Why this chapter matters for UPSC: Chemical reactions are the foundation of industrial chemistry, environmental chemistry, and materials science — all GS3 staples. Corrosion costs India roughly 3.5% of GDP annually. The green hydrogen mission is built on electrolytic decomposition. Redox chemistry underpins smelting, the very basis of India's metals industry. Rancidity connects to food storage policy. Understanding reaction types lets you engage critically with fertilizer production, cement manufacturing, and battery technology questions.

PART 1 — Quick Reference Tables

Types of Chemical Reactions

Type General Form Key Example UPSC Angle
Combination A + B → AB CaO + H₂O → Ca(OH)₂ (slaked lime); Fe + O₂ → Fe₂O₃ Cement setting; lime in agriculture
Decomposition — Thermal AB → A + B (heat) CaCO₃ → CaO + CO₂ Lime kiln / cement manufacturing; CO₂ emissions
Decomposition — Photolytic AB → A + B (light) 2AgCl → 2Ag + Cl₂ (sunlight) Basis of photography; silver recovery
Decomposition — Electrolytic AB → A + B (electricity) 2H₂O → 2H₂ + O₂ Green hydrogen production
Displacement A + BC → AC + B Zn + CuSO₄ → ZnSO₄ + Cu Reactivity series; metal extraction
Double Displacement AB + CD → AD + CB BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl Precipitation; water treatment
Redox Simultaneous oxidation + reduction 2Fe + 3/2 O₂ → Fe₂O₃ (rusting) Corrosion; smelting; bleaching
Exothermic Products have less energy than reactants Combustion, neutralisation, respiration Energy release; fossil fuels
Endothermic Products have more energy than reactants Photosynthesis, evaporation, decomposition of CaCO₃ Solar energy; industrial energy input

Reactivity Series (Most → Least Reactive)

Position Metal Key Property Industrial Relevance
Most reactive K, Na, Ca React violently with water; cannot exist as free metal in nature Stored under oil; used in coolants (Na in fast reactors)
Highly reactive Mg, Al React with hot water/steam; form stable oxides Aluminium extraction requires electrolysis (energy-intensive)
Moderately reactive Zn, Fe, Ni, Sn, Pb React with dilute acids Galvanisation (Zn); steel (Fe); solder (Sn+Pb)
Below hydrogen Cu, Hg, Ag Do not displace H₂ from acids Copper wiring; silver jewellery
Least reactive Au, Pt Occur as native metals; extremely stable Native gold; platinum catalysts

Corrosion: Types and Prevention

Metal Corrosion Product Condition Required Prevention Method
Iron Fe₂O₃·H₂O (rust — hydrated iron oxide) Both O₂ and H₂O needed Galvanisation, painting, oiling, alloying (stainless steel), electroplating
Copper CuCO₃·Cu(OH)₂ (verdigris — green patina) O₂, CO₂, moisture Lacquering; tin plating
Silver Ag₂S (tarnish — black) H₂S in air Silver polish; storage in airtight containers
Aluminium Al₂O₃ (thin protective layer) O₂ Self-passivating — oxide layer protects further corrosion

PART 2 — Detailed Notes

1. What Is a Chemical Reaction?

A chemical reaction involves the rearrangement of atoms — bonds are broken and new bonds are formed. The substances that react are called reactants; the substances produced are called products.

How to identify a chemical reaction has occurred:

  • Change in colour (copper sulphate + iron → iron sulphate turns light green)
  • Evolution of gas (zinc + sulphuric acid → hydrogen gas)
  • Formation of precipitate (lead nitrate + potassium iodide → yellow PbI₂ precipitate)
  • Change in temperature (combustion releases heat; dissolving ammonium chloride absorbs heat)
  • Emission of light (magnesium ribbon burns with a bright white flame)

Balancing equations follows the Law of Conservation of Mass (Lavoisier, 1789): total mass of reactants = total mass of products. Atoms are neither created nor destroyed in a chemical reaction.

2. Types of Reactions

Combination reactions involve two or more substances combining to form a single product. Most are exothermic. Example: quicklime (CaO) reacts vigorously with water releasing heat — this is the basis of cement setting and lime used in agriculture to neutralise acidic soils.

Decomposition reactions break a compound into simpler substances. They require energy input (heat, light, or electricity):

Key Term

Thermal decomposition of limestone (CaCO₃ → CaO + CO₂) is the first step in cement manufacturing. Every tonne of cement clinker produced releases ~0.6 tonnes of CO₂ directly from this reaction — making cement one of the world's largest industrial CO₂ sources (~8% of global emissions). India is the 2nd largest cement producer globally.

UPSC Connect

UPSC GS3 — Green Hydrogen Mission: The electrolytic decomposition of water (2H₂O → 2H₂ + O₂) is the core reaction behind green hydrogen. When powered by renewable electricity, no CO₂ is emitted. India's National Green Hydrogen Mission (launched January 2023) targets 5 MMT (million metric tonnes) green hydrogen production per year by 2030, with an outlay of ₹19,744 crore. Key applications: fertiliser production (replacing grey hydrogen in Haber process), steel manufacturing (replacing coking coal), heavy transport.

Displacement reactions occur when a more reactive element displaces a less reactive one from its compound in solution. The reactivity series predicts whether a reaction will occur — a metal higher in the series will displace one lower. This principle is used in:

  • Extraction of metals — iron displaces copper from copper sulphate in the thermite-type process
  • Galvanic cells (batteries) — voltage depends on the difference in reactivity between electrode metals

Double displacement reactions involve the exchange of ions between two compounds. Precipitation reactions are a subtype — one product is insoluble and forms a solid precipitate. Important example: BaSO₄ precipitate (white, insoluble) is used in medical barium meal X-rays to image the digestive tract.

3. Oxidation and Reduction (Redox Reactions)

Key Term

Oxidation = loss of electrons (or gain of oxygen, or loss of hydrogen) Reduction = gain of electrons (or loss of oxygen, or gain of hydrogen) Mnemonic: OIL RIG — Oxidation Is Loss; Reduction Is Gain (of electrons) Oxidation and reduction always occur simultaneously — one substance is oxidised while another is reduced. The substance that causes oxidation is the oxidising agent (itself gets reduced); the substance causing reduction is the reducing agent (itself gets oxidised).

Corrosion is slow oxidation at the surface of metals. Rusting of iron requires both oxygen and water — neither alone causes rusting. The electrochemical mechanism involves iron acting as an anode (oxidised) while oxygen dissolved in water acts as a cathode (reduced), with water as the electrolyte.

UPSC Connect

UPSC GS3 — Economic cost of corrosion: India loses approximately ₹4–5 lakh crore annually to corrosion (estimated at ~3.5% of GDP) — comparable to what many countries spend on defence. This includes corrosion of infrastructure (bridges, pipelines, railways), industrial equipment, and vehicles. The Bureau of Indian Standards (BIS) and National Corrosion Council of India (NCCI) work on standards and awareness. Galvanised iron pipes (zinc-coated), stainless steel (18% Cr, 8% Ni), and cathodic protection systems (used on ships, underground pipelines) are key mitigation strategies.

Smelting (extraction of metals from ores) is a large-scale reduction reaction. In the blast furnace, iron ore (Fe₂O₃) is reduced by carbon monoxide (CO) — the reducing agent. Steel production — India is the world's 2nd largest producer — depends entirely on this redox chemistry.

Bleaching with chlorine gas exploits redox: Cl₂ + H₂O → HCl + [O] (nascent oxygen). This nascent oxygen oxidises the coloured compounds in cloth/paper, making them colourless. Sodium hypochlorite (NaOCl) in liquid bleach works the same way.

4. Exothermic and Endothermic Reactions

Exothermic reactions release energy (usually as heat and/or light):

  • Combustion (coal, LPG, petrol, wood) — basis of thermal power, heating, transport
  • Neutralisation reactions (acid + base → salt + water) — always exothermic
  • Cellular respiration (glucose + O₂ → CO₂ + H₂O + ATP energy)
  • Condensation (steam → water) and solidification

Endothermic reactions absorb energy from surroundings:

  • Photosynthesis (requires solar energy to build glucose from CO₂ + H₂O)
  • Evaporation (requires heat — basis of cooling towers, sweating)
  • Decomposition reactions (require energy input — see above)
  • Dissolving ammonium chloride (NH₄Cl) in water (feels cold — used in instant cold packs)
Explainer

Respiration is exothermic: Cellular respiration (Chapter 6) is the biological equivalent of combustion — glucose is "burned" with oxygen, releasing energy stored in ATP, plus heat and CO2. This is why your body temperature stays at ~37°C. The reaction: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~2870 kJ of energy per mole of glucose.

5. Rancidity

When fats and oils are oxidised by atmospheric oxygen, they develop an unpleasant smell and taste — this is called rancidity. It is a slow chemical change.

Prevention methods (relevant to food processing industry and FSSAI standards):

  • Antioxidants: BHA (butylated hydroxyanisole), BHT (butylated hydroxytoluene), Vitamin E (tocopherol), Vitamin C (ascorbic acid) are added to packaged foods — they get oxidised preferentially, protecting the fat
  • Vacuum packaging: Removes oxygen
  • Nitrogen flushing: Inert gas (N₂) displaces oxygen in food packets (used for chips, biscuits, dry fruits)
  • Refrigeration: Lower temperature slows the rate of oxidation

6. Industrial Chemistry Connections

Process Reaction Type Product Relevance
Haber Process Combination (N₂ + 3H₂ → 2NH₃, 450°C, Fe catalyst) Ammonia → fertilisers (urea, DAP) India is world's 2nd largest fertiliser consumer
Contact Process Catalytic oxidation (SO₂ → SO₃ → H₂SO₄) Sulphuric acid Most produced industrial chemical globally
Solvay Process Multi-step; NaCl + NH₃ + CO₂ → NaHCO₃ → Na₂CO₃ Soda ash, baking soda Glass, detergent, paper industries
Cement manufacture Thermal decomposition + combination CaO → clinker CO₂ emissions; construction industry
Chlor-alkali process Electrolysis of NaCl(aq) → NaOH + Cl₂ + H₂ Caustic soda, chlorine Soap, paper, PVC, disinfectants

Exam Strategy

Prelims traps:

  • Rusting requires both oxygen and water — not just oxygen alone. Rusting does NOT occur in pure dry oxygen or pure water; it is an electrochemical process requiring both simultaneously.
  • In electrolysis of water, hydrogen is liberated at the cathode (negative electrode) and oxygen at the anode (positive electrode) — not the other way around.
  • Photolytic decomposition of silver chloride (AgCl) → Ag + Cl₂ — the silver turns grey/black (metallic silver). This is why silver jewellery and silverware darkens in light.
  • Oxidising agent itself gets reduced; reducing agent itself gets oxidised — exam questions frequently reverse this.
  • The reactivity series lists metals in decreasing order of reactivity — potassium (K) at the top, platinum (Pt) at the bottom. A metal higher in the series displaces a metal lower in the series from its salt solution.
  • Galvanisation protects iron by sacrificial protection — zinc, being more reactive than iron, oxidises first, protecting the iron even if the zinc coat is scratched.

Mains frameworks:

  • Green hydrogen: electrolytic decomposition → renewable energy → National Green Hydrogen Mission → decarbonisation of fertiliser and steel → import substitution
  • Corrosion costs: ₹4–5 lakh crore/year → infrastructure durability → bridge/pipeline safety → BIS standards
  • Cement and climate: thermal decomposition of limestone → CO₂ emissions → Paris Agreement commitments → green cement alternatives

Previous Year Questions

Prelims:

  1. With reference to the production of green hydrogen, which of the following is/are correct?
    (a) Green hydrogen is produced by electrolysis of water using electricity from fossil fuels
    (b) Green hydrogen production releases carbon dioxide as a by-product
    (c) Green hydrogen is produced when renewable energy is used to electrolyse water
    (d) Green hydrogen is produced by the Haber process

  2. The process of galvanisation protects iron from rusting by coating it with:
    (a) Tin
    (b) Zinc
    (c) Chromium
    (d) Nickel

Mains:

  1. What is green hydrogen? Discuss its potential as a clean fuel and the challenges in scaling up its production in India. (CSE Mains 2023, GS Paper 3, 15 marks)

  2. Explain the major sources of SO₂ and NOₓ emissions in India. How do these contribute to acid rain and what policy measures have been taken to control them? (CSE Mains 2019, GS Paper 3, 15 marks)