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, where the acidic hydrogen atom is bonded to oxygen. Other hydrogens in the molecule are generally not acidic.

Examples of common acids:

• Nitric acid: HNO3

• Sulfuric acid: H2SO4/l

• 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, which are 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 classifies acids as substances that increase the concentration of hydrogen ions (H+) in aqueous solution.

• An acid contains an acidic bond to hydrogen, which ionizes or dissociates to form H+ ions:

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

• Acid strength refers to the ability of an acid to dissociate.

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

Strong and Weak Acids

Classification and Ionization

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:

• Hydrochloric acid (HCl)

• Hydrobromic acid (HBr)

• Hydroiodic acid (HI)

• Chloric acid (HClO3)

• Perchloric acid (HClO4)

• Nitric acid (HNO3)

• Sulfuric acid (H2SO4)

Ionization equations:

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

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

Quantifying Acid Strength

Acid Dissociation Constant ($K_a$) and $pK_a$

The strength of a weak acid is measured by its acid dissociation constant, $K_a$, which quantifies the equilibrium between the acid and its dissociated ions.

• $K_a$ is typically between $1 \times 10^{-2}$ and $1 \times 10^{-12}$ for weak acids.

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

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

• Acid strength is often reported as $pK_a$, the negative logarithm of $K_a$:

$pK_a = -\log K_a$

• The smaller the value of $pK_a$, the stronger the acid.

Examples of Weak Acids and Their $K_a$ Values

Some common weak acids and their dissociation constants are shown below:

Acid	Structural Formula	Conjugate Base	$K_a$
Chlorous acid (HClO2)	ClO2–OH	ClO2-	$1.0 \times 10^{-2}$
Hydrofluoric acid (HF)	H–F	F-	$6.8 \times 10^{-4}$
Nitrous acid (HNO2)	HON=O	NO2-	$4.5 \times 10^{-4}$
Benzoic acid (C6H5COOH)	Ph–COOH	C6H5COO-	$6.3 \times 10^{-5}$
Acetic acid (CH3COOH)	CH3COOH	CH3COO-	$1.8 \times 10^{-5}$

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

Additional info: The $K_a$ and $pK_a$ values are essential for calculating the pH of solutions containing weak acids and for understanding buffer systems in chemistry.