Chapter 17: Acids and Bases

Properties of Acids

Acids are a class of compounds with distinct chemical and physical properties. They are commonly found in foods, industrial processes, and biological systems.

• Sour taste (e.g., citrus fruits contain citric acid)

• Ability to dissolve many metals (e.g., hydrochloric acid dissolves zinc)

• Ability to neutralize bases (react with bases to form water and salts)

• Change blue litmus paper to red

Common Acids and Their Uses

Acids are widely used in industry, food, and biological systems. Some common examples include:

Structures of Acids

• Binary acids: Hydrogen attached to a nonmetal (e.g., HCl, HF)

• Oxyacids: Hydrogen attached to an oxygen atom (e.g., H2SO4, HNO3)

• Carboxylic acids: Contain the carboxyl group (–COOH); only the hydrogen in the –COOH is acidic (e.g., acetic acid, citric acid)

Properties of Bases

Bases are substances with characteristic properties and important industrial and household uses.

• Bitter taste

• Often poisonous

• Slippery feel (e.g., soap)

• Ability to neutralize acids

• Turn red litmus paper blue

Common Bases and Their Uses

Definitions of Acids and Bases

Arrhenius Model

The Arrhenius definition is the earliest and most restrictive model for acids and bases:

• Acid: Produces H+ ions in aqueous solution

• Base: Produces OH− ions in aqueous solution

Example reaction:

Brønsted–Lowry Theory

This model expands the definition to include proton transfer reactions:

• Acid: Proton (H+) donor

• Base: Proton (H+) acceptor

• Amphoteric substances: Can act as either acid or base (e.g., H2O)

Example:

Conjugate Acid–Base Pairs

In Brønsted–Lowry reactions, each acid and base forms a conjugate pair:

• Conjugate acid: Formed when a base gains a proton

• Conjugate base: Formed when an acid loses a proton

Acid and Base Strength

Strong and Weak Acids

• Strong acids: Completely ionize in water (e.g., HCl, HNO3, H2SO4)

• Weak acids: Only partially ionize in water (e.g., HF, acetic acid)

Acid Ionization Constant (Ka)

The strength of an acid is measured by its acid ionization constant, Ka:

• Larger Ka = stronger acid

• Smaller Ka = weaker acid

Autoionization of Water and pH

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

The ion product of water is:

at 25°C

pH and pOH

• pH:

• pOH:

• At 25°C,

• pH < 7: acidic; pH = 7: neutral; pH > 7: basic

Calculating pH of Strong and Weak Acids

• For strong acids:

• For weak acids: Use an ICE table and solve for equilibrium concentrations using Ka

Example for weak acid:

Set up ICE table, solve for x, then

Percent Ionization

Percent ionization measures the fraction of acid molecules that ionize:

• Strong acids: >99%

• Weak acids: <5%

Acid–Base Properties of Salts

Salts in Solution

• Cations from strong bases and anions from strong acids yield neutral solutions (e.g., NaCl)

• Anions from weak acids yield basic solutions (e.g., NaF)

• Cations from weak bases yield acidic solutions (e.g., NH4Cl)

• Highly charged metal cations can also make solutions acidic (e.g., Al(NO3)3)

Relationship Between Ka and Kb

For a conjugate acid–base pair:

Polyprotic Acids

Polyprotic acids can donate more than one proton, with each step having its own Ka value:

Molecular Structure and Acid Strength

Binary Acids

• Acid strength increases across a period (left to right) and down a group (top to bottom) in the periodic table

• More polarized and weaker H–X bonds lead to stronger acids

Oxyacids

• Acid strength increases with the number of oxygen atoms bonded to the central atom

• Greater electronegativity of the central atom also increases acidity

Lewis Acid–Base Theory

The Lewis definition broadens the concept of acids and bases:

• Lewis acid: Electron pair acceptor

• Lewis base: Electron pair donor

All Brønsted–Lowry acids are Lewis acids, but not all Lewis acids are Brønsted–Lowry acids.

Acid Rain and Environmental Impact

Combustion of fossil fuels produces nonmetal oxides (CO2, SO2, NO2) that dissolve in water to form acids, contributing to acid rain. Acid rain damages buildings, aquatic life, and soil quality.