Acids and Bases: Properties, Definitions, Reactions, and Applications

Acids and Bases

Properties of Acids

Acids are a fundamental class of chemical compounds characterized by their ability to donate protons (H+) and exhibit distinctive physical and chemical properties.

• Sour taste: Many acids, such as citric acid in lemons, have a sour flavor.

• Ability to dissolve many metals: Acids can react with metals, often producing hydrogen gas.

• Ability to neutralize bases: Acids react with bases to form water and salts.

• Change blue litmus paper to red: This is a classic test for acidity.

Properties of Bases

Bases are compounds that accept protons or release hydroxide ions (OH-) in solution. They are commonly found in cleaning products and industrial applications.

• Bitter taste: Bases such as sodium bicarbonate have a bitter flavor.

• Alkaloids: Many plant products are alkaline and often poisonous.

• Feel slippery: Bases like soap feel slippery to the touch.

• Ability to turn red litmus paper blue: This is a classic test for basicity.

• Ability to neutralize acids: Bases react with acids to form water and salts.

Litmus Tests for Acids and Bases

Litmus paper is a simple indicator used to distinguish between acids and bases. Blue litmus paper turns red in acidic solutions, while red litmus paper turns blue in basic solutions.

• Acidic solution: Blue litmus turns red.

• Basic solution: Red litmus turns blue.

Definitions of Acids and Bases

Arrhenius Definition

The Arrhenius definition is one of the earliest and simplest ways to classify acids and bases:

• Acid: Increases the concentration of hydrogen ions (H+) in water.

• Base: Increases the concentration of hydroxide ions (OH-) in water.

Example:

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

• $\mathrm{NaOH(s) \rightarrow Na^+(aq) + OH^-(aq)}$

Limitations of the Arrhenius Theory

• Does not explain the basicity of ammonia and similar compounds.

• Applies only to reactions in aqueous solution.

• Protons (H+) do not exist freely in water; they form hydronium ions ($\mathrm{H_3O^+}$).

Brønsted–Lowry Definition

The Brønsted–Lowry theory expands the concept of acids and bases:

• Acid: Proton donor.

• Base: Proton acceptor.

Example: The reaction of hydrochloric acid with water:

• $\mathrm{HCl + H_2O \rightarrow Cl^- + H_3O^+}$

Ammonia as a Brønsted–Lowry base:

• $\mathrm{NH_3 + H_2O \rightarrow NH_4^+ + OH^-}$

Acidic and Basic Anhydrides

Acidic Anhydrides

Nonmetal oxides are acidic anhydrides; when water is added, they form acids.

• Example: $\mathrm{SO_2 + H_2O \rightarrow H_2SO_3}$

Basic Anhydrides

Metal oxides are basic anhydrides; when water is added, they form bases.

• Example: $\mathrm{BaO + H_2O \rightarrow Ba(OH)_2}$

Strong and Weak Acids and Bases

Strong Acids

Strong acids ionize completely in water solution. The seven strong acids are:

• Hydrochloric acid (HCl)

• Hydrobromic acid (HBr)

• Hydroiodic acid (HI)

• Nitric acid (HNO3)

• Sulfuric acid (H2SO4)

• Chloric acid (HClO3)

• Perchloric acid (HClO4)

Weak Acids

Weak acids ionize only partially in water solution. Acetic acid (HC2H3O2) is a common example.

Strong and Weak Bases

Strong bases ionize or dissociate completely in water solution. Weak bases ionize only partially.

Neutralization Reactions

Definition and Example

Neutralization is the reaction of an acid with a base, forming water and a salt.

• $\mathrm{HNO_3 (aq) + KOH (aq) \rightarrow H_2O (l) + KNO_3 (aq)}$

• $\mathrm{NaOH (aq) + HCl (aq) \rightarrow NaCl (aq) + H_2O (l)}$

The pH Scale

Definition and Calculation

The pH scale expresses the acidity or basicity of a solution, ranging from 0 (very acidic) to 14 (very basic). A pH of 7 is neutral.

• Acidic: pH < 7

• Neutral: pH = 7

• Basic: pH > 7

Formula:

• $\mathrm{pH = -\log [H_3O^+]}$

Example: For $[H_3O^+] = 1 \times 10^{-4}$, pH = 4.0

Buffers and Conjugate Acid–Base Pairs

Conjugate Acid–Base Pairs

Conjugate acid–base pairs are compounds or ions that differ by one proton. Reactions between acids and bases always yield their conjugate bases and acids.

• Example: The chloride ion (Cl-) is the conjugate base of hydrochloric acid (HCl).

Buffer Solutions

Buffer solutions contain a weak acid and its conjugate base. They maintain a nearly constant pH when small amounts of acid or base are added.

Acid Rain

Formation and Effects

Acid rain is caused by nonmetal oxides in the air reacting with water to form acidic solutions. Rain with a pH less than 5.6 is considered acid rain. Major contributors include carbon dioxide (CO2), sulfur dioxide (SO2), nitrogen monoxide (NO), and nitrogen dioxide (NO2).

Antacids: A Basic Remedy

Medical Application

Antacids are basic compounds used to neutralize excess stomach acid and relieve symptoms of hyperacidity.

Acids and Bases in Industry and at Home

Industrial and Household Uses

Acids and bases are widely used in industry and at home. Sulfuric acid is used in fertilizer and battery production, while hydrochloric acid is used for cleaning and rust removal. Bases such as sodium hydroxide are used in soap and detergent manufacturing.