Acids and Bases: Key Concepts and Applications

Chapter 14: Acids and Bases

Introduction

This chapter explores the fundamental properties, definitions, and behaviors of acids and bases, essential concepts in introductory chemistry. Understanding acids and bases is crucial for interpreting chemical reactions, biological processes, and industrial applications.

Properties of Acids

General Characteristics

• Sour Taste: Acids typically taste sour, as experienced with foods like citrus fruits and candies coated with citric and tartaric acids.

• Dissolve Many Metals: Acids can react with and dissolve metals, a property often dramatized in popular culture (e.g., spy movies).

• Litmus Test: Acids turn blue litmus paper red due to the presence of hydrogen ions.

Examples and Applications

• Hydrochloric Acid (HCl): Found in laboratories and stomach acid; used in cleaning metals and food processing.

• Sulfuric Acid (H2SO4): Most produced chemical in the US; used in fertilizers, batteries, and explosives.

• Acetic Acid (CH3COOH): Responsible for vinegar's sour taste; example of a carboxylic acid.

• Carboxylic Acids: Includes citric acid (lemons/limes) and malic acid (apples, grapes, wine).

Properties of Bases

General Characteristics

• Bitter Taste: Bases are less common in foods due to their bitterness (e.g., caffeine in coffee).

• Slippery Feel: Bases react with skin oils to form soap-like substances, giving a slippery sensation.

• Litmus Test: Bases turn red litmus paper blue.

Examples and Applications

• Sodium Hydroxide (NaOH) and Potassium Hydroxide (KOH): Used in soap, cleaning products, and manufacturing.

• Sodium Bicarbonate (NaHCO3): Commonly known as baking soda; neutralizes stomach acid.

Definitions of Acids and Bases

Arrhenius Definition

• Acid: Produces H+ ions in aqueous solution.

• Base: Produces OH- ions in aqueous solution.

Example Equation:

• $ext{HCl (aq)} ightarrow ext{H}^+ ( ext{aq}) + ext{Cl}^- ( ext{aq})$

• $ext{NaOH (aq)} ightarrow ext{Na}^+ ( ext{aq}) + ext{OH}^- ( ext{aq})$

Brønsted–Lowry Definition

• Acid: Proton donor.

• Base: Proton acceptor.

Example Equation:

• $ext{HCl (aq)} + ext{H}_2 ext{O (l)} ightarrow ext{H}_3 ext{O}^+ ( ext{aq}) + ext{Cl}^- ( ext{aq})$

• $ext{NH}_3 ( ext{aq}) + ext{H}_2 ext{O (l)} ightleftharpoons ext{NH}_4^+ ( ext{aq}) + ext{OH}^- ( ext{aq})$

Acid-Base Reactions

Neutralization

• Acids and bases react to form water and a salt.

• Net Ionic Equation: $ext{H}^+ ( ext{aq}) + ext{OH}^- ( ext{aq}) ightarrow ext{H}_2 ext{O (l)}$

Conjugate Acid-Base Pairs

• Two substances related by the transfer of a proton.

• Example: $ext{NH}_3$ and $ext{NH}_4^+$

Strength of Acids and Bases

Strong Acids and Bases

• Strong Acid: Completely ionizes in solution (e.g., HCl).

• Strong Base: Completely dissociates in solution (e.g., NaOH).

Weak Acids and Bases

• Weak Acid: Partially ionizes in solution (e.g., HF).

• Weak Base: Partially ionizes or reacts with water to produce OH- (e.g., NH3).

Self-Ionization of Water

Concept and Equation

• Water can act as both an acid and a base (amphoteric).

• Self-ionization produces equal concentrations of H3O+ and OH- in pure water.

• $ext{H}_2 ext{O (l)} + ext{H}_2 ext{O (l)} ightleftharpoons ext{H}_3 ext{O}^+ ( ext{aq}) + ext{OH}^- ( ext{aq})$

• At 25°C: $[ ext{H}_3 ext{O}^+] = [ ext{OH}^-] = 1.0 imes 10^{-7} ext{M}$

• Ion product constant: $K_w = [ ext{H}_3 ext{O}^+][ ext{OH}^-] = 1.0 imes 10^{-14}$

The pH Scale

Definition and Calculation

• pH expresses the acidity or basicity of a solution based on H3O+ concentration.

• $ext{pH} = - ext{log}[ ext{H}_3 ext{O}^+]$

• At 25°C:

  • pH < 7: acidic

  • pH = 7: neutral

  • pH > 7: basic

• Each unit change in pH represents a tenfold change in H3O+ concentration.

Summary Table: Properties of Acids and Bases

Property	Acids	Bases
Taste	Sour	Bitter
Feel	---	Slippery
Litmus Test	Turns blue litmus red	Turns red litmus blue
Reaction with Metals	Dissolves many metals	---

Key Equations

• $ext{HCl (aq)} ightarrow ext{H}^+ ( ext{aq}) + ext{Cl}^- ( ext{aq})$

• $ext{NaOH (aq)} ightarrow ext{Na}^+ ( ext{aq}) + ext{OH}^- ( ext{aq})$

• $ext{H}_2 ext{O (l)} + ext{H}_2 ext{O (l)} ightleftharpoons ext{H}_3 ext{O}^+ ( ext{aq}) + ext{OH}^- ( ext{aq})$

• $K_w = [ ext{H}_3 ext{O}^+][ ext{OH}^-] = 1.0 imes 10^{-14}$

• $ext{pH} = - ext{log}[ ext{H}_3 ext{O}^+]$

Additional info:

• Amphoteric substances can act as both acids and bases (e.g., water).

• Conjugate acid-base pairs are central to understanding acid-base reactions.

• Strong electrolytes conduct electricity well due to complete ionization/dissociation.