Learning Objectives
- Write and balance chemical equations
- Identify types of chemical reactions
- Understand the effects of oxidation and reduction in everyday life
- Recognise observable changes during chemical reactions
Key Concepts
Chemical Equations
A chemical equation is a symbolic representation of a chemical reaction using formulae and symbols. Reactants are written on the left and products on the right, separated by an arrow (→).
Balancing: A balanced equation has equal numbers of atoms of each element on both sides, satisfying the Law of Conservation of Mass.
Physical states are indicated as: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution.
Types of Chemical Reactions
- Combination Reaction: Two or more reactants combine to form a single product. Example: 2Mg + O₂ → 2MgO
- Decomposition Reaction: A single reactant breaks down into two or more products. Can be thermal, electrolytic, or photolytic. Example: 2H₂O → 2H₂ + O₂ (electrolysis)
- Displacement Reaction: A more reactive element displaces a less reactive one from its compound. Example: Fe + CuSO₄ → FeSO₄ + Cu
- Double Displacement Reaction: Exchange of ions between two compounds, often forming a precipitate. Example: Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl
Oxidation and Reduction (Redox Reactions)
Oxidation: Gain of oxygen or loss of hydrogen by a substance.
Reduction: Loss of oxygen or gain of hydrogen by a substance.
In a redox reaction, oxidation and reduction occur simultaneously. The substance that gets oxidised is the reducing agent, and the substance that gets reduced is the oxidising agent.
Effects of Oxidation in Daily Life
Corrosion: Metals are attacked by moisture, acids, or gases. Example: Rusting of iron (Fe₂O₃·xH₂O).
Rancidity: Oxidation of fats and oils in food, causing bad taste and smell. Prevented by adding antioxidants, storing in airtight containers, or filling with nitrogen gas.
Summary
Chemical reactions involve the transformation of reactants into products with new properties. They are represented by balanced chemical equations. Reactions are classified as combination, decomposition, displacement, double displacement, and redox reactions. Oxidation and reduction always occur together. Corrosion and rancidity are everyday effects of oxidation.
Important Terms
- Reactants
- Substances that undergo a chemical change in a reaction
- Products
- New substances formed as a result of a chemical reaction
- Precipitate
- An insoluble solid formed during a double displacement reaction
- Redox Reaction
- A reaction in which both oxidation and reduction occur simultaneously
- Corrosion
- Slow deterioration of a metal surface due to reaction with environment
Quick Revision
- Always balance chemical equations to satisfy conservation of mass
- Combination: A + B → AB; Decomposition: AB → A + B
- Displacement: A + BC → AC + B; Double displacement: AB + CD → AD + CB
- Oxidation = gain of O₂ or loss of H₂; Reduction = loss of O₂ or gain of H₂
- Rancidity is prevented by antioxidants, airtight containers, refrigeration, or nitrogen flushing