Acids and Bases

Properties of Acids and Bases

Acids and bases are fundamental chemical species with distinct properties that are easily observed in laboratory and everyday contexts.

• Acids: Sour taste, ability to dissolve metals, turn litmus paper red, neutralize bases.

• Bases: Bitter taste, slippery feel, turn litmus paper blue, neutralize acids.

Definitions of Acids and Bases

There are several definitions for acids and bases, each with its own scope and application:

• Arrhenius Definition: An acid produces H+ ions in aqueous solution; a base produces OH– ions in aqueous solution.

• Brønsted–Lowry Definition: An acid donates H+ in a chemical reaction; a base accepts H+. This definition is more broadly applicable.

• Lewis Definition: (To be covered later) Focuses on electron pair donation and acceptance.

Acid Dissociation and Conjugate Pairs

Acids dissociate in water, forming conjugate acid-base pairs. For example:

• CH3COOH(aq) + H2O(l) → CH3COO–(aq) + H3O+(aq)

• Acid (donates H+): CH3COOH

• Base (accepts H+): H2O

• Conjugate base: CH3COO–

• Conjugate acid: H3O+

The weaker the acid, the stronger its conjugate base.

Acid and Base Strength

Acid Ionization Constant (Ka)

The equilibrium constant for acid dissociation is called the acid ionization constant, Ka:

• Strong acids: Equilibrium lies far to the right; reaction goes to completion. Ka is not discussed for strong acids.

• Weak acids: Equilibrium exists; Ka is used to quantify strength.

• Stronger acid: Larger Ka value.

Base Ionization Constant (Kb)

Bases also have an equilibrium constant, Kb:

• Stronger base: Larger Kb value.

Relationship Between Ka and Kb

For a conjugate acid-base pair:

Where is the ion-product constant for water.

Autoionization of Water and Kw

Autoionization Reaction

Water can act as both an acid and a base (amphoteric). When two water molecules react:

The equilibrium constant is:

At 25°C, .

• If , the solution is acidic.

• If , the solution is basic.

• If , the solution is neutral.

Hydronium and Hydroxide Concentrations in Pure Water

In pure water at 25°C:

The pH and pOH Scales

pH Scale

The pH scale is a logarithmic scale used to quantify acidity:

• pH < 7.0: Acidic

• pH = 7.0: Neutral

• pH > 7.0: Basic

pOH Scale

pOH is defined similarly:

Relationship between pH and pOH:

(at 25°C)

Converting Between pH, pOH, [H3O+], and [OH–]

• Given one value, others can be calculated using and logarithmic relationships.

pKa and pKb

pKa and pKb Definitions

pKa and pKb are logarithmic measures of Ka and Kb:

Lower pKa means stronger acid.

Percent Ionization of Weak Acids

Definition and Calculation

Percent ionization quantifies how much a weak acid dissociates:

• As acid concentration increases, percent ionization typically decreases.

Example Calculation

For a weak acid with and , :

Summary Table: pH and pOH Values for Common Substances

Practice and Application

• Write balanced acid-base reactions and identify conjugate pairs.

• Use equilibrium tables and Ka/Kb expressions to solve for unknown concentrations.

• Calculate pH, pOH, and percent ionization for strong and weak acids.

Example: For HNO2 with Ka = 4.6 x 10–4 and [HNO2]initial = 0.200 M, pH = 2.02.

Additional info: The notes cover all major aspects of acid-base chemistry relevant to a general chemistry course, including equilibrium, calculations, and conceptual understanding.