A chemical reaction is a process in which one or more substances (reactants) are converted into one or more new substances (products) with different properties. Chemical reactions are represented using chemical equations.
Key Concepts
Chemical Equation: A symbolic representation of a chemical reaction using formulae of reactants and products.
Balanced Chemical Equation: An equation where the number of atoms of each element is equal on both sides (Law of Conservation of Mass).
- Types of Chemical Reactions:
- Combination Reaction: Two or more substances combine to form a single product. A + B → AB
- Decomposition Reaction: A single compound breaks into two or more simpler substances. AB → A + B
- Displacement Reaction: A more reactive element displaces a less reactive element from its compound. A + BC → AC + B
- Double Displacement Reaction: Exchange of ions between two compounds. AB + CD → AD + CB
- Oxidation and Reduction (Redox): Simultaneous gain and loss of oxygen/hydrogen or electrons.
Exothermic Reaction: Reaction that releases energy (heat). Example: Burning of coal.
Endothermic Reaction: Reaction that absorbs energy. Example: Decomposition of calcium carbonate.
Corrosion: Slow eating away of metals due to reaction with moisture and oxygen (e.g., rusting of iron).
Rancidity: Oxidation of fats and oils in food causing bad smell and taste.
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Write the balanced equation for burning of magnesium in air.
- Unbalanced: Mg + O2 → MgO
- Balanced: 2Mg + O2 → 2MgO
- This is a combination reaction and is also exothermic.
Identify the type of reaction: CaCO3 → CaO + CO2 (on heating)
- One compound breaks into two simpler substances → Decomposition Reaction.
- Heat is absorbed → Endothermic reaction.
Balance the equation: Fe + H2O → Fe3O4 + H2
- Balanced: 3Fe + 4H2O → Fe3O4 + 4H2
- Iron displaces hydrogen from water → Displacement Reaction.
Identify oxidation and reduction in: CuO + H2 → Cu + H2O
- CuO loses oxygen → CuO is reduced (gains electrons).
- H2 gains oxygen → H2 is oxidised (loses electrons).
- This is a redox reaction.
Write the double displacement reaction between sodium sulphate and barium chloride.
- Na2SO4 + BaCl2 → BaSO4 + 2NaCl
- White precipitate of BaSO4 is formed (insoluble salt).
Explain why chips manufacturers flush nitrogen gas into packets.
- Chips (fried food) contain fats and oils that can get oxidised (rancidity).
- Nitrogen is an inert gas that prevents oxidation, thus keeping chips fresh longer.
Silver chloride turns grey in sunlight. Write the equation and identify the type.
- 2AgCl → 2Ag + Cl2 (sunlight)
- One substance breaks down → Decomposition reaction (photolytic decomposition).
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Key Formulas
- Combination: A + B → AB
- Decomposition: AB → A + B
- Displacement: A + BC → AC + B (if A is more reactive than B)
Common mistakes
- Forgetting to balance equations: Always count atoms on both sides.
- Confusing oxidation and reduction: Oxidation = loss of electrons / gain of oxygen; Reduction = gain of electrons / loss of oxygen.
- Misidentifying reaction types: Check if one or two reactants produce one or multiple products.
Summary
Chemical equations must be balanced to satisfy the Law of Conservation of Mass. Reactions are classified as combination, decomposition, displacement, double displacement, and redox. Energy changes (exo/endothermic) are key features. Corrosion and rancidity are harmful effects of oxidation in daily life.