Chemical Reactions and Equations

A chemical reaction forms a NEW substance. Signs: change of state, change of colour, evolution of a gas, change in temperature — any of these hints a reaction happened.

Magnesium ribbon burns with a dazzling white flame forming white MgO powder. Clean the ribbon with sandpaper first — to remove the MgO layer so the fresh metal burns readily.

Word equation → skeletal equation (symbols, unbalanced) → balanced equation. Balancing uses the hit-and-trial method.

Balance equations because of the law of conservation of mass: mass can neither be created nor destroyed in a chemical reaction — so the number of atoms of each element stays the same on both sides. NEVER change a formula to balance — only adjust coefficients.

State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water). Reaction conditions (heat Δ, catalyst, light) are written above/below the arrow.

Combination reaction: A + B → AB. Example: CaO (quick lime) + H₂O → Ca(OH)₂ (slaked lime) + heat — the vessel gets hot. Also: C + O₂ → CO₂, 2H₂ + O₂ → 2H₂O.

Exothermic = heat released (burning fuels, respiration!). Endothermic = energy absorbed (most decompositions need heat, light or electricity).

Decomposition reaction: AB → A + B. Thermal: green FeSO₄ → brown Fe₂O₃ + SO₂ + SO₃ (burning-sulphur smell); CaCO₃ → CaO + CO₂; and 2Pb(NO₃)₂ →Δ 2PbO + 4NO₂ + O₂ — BROWN FUMES of nitrogen dioxide (favourite board question). Electrolytic: water → H₂ + O₂ (hydrogen volume is DOUBLE oxygen). Photolytic: 2AgCl → 2Ag + Cl₂ (white → grey in light; basis of B&W photography).

Displacement reaction: a MORE reactive element displaces a less reactive one. Fe + CuSO₄ → FeSO₄ + Cu: blue solution turns green, brown copper coats the nail. Also Zn + CuSO₄, Pb + CuCl₂.

Double displacement: two compounds exchange ions. Na₂SO₄ + BaCl₂ → BaSO₄↓ (white precipitate) + 2NaCl. A precipitate is an insoluble solid — hence 'precipitation reaction'.

Oxidation = gain of oxygen OR loss of hydrogen. Reduction = loss of oxygen OR gain of hydrogen. They ALWAYS occur together → redox. Example: CuO + H₂ → Cu + H₂O (CuO reduced, H₂ oxidised).

Copper powder heated in air turns black (2Cu + O₂ → 2CuO); pass hydrogen over hot CuO and it turns brown again — the reverse redox pair.

Corrosion: metals attacked by moisture + air — iron forms reddish-brown rust (needs BOTH air and water), silver turns black, copper turns green. Prevention: painting, oiling, greasing, galvanisation (the process of coating with zinc), chrome plating, alloying.

Rancidity: fats and oils in food get oxidised → bad smell and taste. Prevention: antioxidants, airtight containers, refrigeration, and flushing packets with nitrogen (why chips packets are puffed).

Respiration is an exothermic reaction: glucose + oxygen → carbon dioxide + water + energy — a favourite MCQ.

Burning magnesium (combination)

2Mg(s) + O₂(g) → 2MgO(s)

Dazzling white flame; white MgO powder.

Quick lime + water (combination, exothermic)

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat

Quick lime → slaked lime; vessel becomes hot.

Whitewash shine

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

Slaked lime slowly forms a shiny calcium carbonate layer on walls.

Thermal decomposition of ferrous sulphate

2FeSO₄(s) →Δ Fe₂O₃(s) + SO₂(g) + SO₃(g)

Green crystals → brown solid; smell of burning sulphur.

Limestone decomposition

CaCO₃(s) →Δ CaO(s) + CO₂(g)

Used in cement industry; CaO is quick lime.

Lead nitrate decomposition

2Pb(NO₃)₂(s) →Δ 2PbO(s) + 4NO₂(g) + O₂(g)

Brown fumes of nitrogen dioxide evolve; the residue is lead oxide.

Electrolysis of water

2H₂O(l) →electricity→ 2H₂(g) + O₂(g)

Hydrogen collected is double the volume of oxygen.

Photolytic decomposition

2AgCl(s) →light→ 2Ag(s) + Cl₂(g)

White AgCl turns grey; AgBr behaves the same — basis of B&W photography.

Displacement

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Blue → green; brown copper deposits on the iron.

Double displacement / precipitation

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)

BaSO₄ is the white precipitate.

Redox

CuO + H₂ →Δ Cu + H₂O

CuO is reduced (loses O); H₂ is oxidised (gains O).