Acids, Bases, and Salts (Chemistry)

4th Form Chemistry: Acids, Bases, and Salts

Table of Contents

• About Acids - Covers definitions, types, and reactions of acids.

• The Bases - Explains the definition, types, and reactions of bases.

• pH Scale - Discusses methods for testing acidity and basicity.

• Salts - Defines salts, types, and preparation methods.

01. About Acids

Definition of an Acid

• A substance that reacts with a base to form salt and water, typically described as:

  • Corrosive - Can damage or destroy other materials.

  • pH Level - Less than 7 indicates acidity.

  • Litmus Test - Turns blue litmus paper red.

  • Taste - Often has a sour taste.

  • Electrolytic Properties - Can conduct electric current.

Examples of Acids

• Carbonic Acid - Used in aerated drinks.

• Sulphuric Acid - Used in car batteries and paint.

• Tartaric Acid - Important in wine manufacturing and as an ingredient in aspirin.

• Nitric Acid - Commonly used in fertilizers.

• Hydrochloric Acid - Plays a role in digestion.

• Salicylic Acid - Found in aspirin.

• Acetic Acid - Known for vinegar.

• Citric Acid - Present in citrus fruits.

• Laboratory Acids - Used in various lab settings.

Composition of Acids

• All acids contain Hydrogen (e.g., HCl, HF, HNO3).

• Can exist in anhydrous forms as solid, liquid, or gas at room temperature:

  • Solids: Citric and tartaric acid.

  • Liquids: Nitric and sulfuric acid.

  • Gas: Hydrogen chloride.

Ionization of Acids

• To carry an electric current, ions must exist and move freely. In water:

  • Acids ionize to form H+ (hydrogen ions) and negative anions.

  • Hydronium Ions form when H+ ions associate with water molecules.

Example of Ionization

• Hydrochloric Acid Ionization:

  • Reaction: HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq).

  • Simplified formula: HCl(aq) ⟶ H+(aq) + Cl-(aq).

Other Ionization Examples

• Nitric Acid: HNO3(aq) ⟶ H+(aq) + NO3-(aq).

• Sulfuric Acid: H₂SO4(aq) ⟶ 2H+(aq) + SO4^(2-)(aq).

• Phosphoric Acid: H3PO4(aq) ⟶ 3H+(aq) + PO4^(3-)(aq).

• Acetic Acid: CH3COOH(aq) ⟶ CH3COO-(aq) + H+(aq).

Acids as Proton Donors

• Acids donate protons (H+) in reactions, known as proton donors.

  • When hydrogen loses an electron, it becomes a proton.

Classification of Acids

• Acids are categorized as:

  • Strong vs Weak Acids - Complete vs partial ionization in water.

  • Dilute vs Concentrated Acids - Amount of solute in solution.

  • Basicity - Monobasic, dibasic, tribasic.

  • Acid Anhydrides - Compounds forming acids when reacting with water.

Strong vs Weak Acids

• Strong Acids: Fully ionize in water (e.g., sulfuric, nitric).

• Weak Acids: Partially ionize (e.g., hydrofluoric, citric).

Dilute vs Concentrated Acids

• Dilute Acid: High water content, e.g., hydrochloric acid (0.1 mol dm⁻³).

Basicity/Proticity

• Basicity refers to the number of H+ ions produced per molecule in solution.

Acid Anhydrides

• Substances that react with water to form acids. Examples include carbon dioxide and sulfur trioxide.

Organic vs Inorganic Acids

• Inorganic Acids: Contain hydrogen and non-metallic elements (e.g., HCl, H2SO4).

• Organic Acids: Contain carboxyl groups (COOH), e.g., acetic acid.

02. About Bases

Definition of a Base

• Reacts with an acid to form salt and water (neutralization).

  • Includes metal oxides, metal hydroxides, and metal carbonates.

Reactions with Acids

• Bases act as proton acceptors, forming water when OH^- ions combine with H+ ions.

• Classifications include strong and weak bases.

Strong vs Weak Bases

• Strong Bases: Fully ionize in solution (e.g., sodium hydroxide).

• Weak Bases: Partially ionize; e.g., ammonia.

Differences Between Bases and Alkalis

• Bases: Insoluble in water.

• Alkalis: Soluble bases (e.g., NaOH).

Properties of Aqueous Alkalis

• Presence of OH- ions confers characteristic properties:

  • Bitter taste, changes red litmus to blue, pH > 7.

  • Corrosive and feel soapy when touched.

Amphoteric Oxides and Hydroxides

• Certain metal oxides/hydroxides that react as either acids or bases (amphoteric).

Examples and Reactions of Amphoteric Oxides

• Aluminium Hydroxide:

  • Reacts with HCl → AlCl3 + H2O.

  • Reacts with NaOH → NaAlO2 + H2O.

• Zinc Oxide:

  • Reacts with HCl → ZnCl2 + H2O.

  • Reacts with NaOH → Na2ZnO2 + H2O.

Classification of Oxides

• Acidic Oxides: React with bases.

• Basic Oxides: React with acids.

• Neutral Oxides: Do not react with acids or bases.

• Amphoteric Oxides: React with both.

Acidic Oxides

• Non-metals that produce salts and water upon reaction with bases.

• Example reactions include:

  • NaOH + CO2 ⟶ Na2CO3 + H2O.

Basic Oxides

• Metal oxides responding with acids to yield salts and water.

• Example: CuO + 2HNO3 ⟶ Cu(NO3)2 + H2O.

Amphoteric Oxides

• Metal oxides reacting with both acids and strong alkalis.

• Example: PbO with HNO3 and NaOH producing salts and water.

Neutral Oxides

• Do not participate in acid or base reactions (e.g., CO, NO).

03. pH Scale

Understanding the pH Scale

• Measures acidity/alkalinity; ranges from 0 to 14:

  • pH < 7: Acidic (stronger with lower values).

  • pH = 7: Neutral.

  • pH > 7: Alkaline (stronger with higher values).

Measurement Methods

• Universal Indicator: Color indicates pH level.

• pH Meter: An electronic instrument providing precise pH readings.

• Indicators: Determine whether a solution is acidic/alkaline.

Indicators Used in Laboratory

• Litmus: Blue in alkaline, red in acidic.

• Methyl Orange: Red in acidic, yellow in alkaline.

• Phenolphthalein: Colorless in acidic, pink in alkaline.