9.2 Chemical Reactions & Equations

A chemical reaction is the process by which one or more substances (the reactants) change into new substances (the products) with different properties. The Law of Conservation of Mass — atoms are neither created nor destroyed in a chemical reaction — is why every equation must be balanced: the same number of each type of atom on each side.

Writing and Balancing Equations

A skeletal equation lists the reactants and products; a balanced equation adds coefficients so atoms match. Steps:

1. Write correct formulas for reactants and products.
2. Count atoms of each element on both sides.
3. Adjust coefficients (never change subscripts) until atoms balance.
4. Ensure coefficients are the smallest whole numbers.

Worked example — balance Fe + H₂O → Fe₃O₄ + H₂.

• Fe: 1 left, 3 right → put 3 Fe on the left: 3Fe + H₂O → Fe₃O₄ + H₂.
• O: 1 left, 4 right → put 4 H₂O on the left: 3Fe + 4H₂O → Fe₃O₄ + H₂.
• H: 8 left, 2 right → put 4 H₂ on the right: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
• Check: Fe 3=3, O 4=4, H 8=8. Balanced.

State symbols matter too: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution. CaO(s) + H₂O(l) → Ca(OH)₂(aq) tells you a solid reacts with liquid water to give a soluble product.

Types of Chemical Reactions

Combination reactions

Two or more reactants form a single product. Burning of magnesium (2Mg + O₂ → 2MgO, white dazzling flame) and slaking of lime (CaO + H₂O → Ca(OH)₂, heat released) are classics. Reactions that release heat are exothermic; those that absorb heat are endothermic.

Decomposition reactions

A single compound breaks into two or more substances. Three sub-types recur on RRB papers:

• Thermolysis (heat): CaCO₃(s) → CaO(s) + CO₂(g) (limestone to quicklime — used in cement). 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (green ferrous sulphate turns reddish-brown ferric oxide with smell of burning sulphur).
• Photolysis (light): 2AgCl → 2Ag + Cl₂ (white silver chloride turns grey in sunlight — basis of black-and-white photography). 2AgBr → 2Ag + Br₂.
• Electrolysis (electricity): 2H₂O → 2H₂ + O₂ (acidulated water). 2NaCl → 2Na + Cl₂ (molten NaCl).

Displacement reactions

A more reactive element displaces a less reactive one from its compound. Zinc displacing copper from CuSO₄ (colour change blue → colourless, brown copper deposits) shows Zn is above Cu in the reactivity series. Iron displacing copper from CuSO₄ (blue → pale green FeSO₄, reddish Cu deposit) is the textbook lab example.

Double displacement reactions

Two compounds exchange ions to form two new compounds, often with a precipitate, gas or water. AgNO₃(aq) + NaCl(aq) → AgCl(s)↓ + NaNO₃(aq) — the white curdy precipitate of AgCl darkens in sunlight (photolysis). Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl — the white precipitate is used as 'barium meal' in X-ray imaging.

Oxidation and Reduction (Redox)

• Oxidation: gain of oxygen, or loss of hydrogen/electrons.
• Reduction: loss of oxygen, or gain of hydrogen/electrons.
• Both happen together — hence redox.

In 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃, Fe²⁺ is oxidised to Fe³⁺ (loses an electron) while sulphate is reduced; the reaction is internally redox.

In CuO + H₂ → Cu + H₂O, CuO loses oxygen (is reduced, the oxidising agent) and H₂ gains oxygen (is oxidised, the reducing agent).

Effects of Heat, Light and Electricity

RRB Exam Scenarios and Traps

• Trap: changing subscripts to balance. Mg + O₂ → MgO₂ is wrong; the correct balanced form is 2Mg + O₂ → 2MgO.
• Trap: misclassifying displacement as double displacement. Single swap = displacement; ion exchange between two compounds = double displacement.
• Trap: forgetting that a redox reaction requires both oxidation and reduction — if only oxidation is mentioned, something must also be reduced.
• Expect: 'Which reaction is endothermic?' Decomposition of CaCO₃ absorbs heat (endothermic); respiration is exothermic.
• Expect: 'Identify the oxidising agent in CuO + H₂ → Cu + H₂O.' Answer: CuO (it gives oxygen to H₂).
• Expect: 'White silver chloride turns grey in sunlight due to …' photolytic decomposition into silver metal.

RRB often asks you to identify reaction types from the equation alone. Practise spotting combination (one product), decomposition (one reactant), displacement (one element swap), double displacement (two compounds swapping ions) and redox (oxygen/hydrogen transfer).