Acids and Bases: Structures and Types

Common Acids and Their Uses

Acids are substances that can donate protons (H+) or accept electron pairs. They are widely used in industry, food, and biological systems. The table below summarizes some common acids and their applications:

Acid Structures

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

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

• Carboxylic acids: Contain a –COOH group; only the first H is acidic (e.g., acetic acid, citric acid).

Common Bases and Their Uses

Bases are substances that can accept protons or donate hydroxide ions (OH−). They are important in cleaning, manufacturing, and biological systems.

Acid-Base Definitions and Theories

Arrhenius Definition

• Acid: Produces H+ ions in solution (which associate with water to form H3O+).

• Base: Produces OH− ions in solution.

Brønsted–Lowry Definition

• Acid: Proton (H+) donor.

• Base: Proton (H+) acceptor.

• All Arrhenius acids and bases are also Brønsted–Lowry acids and bases, but the Brønsted–Lowry theory is more general.

Conjugate Acid–Base Pairs

In Brønsted–Lowry reactions, acids and bases form conjugate pairs:

• When an acid donates a proton, it becomes its conjugate base.

• When a base accepts a proton, it becomes its conjugate acid.

Amphoteric Substances

Amphoteric substances can act as either acids or bases. Water is a classic example:

• As a base: HCl(aq) + H2O(l) → Cl−(aq) + H3O+(aq)

• As an acid: NH3(aq) + H2O(l) → NH4+(aq) + OH−(aq)

Acid and Base Strength

Strong vs. Weak Acids

• Strong acids: Ionize completely in water; strong electrolytes.

• Weak acids: Ionize only partially; weak electrolytes.

Acid Strength and Ionization Constant (Ka)

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

• Larger Ka = stronger acid

• Smaller Ka = weaker acid

The equilibrium expression for acid ionization is:

Strong vs. Weak Bases

• Strong bases: Ionize completely in water; strong electrolytes (e.g., NaOH, KOH).

• Weak bases: Ionize only partially; weak electrolytes (e.g., NH3).

Base Strength and Ionization Constant (Kb)

The strength of a base is measured by its base ionization constant, Kb:

Autoionization of Water and pH

Autoionization of Water

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

• The ion product constant for water is at 25°C.

pH and pOH

• pH measures acidity:

• pOH measures basicity:

• Relationship:

• Lower pH = more acidic; higher pH = more basic.

pH of Common Substances

Polyprotic Acids

Definition and Ionization Steps

• Polyprotic acids can donate more than one proton, ionizing in steps.

• Each step has its own Ka value: Ka1 > Ka2 > Ka3.

• For most calculations, only the first ionization is significant except for sulfuric acid (H2SO4).

Conjugate Acid-Base Pairs: Examples and Practice

Identifying Conjugate Pairs

• Conjugate base: Remove one H+ from the acid.

• Conjugate acid: Add one H+ to the base.

Acid-Base Properties of Salts

Classification of Salt Solutions

• Neutral: Salts from strong acid and strong base (e.g., NaCl).

• Basic: Salts from strong base and weak acid (e.g., NaCH3COO).

• Acidic: Salts from strong acid and weak base (e.g., NH4Cl).

Summary Table: Acid and Base Strength

Practice Problems and Examples

Sample Calculations

• Calculate [OH−] in a solution given [H3O+].

• Find pH or pOH for a given concentration.

• Determine Ka or Kb from pH and concentration data.

Additional info:

• Acid and base strength is determined by the extent of ionization and the equilibrium constant (Ka or Kb).

• Polyprotic acids require consideration of multiple ionization steps for accurate pH calculations.

• Conjugate pairs are fundamental to understanding acid-base reactions and equilibrium.