BharatNotes 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 Hâ‚‚ 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):
Thermal decomposition of limestone (CaCO₃ → CaO + COâ‚‚) is the first step in cement manufacturing. Every tonne of cement clinker produced releases ~0.6 tonnes of COâ‚‚ directly from this reaction — making cement one of the world’s largest industrial COâ‚‚ sources (~8% of global emissions). India is the 2nd largest cement producer globally.
[Additional] Green Cement — Reducing the Decomposition Penalty: Since the CO₂ from cement comes directly from the thermal decomposition of CaCO₃ (not just from burning fuel), switching to renewable energy alone cannot decarbonise cement. Three approaches reduce this “process emission”:
- Blended cements / SCMs (Supplementary Cementitious Materials): Partially replace clinker with industrial by-products — fly ash (from coal power plants), ground granulated blast furnace slag (GGBS from steel plants), and silica fume — which react with Ca(OH)₂ without requiring calcination. Reduces clinker factor and CO₂ by 20—40%.
- LC3 (Limestone Calcined Clay Cement): Uses calcined clay + limestone to replace 50% of clinker; CO₂ savings of ~40%; India has abundant clay deposits; large pilot projects underway (including housing projects that saved 80,000 tonnes CO₂ in 2024).
- Geopolymer cement: Uses fly ash or slag activated with alkali solutions — no limestone calcination needed; 75—90% lower CO₂ than Portland cement. Still at pre-commercial stage in India.
UPSC angle: India’s PAT (Perform Achieve Trade) scheme under BEE targets energy efficiency in cement; green cement is central to India’s hard-to-abate sector decarbonisation strategy under the LTS (Long-Term Strategy) submitted to UNFCCC.
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 — more reactive metals reduce oxides of less reactive metals
- Galvanic cells (batteries) — voltage depends on the difference in reactivity between electrode metals
[Additional] Thermite Reaction — Welding Railway Tracks: Feâ‚‚O₃ + 2Al â†' 2Fe + Alâ‚‚O₃ + enormous heat (~3,000°C)
Aluminium (higher in reactivity series) displaces iron from iron oxide — an intensely exothermic displacement reaction. The molten iron produced flows into the gap between two rail ends and solidifies, creating a seamless joint. This thermite welding (Goldschmidt process) is the standard method used by Indian Railways to join rails, producing Long Welded Rail (LWR) track that eliminates clickety-clack joints and allows higher train speeds. No external power or electricity is needed — the chemical reaction itself provides ~3,000°C heat. Also used to repair heavy machinery and cracked metal structures on-site.
UPSC angle: Direct application of the reactivity series; appears in questions on Indian Railways infrastructure, high-speed rail corridor technology, and metallurgy.
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)
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 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.
[Additional] Cathodic Protection — How It Works: Cathodic protection exploits the reactivity series directly. A sacrificial anode (block of zinc or aluminium, which are more reactive than iron/steel) is bolted to a ship's hull or buried alongside an underground pipeline. Because zinc/aluminium are higher in the reactivity series, they preferentially oxidise (corrode) instead of the iron structure — the structure becomes the cathode, the sacrificial anode corrodes away and is periodically replaced. Used on: Indian Navy warships, merchant vessels, offshore oil platforms (ONGC), buried oil/gas pipelines (GAIL, IOC), bridge pilings in coastal areas. The same principle explains galvanisation — even if the zinc coating is scratched, zinc still protects the exposed iron underneath.
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 COâ‚‚ + Hâ‚‚O)
- 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)
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: C₆Hâ‚â‚‚O₆ + 6Oâ‚‚ → 6COâ‚‚ + 6Hâ‚‚O + ~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 (Nâ‚‚) 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
Practice Questions
Prelims:
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 processThe process of galvanisation protects iron from rusting by coating it with:
(a) Tin
(b) Zinc
(c) Chromium
(d) Nickel
Mains:
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)
Explain the major sources of SOâ‚‚ and NOâ‚“ 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)
