Chapter 17: Acids and Bases

Introduction to Acids and Bases

Acids and bases are fundamental classes of compounds in chemistry, playing crucial roles in chemical reactions, biological systems, and industrial processes. This chapter explores their properties, definitions, strengths, and the equilibria they establish in aqueous solutions.

Properties and Definitions

Properties of Acids

• Sour taste (e.g., citric acid in lemons)

• Ability to dissolve many metals (e.g., HCl dissolving Zn)

• Ability to neutralize bases

• Change blue litmus paper to red

Properties of Bases

• Bitter taste (e.g., caffeine, quinine)

• Slippery feel (e.g., soap)

• Ability to neutralize acids

• Turn red litmus paper blue

Common Acids and Bases

Acids and bases are found in many everyday substances. For example, hydrochloric acid (HCl) is found in stomach acid, while sodium hydroxide (NaOH) is a common base in drain cleaners.

Structures of Acids

Binary Acids

Binary acids consist of hydrogen and a nonmetal (e.g., HCl, HF). The acidic hydrogen is bonded directly to the nonmetal atom.

Oxyacids

Oxyacids have the acidic hydrogen attached to an oxygen atom (e.g., HNO3, H2SO4).

Carboxylic Acids

Carboxylic acids contain the –COOH group. Only the hydrogen in the –COOH group is acidic (e.g., acetic acid, CH3COOH).

Definitions of Acids and Bases

Arrhenius Definition

• Acid: Produces H+ ions in aqueous solution.

• Base: Produces OH− ions in aqueous solution.

Example: HCl(aq) → H+(aq) + Cl−(aq)

OH− ions from bases combine with H+ ions from acids to form water in neutralization reactions.

Brønsted-Lowry Definition

• Acid: Proton (H+) donor

• Base: Proton (H+) acceptor

This definition is broader and includes reactions not limited to aqueous solutions.

Amphoteric Substances

Substances like water can act as either an acid or a base, depending on the reaction partner.

Conjugate Acid-Base Pairs

In Brønsted-Lowry theory, acids and bases exist in conjugate pairs. When an acid donates a proton, it becomes its conjugate base; when a base accepts a proton, it becomes its conjugate acid.

Acid and Base Strength

Strong and Weak Acids/Bases

• Strong acids/bases: Completely ionize in solution (strong electrolytes).

• Weak acids/bases: Partially ionize in solution (weak electrolytes).

Acid Ionization Constant (Ka)

The strength of an acid is measured by its equilibrium constant for ionization in water:

Larger Ka means a stronger acid.

Base Ionization Constant (Kb)

The strength of a base is measured by its equilibrium constant for reaction with water:

Larger Kb means a stronger base.

Examples of Weak Acids

Examples of Weak Bases

Autoionization of Water and pH Scale

Autoionization of Water

Water can act as both an acid and a base, leading to the autoionization equilibrium:

The equilibrium constant for this process is called the ion product of water, :

at 25°C

pH and pOH

• pH:

• pOH:

• Relationship: at 25°C

pH < 7 is acidic, pH > 7 is basic, and pH = 7 is neutral.

Acid–Base Equilibria and Calculations

Calculating pH of Strong and Weak Acids

• Strong acids:

• Weak acids: Use an ICE table and the expression to solve for

Percent Ionization

Percent ionization is the fraction of acid molecules that ionize in solution:

Higher percent ionization means a stronger acid.

Acid–Base Properties of Salts

Salts in Solution

• Cations of strong bases and anions of strong acids are pH neutral.

• Anions of weak acids are basic; cations of weak bases are acidic.

• Small, highly charged metal cations are weakly acidic.

Classifying Salt Solutions

• Neutral: Cation of strong base + anion of strong acid

• Basic: Cation of strong base + anion of weak acid

• Acidic: Cation of weak base + anion of strong acid

• Depends: Cation of weak base + anion of weak acid (compare and )

Polyprotic Acids

Ionization Steps

Polyprotic acids ionize in steps, each with its own . Usually, the first ionization is the most significant for pH calculations.

Examples of Polyprotic Acids

• Sulfuric acid (H2SO4)

• Phosphoric acid (H3PO4)

Factors Affecting Acid Strength

Binary Acids

• Acid strength increases with bond polarity and decreases with bond strength.

• Trends: Increases across a period and down a group.

Oxyacids

• Acid strength increases with the electronegativity of the central atom and the number of oxygen atoms attached.

• Higher oxidation number of the central atom increases acidity.

Lewis Acid–Base Theory

Lewis Acids and Bases

• Lewis acid: Electron pair acceptor (often electron-deficient species)

• Lewis base: Electron pair donor (must have a lone pair)

Lewis theory expands the definition of acids and bases beyond proton transfer to include electron pair transfer.

Summary Table: Key Concepts

Additional info: The images included above are directly relevant to the structures and tables discussed in the corresponding paragraphs, reinforcing the academic explanations of weak acids, weak bases, and polyprotic acids.