Lesson 2 - Acids, Bases, Salts, Oxides and pH Scale

• Acids: Substances that dissociate in aqueous solution to release positive hydrogen ions (H+ or hydronium H3O+), producing a sour taste and turning blue litmus paper red.
• Bases and Alkalis: Bases are metal oxides or hydroxides that neutralize acids; an alkali is a water-soluble base that dissociates to release hydroxide ions (OH-), turning red litmus blue.
• Neutralization Reaction: A fundamental chemical reaction between an acid and a base to produce salt and water: Acid + Base -> Salt + Water.
• pH Scale: A numerical scale ranging from 0 to 14 measuring the acidity or alkalinity of an aqueous solution: pH < 7 is acidic, pH = 7 is neutral, and pH > 7 is basic/alkaline.
• Basic Oxides: Metal oxides that react with acids to form salt and water (e.g., Na2O, CaO, CuO); those that dissolve in water form alkalis.
• Acidic Oxides (Acid Anhydrides): Non-metal oxides that dissolve in water to form acids (e.g., CO2, SO3, P2O5) and react with bases to form salt and water.

• Arrhenius Acid-Base Dissociation: [ Formula: Acid: HCl(aq) -> H+(aq) + Cl-(aq) | Base: NaOH(aq) -> Na+(aq) + OH-(aq) ] Defines acids and bases in aqueous media.
• Neutralization General Formulation: [ Formula: H+(aq) + OH-(aq) ----> H2O(l) ] Net ionic equation of acid-base neutralization.
• pH Definition: [ Formula: pH = -log10 [H+] | pH + pOH = 14 ] At 25°C in aqueous solutions.

Chemical compounds are categorized into four major families: acids, bases, oxides, and salts.

• Acids: Characterized by a chemical formula beginning with hydrogen (except organic acids which end in -COOH).
• Examples: Hydrochloric acid (HCl), Sulfuric acid (H2SO4), Nitric acid (HNO3).
• Physical and chemical properties: sour taste, corrosive to metals (releasing flammable hydrogen gas: Zn + 2HCl -> ZnCl2 + H2↑), and turning litmus paper red.

• Bases and Alkalis: An alkali is a subset of bases that dissolve in water to produce hydroxide ions (OH-).
• Famous rule: All alkalis are bases, but not all bases are alkalis!
• (For example, CuO and Fe(OH)3 are bases that do not dissolve in water, hence they are not alkalis).
• Properties: bitter taste, soapy feel, and turning litmus paper blue.

Oxides are binary compounds formed by the combination of an element with oxygen:

1. • Basic Oxides: Oxides of metals (e.g., MgO, CaO, Na2O).
   • Basic oxides that dissolve in water form alkaline solutions: Na2O + H2O -> 2NaOH.
   • Basic oxides react with acids to produce salt and water: CuO + H2SO4 -> CuSO4 + H2O.
2. • Acidic Oxides: Oxides of non-metals (e.g., CO2, SO2, SO3).
   • When dissolved in water, they yield acids: CO2 + H2O -> H2CO3 (carbonic acid), SO3 + H2O -> H2SO4 (sulfuric acid).
   • Acidic oxides react with alkalis to form salt and water: CO2 + Ca(OH)2 -> CaCO3↓ + H2O (turbidity in limewater).

3. Amphoteric Oxides: Oxides that react with acids as bases and with bases as acids, producing salt and water in both cases (e.g., Al2O3, ZnO, SnO, PbO).

The pH Scale: Developed by Sørensen to express hydrogen ion concentration conveniently:

• Strongly acidic: pH 0 to 3 (e.g., gastric juice, battery acid).

• Weakly acidic: pH 4 to 6 (e.g., vinegar, coffee, milk).

• Neutral: pH 7.0 (pure distilled water at 25°C).

• Weakly basic: pH 8 to 10 (e.g., baking soda, sea water).

• Strongly basic: pH 11 to 14 (e.g., bleach, drain cleaner, NaOH).

Salts: Ionic compounds composed of a positive metallic cation and a negative acid radical anion. Classification by solubility:

• Soluble Salts: All nitrates (NO3-), all acetates (CH3COO-), all sodium (Na+), potassium (K+), and ammonium (NH4+) salts.

• Insoluble Salts (Precipitates): Silver chloride (AgCl, white), Lead iodide (PbI2, yellow), Barium sulfate (BaSO4, white), Calcium carbonate (CaCO3, white).

Table: Comparative Distinction Between Acids and Bases (Alkalis)

• Identifying Reaction Types and Predicting Salt FormationProblem Statement & Given Data: Write the balanced chemical equations for the following reactions and state the type of reaction: (1) Hydrochloric acid with sodium hydroxide, (2) Carbon dioxide with limewater, (3) Zinc metal with dilute sulfuric acid. Step-by-Step Mathematical & Chemical Solution: [1] Reaction 1: Acid + Base Neutralization: [2] HCl(aq) + NaOH(aq) ----> NaCl(aq) + H2O(l). [3] Type: Neutralization / Double displacement; produces sodium chloride salt and water. [4] Reaction 2: Acidic Oxide with Base: [5] CO2(g) + Ca(OH)2(aq) ----> CaCO3(s)↓ + H2O(l). [6] Type: Precipitation; forms white insoluble calcium carbonate precipitate causing limewater turbidity. [7] Reaction 3: Metal with Acid Single Displacement: [8] Zn(s) + H2SO4(aq) ----> ZnSO4(aq) + H2(g)↑. [9] Type: Single displacement; produces zinc sulfate salt and flammable hydrogen gas which pops with a lighted splint.💡 High-Yield Exam Shortcut & Pitfall Alert: Exam Tip: All neutralization reactions produce Salt + Water!

• All alkalis are bases, but not all bases are alkalis (only water-soluble bases are alkalis).
• The pH of acids is less than 7, neutral is 7, and alkalis is greater than 7.
• Acids turn blue litmus red; alkalis turn red litmus blue.
• Metal oxides are basic; non-metal oxides are acidic; Al2O3 and ZnO are amphoteric.
• All sodium, potassium, ammonium, and nitrate salts dissolve completely in water.