Unit 1 - Lesson 1: Types of Chemical Reactions

Chemical reactions constitute the foundation of vital biological processes and modern industrial synthesis. Thermal decomposition reactions involve the breaking down of complex chemical compounds under heat into their constituent elements or simpler compounds. When red mercuric oxide (2HgO) is heated in a dry test tube, it thermally decomposes into liquid silvery mercury (2Hg) while releasing colorless oxygen gas (O₂), which rekindles a glowing wooden splint. Similarly, green copper carbonate (CuCO₃) decomposes upon heating into black copper oxide (CuO) and carbon dioxide gas (CO₂), which turns clear limewater turbid. Blue copper hydroxide (Cu(OH)₂) turns black releasing water vapor, and white sodium nitrate (2NaNO₃) decomposes into yellowish-white sodium nitrite (2NaNO₂) with oxygen liberation.

Chemists arranged metals in descending order according to their degree of chemical reactivity into the (Chemical Activity Series). Highly active alkali and alkaline earth metals such as potassium (K) and sodium (Na) lead the series, while less active metals like copper (Cu), silver (Ag), and gold (Au) lie below hydrogen. In (Simple Substitution Reactions), an active element displaces a less active element from its aqueous salt solution or acid. For example, zinc (Zn) or magnesium (Mg) rapidly displaces hydrogen from dilute hydrochloric acid (2HCl), forming zinc chloride (ZnCl₂) and effervescing hydrogen gas (H₂) that burns with a characteristic pop sound. When magnesium strip is submerged in blue copper sulfate (CuSO₄) solution, magnesium displaces copper, forming colorless magnesium sulfate (MgSO₄) and depositing red elemental copper precipitate.

(Double Substitution Reactions) occur through mutual ion exchange between the radicals (ions) of two reacting compounds to generate two entirely new compounds without changes in oxidation states. These reactions are categorized into three primary types: 1. (Neutralization Reaction): The reaction between an acid and an alkali to form salt and water: [HCl + NaOH -> NaCl + H₂O]. 2. (Acid-Carbonate Reaction): Reaction of dilute hydrochloric acid with solid sodium carbonate: [2HCl + Na₂CO₃ -> 2NaCl + H₂O + CO₂^], producing active effervescence of carbon dioxide. 3. (Precipitation Reaction): Mutual interaction between two soluble aqueous salts producing an insoluble solid precipitate: [AgNO₃ + NaCl -> NaNO₃ + AgCl v], instantly yielding a dense white precipitate of silver chloride that turns violet upon prolonged exposure to sunlight.

Oxidation and reduction are interpreted through two fundamental perspectives: 1. (Traditional Concept): Oxidation is a chemical process leading to an increase in oxygen percentage or decrease in hydrogen (e.g., passing H₂ over heated CuO: H₂ is oxidized to H₂O while CuO is reduced to Cu). The substance providing oxygen is the oxidizing agent, while the substance taking oxygen is the reducing agent. 2. (Modern Electronic Concept): Oxidation is a process in which an atom loses one or more valence electrons (such as 2Na -> 2Na⁺ + 2e⁻, where sodium is oxidized and acts as the reducing agent). Reduction is the acquisition of one or more electrons (such as Cl₂ + 2e⁻ -> 2Cl⁻, where chlorine is reduced and acts as the oxidizing agent). Oxidation and reduction are concurrent, complementary processes.