Acids and Bases: Structure, Properties, and Reactions

Acids and Bases

Overview and Importance

Acids and bases are fundamental to chemistry, impacting health, household products, foods, and biological processes. Their behavior in aqueous solutions is central to understanding chemical reactions and molecular biology.

• Acids are substances that produce H+ ions in solution.

• Bases are substances that produce OH- ions in solution.

• Acids and bases are present in many foods, medicines, and cleaning products.

Stomach Acid and Heartburn

Stomach acid, primarily hydrochloric acid (HCl), is produced by cells lining the stomach to kill bacteria and activate digestive enzymes. Heartburn occurs when this acid backs up into the esophagus, causing irritation and a burning sensation.

• Large meals and lying down after eating can trigger heartburn.

• Chronic heartburn is known as GERD (gastroesophageal reflux disease).

The Nature and Properties of Acids

Physical and Chemical Properties

Acids exhibit several characteristic properties:

• Sour taste

• Ability to dissolve many metals

• Ability to neutralize bases

• Change blue litmus paper to red

Common Acids and Their Structures

Acids can be classified by their structure:

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

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

• Carboxylic acids: Contain a COOH group; only the first H is acidic (e.g., HC2H3O2, citric acid).

The Nature and Properties of Bases

Physical and Chemical Properties

Bases are substances with the following properties:

• Bitter taste

• Slippery feel

• Ability to turn red litmus paper blue

• Ability to neutralize acids

Common Bases

Bases are found in many household products, such as cleaning agents and antacids.

Definitions of Acids and Bases

Arrhenius, Brønsted-Lowry, and Lewis Definitions

There are three main definitions of acids and bases, each useful in different contexts:

• Arrhenius definition: Acids produce H+ ions; bases produce OH- ions in aqueous solution.

• Brønsted-Lowry definition: Acids are proton (H+) donors; bases are proton acceptors.

• Lewis definition: Acids are electron pair acceptors; bases are electron pair donors.

Arrhenius Acid and Base Reactions

• Arrhenius acids dissociate in water to produce H+ ions.

• Arrhenius bases dissociate in water to produce OH- ions.

$\mathrm{HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)}$

Hydronium Ion Formation

H+ ions are highly reactive and do not exist freely in water; they combine with water molecules to form hydronium ions (H3O+). $\mathrm{H^+ + H_2O \rightarrow H_3O^+}$

Brønsted-Lowry Theory

Acid-Base Reactions and Conjugate Pairs

In Brønsted-Lowry theory, acids donate protons and bases accept protons. Each acid-base reaction produces a conjugate acid and a conjugate base.

• Acid: H+ donor

• Base: H+ acceptor

• Conjugate acid: formed when a base gains a proton

• Conjugate base: formed when an acid loses a proton

$\mathrm{NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)}$

Acid Strength and Ionization

Strong vs. Weak Acids

The strength of an acid is determined by its degree of ionization in water:

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

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

Acid Ionization Constant (Ka)

The acid ionization constant ($K_a$) quantifies the strength of an acid:

• Larger $K_a$ means stronger acid.

• Smaller $K_a$ means weaker acid.

$\mathrm{HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq)}$

Autoionization of Water and pH

Autoionization of Water

Water is amphoteric and can act as both an acid and a base, undergoing autoionization: $\mathrm{2H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)}$

• All aqueous solutions contain both H3O+ and OH- ions.

• Neutral solutions: [H3O+] = [OH-]

Measuring Acidity: pH

The pH scale quantifies the acidity or basicity of a solution:

• pH < 7: acidic

• pH > 7: basic

• pH = 7: neutral

$\mathrm{pH = -\log_{10}[H_3O^+]}$ $\mathrm{[H_3O^+] = 10^{-pH}}$

Summary Table: Common Acids and Bases

Name of Acid	Formula	Name of Base	Formula
Hydrochloric acid	HCl	Sodium hydroxide	NaOH
Hydrobromic acid	HBr	Lithium hydroxide	LiOH
Hydroiodic acid	HI	Potassium hydroxide	KOH
Nitric acid	HNO3	Calcium hydroxide	Ca(OH)2
Sulfuric acid	H2SO4	Barium hydroxide	Ba(OH)2
Acetic acid (weak acid)	HC2H3O2	Ammonia (weak base)	NH3
Hydrofluoric acid (weak acid)	HF

Additional info:

• Acids and bases are classified by their strength, structure, and behavior in solution.

• Understanding acid-base reactions is essential for predicting the outcome of chemical processes in biological, environmental, and industrial contexts.