Types of reactions, balancing equations, oxidation-reduction, and corrosion
A chemical reaction is a process in which substances (reactants) are converted into new substances (products) with different properties. During a chemical reaction, bonds in reactants break and new bonds form in products.
• Change in colour (e.g., copper turns black when heated in air)
• Change in state (e.g., liquid → gas, solid → liquid)
• Evolution of gas (e.g., bubbles when vinegar + baking soda)
• Formation of precipitate (insoluble solid formed in solution)
• Change in temperature (heat absorbed or released)
• Change in smell (new odour produced)
A chemical equation is a symbolic representation of a chemical reaction.
Format: Reactants → Products
Example: Magnesium + Oxygen → Magnesium Oxide
Word equation: 2Mg + O₂ → 2MgO
States: (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous (dissolved in water)
A balanced equation obeys the Law of Conservation of Mass: atoms of each element must be equal on both sides.
Method: Hit and Trial
Example: H₂ + O₂ → H₂O
Count: Left: 2H, 2O | Right: 2H, 1O → Oxygen not balanced
Balanced: 2H₂ + O₂ → 2H₂O ✓ (4H, 2O on both sides)
Start with Fe₃O₄ (most complex): needs 3Fe, 4O on left
3Fe + 4H₂O → Fe₃O₄ + H₂
Count: Left: 3Fe, 4O, 8H | Right: 3Fe, 4O, 2H → H not balanced
Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂
Verify: 3Fe, 4O, 8H on both sides ✓
Two or more substances combine to form a single product: A + B → AB
A single compound breaks into two or more substances: AB → A + B
Requires energy input: heat (thermal), electricity (electrolysis), or light (photolysis)
A more reactive element displaces a less reactive element from its compound: A + BC → AC + B
Two compounds exchange ions to form two new compounds. Often one product is a precipitate (insoluble solid): AB + CD → AD + CB
Oxidation: Gain of oxygen OR loss of hydrogen OR loss of electrons
Reduction: Loss of oxygen OR gain of hydrogen OR gain of electrons
Redox reaction: Oxidation and reduction occur simultaneously
CuO loses oxygen → CuO is reduced (it's an oxidising agent)
H₂ gains oxygen → H₂ is oxidised (it's a reducing agent)
Remember: OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons)
Corrosion is the slow deterioration of a metal due to chemical reactions with moisture and air in the environment.
• Rusting of iron: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃.xH₂O (rust, reddish-brown)
• Blackening of silver: 2Ag + H₂S → Ag₂S + H₂ (tarnishing)
• Greenish layer on copper: Cu + CO₂ + H₂O → basic copper carbonate
Rancidity is the oxidation of oils and fats in food, producing an unpleasant smell and taste.
Prevention:
• Store food in airtight containers
• Add antioxidants (BHA, BHT) — used in chips, fried snacks
• Refrigerate food (low temperature slows oxidation)
• Replace air with nitrogen gas (nitrogen flushing in packaged food)