Overview
This lecture introduces chemical reactions and equations, explains how to represent and balance them, and details the types and characteristics of chemical reactions observed in daily life.
Chemical Reactions and Their Indicators
- A chemical reaction leads to a change in the nature and identity of substances.
- Indicators of a chemical reaction: change in state, color, temperature, or evolution of gas.
- Examples from daily life include rusting, fermentation, digestion, and respiration.
Writing and Balancing Chemical Equations
- Word equations describe reactants and products using words (e.g., Magnesium + Oxygen → Magnesium oxide).
- Chemical equations use chemical symbols and formulas for brevity.
- A balanced equation has equal numbers of each type of atom on both sides, following the law of conservation of mass.
- The "hit-and-trial" method is used to balance equations by adjusting coefficients.
Representing Chemical Equations
- Reactants are on the left and products on the right; arrows indicate the direction of the reaction.
- Physical states are shown: (s) solid, (l) liquid, (g) gas, (aq) aqueous.
- Reaction conditions (temperature, pressure, catalysts) can be noted above/below the arrow.
Types of Chemical Reactions
Combination Reaction
- Two or more substances combine to form a single product (e.g., CaO + H₂O → Ca(OH)₂).
- Often exothermic, releasing heat.
Decomposition Reaction
- A single compound breaks down into two or more simpler substances (e.g., CaCO₃ → CaO + CO₂).
- Can be triggered by heat (thermal), light (photolytic), or electricity (electrolytic).
- Endothermic as energy is absorbed to break bonds.
Displacement Reaction
- A more reactive element displaces a less reactive element from its compound (e.g., Fe + CuSO₄ → FeSO₄ + Cu).
Double Displacement Reaction
- Exchange of ions between reactants forms two new compounds (e.g., Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl).
- Precipitation reactions produce insoluble products.
Oxidation and Reduction (Redox Reactions)
- Oxidation: Gain of oxygen or loss of hydrogen.
- Reduction: Loss of oxygen or gain of hydrogen.
- One substance is oxidised and another reduced in redox reactions.
Effects of Oxidation in Daily Life
Corrosion
- Metals react with environmental substances leading to rust or tarnish (e.g., iron rusting).
Rancidity
- Fats/oils oxidise, causing spoilage; prevented by antioxidants or by storing in nitrogen.
Key Terms & Definitions
- Reactants — Substances that undergo change in a reaction.
- Products — New substances produced in a reaction.
- Balanced Equation — Equation with equal numbers of each atom on both sides.
- Combination Reaction — Two or more reactants form one product.
- Decomposition Reaction — One compound breaks into two or more products.
- Displacement Reaction — An element displaces another from a compound.
- Double Displacement Reaction — Ions are exchanged between two compounds.
- Oxidation — Gain of oxygen or loss of hydrogen.
- Reduction — Loss of oxygen or gain of hydrogen.
- Redox Reaction — Involves both oxidation and reduction.
- Precipitation — Formation of an insoluble solid in solution.
- Corrosion — Deterioration of metals via chemical reactions.
- Rancidity — Spoilage of fats/oils due to oxidation.
Action Items / Next Steps
- Practice balancing chemical equations as per the "hit-and-trial" method.
- Complete assigned exercises on identifying and writing types of reactions.
- Perform suggested group activities to distinguish between exothermic and endothermic reactions.
- Review definitions and examples of key reaction types for exam preparation.