the value of pKa, the stronger the acid.

Quantifying Acid Strength: Examples

Comparison of Weak Acids

Strong acids are fully ionized and not described by a Ka. Among weak acids, those with higher Ka and smaller pKa are stronger.

Acid	Structural Formula	Conjugate Base	Ka
Chlorous (HClO2)	H–O–Cl–O	ClO2−	1.0 × 10−2
Hydrofluoric (HF)	H–F	F−	6.8 × 10−4
Nitrous (HNO2)	H–O–N=O	NO2−	4.5 × 10−4
Benzoic (C6H5COOH)	See structure	C6H5COO−	6.3 × 10−5
Acetic (CH3COOH)	See structure	CH3COO−	1.8 × 10−5

Example: Acetic acid (CH3COOH) is a weak acid with $K_a = 1.8 \times 10^{-5}$, meaning it only partially ionizes in water.