Chemical Reactions and Equations | Class 10 Science Notes

1.2 Types of Chemical Reactions

Chemical reactions can be classified into several types based on the nature of reactants and products and the changes occurring during the reaction. The main types include combination (synthesis), decomposition, displacement, and double displacement reactions. Combination reactions involve two or more substances combining to form a single product. For example, calcium oxide reacts with water to form calcium hydroxide (slaked lime), releasing heat, making it an exothermic reaction. This reaction is represented as CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat. Other examples include burning of coal (C + O₂ → CO₂) and formation of water (2H₂ + O₂ → 2H₂O). Decomposition reactions are the opposite, where a single compound breaks down into two or more simpler substances, often requiring energy input such as heat, light, or electricity. For instance, heating ferrous sulphate crystals leads to formation of ferric oxide, sulfur dioxide, and sulfur trioxide. Thermal decomposition of calcium carbonate produces calcium oxide and carbon dioxide. Electrolysis of water splits water into hydrogen and oxygen gases. Silver chloride decomposes under sunlight into silver and chlorine gas, a reaction used in photography. Displacement reactions occur when a more reactive element displaces a less reactive element from its compound, such as iron displacing copper from copper sulphate solution (Fe + CuSO₄ → FeSO₄ + Cu). Double displacement reactions involve exchange of ions between two compounds, resulting in formation of new compounds, often producing a precipitate. For example, mixing sodium sulphate and barium chloride solutions forms insoluble barium sulphate precipitate and sodium chloride solution (Na₂SO₄ + BaCl₂ → BaSO₄(s) + 2NaCl). Understanding these types helps in predicting products and understanding reaction mechanisms.

📊 Diagram: Figure 1.3 Formation of slaked lime by the reaction of calcium oxide with water; Figure 1.4 Correct way of heating the boiling tube containing crystals of ferrous sulphate and of smelling the odour; Figure 1.5 Heating of lead nitrate and emission of nitrogen dioxide; Figure 1.6 Electrolysis of water; Figure 1.7 Silver chloride turns grey in sunlight to form silver metal; Figure 1.8 (a) Iron nails dipped in copper sulphate solution; Figure 1.8 (b) Iron nails and copper sulphate solutions compared before and after the experiment; Figure 1.9 Formation of barium sulphate and sodium chloride.

🧪 Activity: Activities 1.4 to 1.10 demonstrating combination, decomposition, displacement, and double displacement reactions including heating ferrous sulphate, lead nitrate, electrolysis of water, silver chloride decomposition, iron nails in copper sulphate, and mixing sodium sulphate with barium chloride.

🔗 Connection: Understanding types of chemical reactions leads to the study of oxidation and reduction processes, which are important in many chemical changes.