NCERT Class 10 Science | Chapter: Carbon and Its Compounds | Texcellency Book Series

---

✅ Answer: Hydrogenation is the Addition of Hydrogen Gas Across a C=C Double Bond in the Presence of a Nickel Catalyst

Definition: Hydrogenation is a chemical reaction in which hydrogen gas (H₂) is added to an unsaturated compound (one containing C=C double bonds or C≡C triple bonds) in the presence of a nickel (Ni) catalyst at suitable temperature and pressure. The multiple bond breaks open, hydrogen atoms attach to the carbons on either side, and the unsaturated compound becomes saturated.

The one-line definition for the exam: Hydrogenation is the addition of hydrogen to an unsaturated compound using a nickel catalyst, converting it into a saturated compound.

Principal Industrial Application: Hydrogenation is used to convert liquid vegetable oils (unsaturated) into solid vegetable fats — commercially sold as Vanaspati ghee / Dalda — used in cooking and food production.

---

🏭 The Renovation Analogy — Filling the Gaps

Imagine an unsaturated compound as a building under construction — it has some floors (carbon atoms) that are connected by temporary double scaffolding (double bonds). These double-scaffolded floors have unused connection points — gaps that are not yet filled.

🔵 Hydrogenation is like sending in a construction crew (the Ni catalyst) with bricks (hydrogen atoms) to fill every gap — every double scaffolding is replaced by a solid single connection, with a brick (H atom) added on each side. 🔵 Once all gaps are filled, the building is complete — fully saturated — no more double scaffolding anywhere. 🔵 The building’s character changes: it was flexible and liquid-like (oil) — now it is rigid and solid-like (fat / Vanaspati).

---

🔴 The Chemistry — What Exactly Happens

The Reaction:

—CH=CH— + H₂ →(Ni catalyst, 150–200°C)→ —CH₂–CH₂—

🔵 The C=C double bond breaks open 🔵 One hydrogen atom from H₂ attaches to each of the two carbons 🔵 The double bond becomes a single bond 🔵 The compound goes from unsaturated → saturated

For a Specific Example — Hydrogenation of Ethene:

H₂C=CH₂ + H₂ →(Ni, heat)→ H₃C–CH₃ (Ethene + Hydrogen → Ethane)

🔵 Ethene (unsaturated, alkene, C₂H₄) + H₂ → Ethane (saturated, alkane, C₂H₆) 🔵 The double bond is gone — a fully saturated product forms

For a Compound with Multiple Double Bonds (like in vegetable oil):

R–CH=CH–R' + H₂ →(Ni, heat)→ R–CH₂–CH₂–R' (where R and R’ represent the rest of the long fatty acid chain)

🔵 Each C=C double bond in the vegetable oil reacts with one H₂ molecule 🔵 If the oil has multiple double bonds (polyunsaturated), it reacts with multiple H₂ molecules 🔵 Eventually all double bonds are eliminated — oil (liquid) → solid fat

---

🔶 The Role of the Nickel Catalyst — Why Is It Essential?

H₂ and vegetable oil do NOT react on their own at room temperature — the reaction needs activation energy. The nickel catalyst provides a surface on which both the H₂ molecule and the oil molecule can adsorb (stick), bringing them close together and lowering the activation energy so the reaction can proceed at a practical temperature (~150–200°C).

🔵 Ni (nickel) is the most commonly used catalyst in industrial hydrogenation — cheap, effective, widely available 🔵 Pt (platinum) and Pd (palladium) are also used — more efficient but far more expensive — used in laboratory-scale reactions and pharmaceutical hydrogenation 🔵 The catalyst is NOT consumed in the reaction — it speeds up the reaction without being used up itself 🔵 Without the catalyst: the reaction would require extremely high temperatures, making it industrially unviable

Why Ni Works:

Nickel has a crystal surface with spaces that can adsorb both H₂ and the unsaturated compound. H₂ molecules split into individual H atoms on the Ni surface (chemisorption). These H atoms are then transferred to the double bond of the oil molecule. The saturated product desorbs (leaves) the surface and the Ni surface is free for the next reaction cycle.

---

🔴 Principal Industrial Application — Vegetable Oil to Vanaspati / Dalda

This is the NCERT-expected main application and must be written clearly.

The Problem Hydrogenation Solves:

🔵 Vegetable oils (sunflower oil, groundnut oil, soybean oil, cottonseed oil) are liquid at room temperature — because they contain unsaturated fatty acids with C=C double bonds 🔵 These C=C double bonds create “kinks” in the fatty acid chains, preventing them from packing tightly → the fat stays liquid 🔵 For many food applications — baking, frying, making biscuits/pastries/bread — a solid fat is preferred because: it is easier to handle, has a longer shelf life, gives better texture to baked goods, and is cheaper than animal ghee/butter

The Hydrogenation Solution:

🔵 Vegetable oil is heated to ~150–200°C with hydrogen gas (H₂) passed through it 🔵 A nickel catalyst is present (finely divided Ni powder gives maximum surface area) 🔵 H₂ adds across the C=C double bonds in the fatty acid chains 🔵 The unsaturated fatty acids (oleic acid, linoleic acid etc.) → become saturated fatty acids (stearic acid, palmitic acid etc.) 🔵 As the degree of saturation increases, the melting point rises — the oil (liquid) gradually becomes semi-solid → solid 🔵 Product: Vanaspati ghee / Dalda / Hydrogenated vegetable fat — solid at room temperature, white/cream coloured, used as a cheaper substitute for ghee/butter

The Chemical Transformation of Oleic Acid (Example):

CH₃(CH₂)₇CH=CH(CH₂)₇COOH + H₂ →(Ni)→ CH₃(CH₂)₁₆COOH (Oleic acid — unsaturated, liquid — → Stearic acid — saturated, solid)

🔵 Oleic acid has one C=C double bond — one H₂ molecule converts it to stearic acid 🔵 Stearic acid is a saturated fatty acid — found naturally in butter and animal fats 🔵 This transformation is exactly how cooking oil (liquid) becomes Dalda (solid)

Real-Life Products Made by Hydrogenation of Oils:

🔵 Vanaspati ghee / Dalda — used in Indian cooking, bakery, mithai-making 🔵 Margarine — used as a butter substitute in Western countries 🔵 Shortening — used in baking (biscuits, pastries, pie crusts) 🔵 Peanut butter — partially hydrogenated to prevent the oil from separating and to improve texture/shelf life

---

🔷 Other Industrial Applications of Hydrogenation (Bonus — For Extra Marks)

Hydrogenation is not limited to food. It has wide industrial applications:

🔵 Petroleum refining (hydrocracking): Heavy petroleum fractions are hydrogenated to break them into lighter, more useful fuels like petrol and diesel. Also used to remove sulphur, nitrogen, and other impurities from crude oil fractions (hydrodesulphurisation) — reducing pollution from fuels.

🔵 Ammonia production (Haber Process): N₂ + 3H₂ →(Fe catalyst, high T and P)→ 2NH₃. Technically this is hydrogenation of nitrogen — the most important industrial chemical reaction in history (produces fertilisers that feed billions of people).

🔵 Pharmaceutical manufacturing: Many drugs and active pharmaceutical ingredients are synthesised using hydrogenation reactions — to convert unsaturated precursors to specific saturated target compounds with precise molecular structure.

🔵 Production of cyclohexane: Benzene (C₆H₆) + 3H₂ →(Ni/Pt, heat/pressure)→ Cyclohexane (C₆H₁₂). Cyclohexane is used to make nylon — one of the most important synthetic fibres.

🔵 Hydrogenation of coal (coal liquefaction): Coal can be converted to liquid synthetic fuels by hydrogenation — used during World War II by Germany when petroleum was scarce (Bergius process).

---

🔶 Health Note — Why Hydrogenation and Trans Fats Matter

This is a bonus point that connects chemistry to real life — examiners love it.

During partial hydrogenation (when the reaction is not taken to completion — some double bonds remain unreacted), a side reaction can produce trans fatty acids — a different geometric form of the remaining double bonds.

🔵 Natural unsaturated fats have cis double bonds (both H atoms on the same side of the double bond) — this gives the kink/bend in the chain 🔵 Trans fats have trans double bonds (H atoms on opposite sides) — these chains are straighter, pack more tightly, and raise LDL (bad cholesterol) while lowering HDL (good cholesterol) 🔵 Trans fats are strongly linked to cardiovascular disease — which is why many countries now require food labels to declare trans fat content, and many food manufacturers have moved away from partial hydrogenation toward full hydrogenation or alternative processes 🔵 Full hydrogenation (all double bonds removed) does NOT produce trans fats — it produces only saturated fat

This is why “partially hydrogenated vegetable oil” on a food label is a warning sign — while “fully hydrogenated” is different.

---

📊 Summary Table — Hydrogenation at a Glance

---

🎵 Rhyme to Remember

“Hydrogenation — add H₂ to a double bond, With nickel as catalyst — of which industry is fond,* The C=C breaks open, hydrogen fills the gap,* Unsaturated to saturated — a chemical snap!* Oil is liquid — chains that bend and kink,* Add H₂ across — and watch the chains re-link,* Straight and packed and solid — Dalda on your plate,* Hydrogenation did it — chemistry that’s great!”*

---

🧩 Mnemonics

🔵 “H₂ + Ni + Heat = HARD fat from SOFT oil” — hydrogen, nickel catalyst, heat → turns soft liquid oil into hard solid fat. 🔵 “Ni = Nickel = Nice catalyst — makes the reaction go” — nickel is the workhorse catalyst of hydrogenation. 🔵 “Dalda = Double bonds Axed, Liquid → Definite solid Added” — each double bond is axed by H₂ addition, turning liquid oil into solid Dalda. 🔵 “Unsaturated oils → add H₂ → Saturated fats → Solid. Same as: cooking oil + H₂ → butter-like solid.” 🔵 “Partial hydrogenation = TRANS FATS = Trouble” — incomplete hydrogenation creates unhealthy trans fats.

---

✅ Exam-Ready Answer (Write This in Board Exam)

What is hydrogenation? What is its industrial application?

Hydrogenation: Hydrogenation is a chemical reaction in which hydrogen gas (H₂) is added to an unsaturated compound (containing C=C double bonds) in the presence of a nickel (Ni) catalyst at approximately 150–200°C. The double bond breaks and hydrogen atoms attach to the carbon atoms on either side, converting the unsaturated compound into a saturated one.

Chemical equation: —CH=CH— + H₂ →(Ni, ~150–200°C)→ —CH₂–CH₂—

Industrial Application: The most important industrial application of hydrogenation is the conversion of vegetable oils (liquid, unsaturated) into solid vegetable fats (Vanaspati ghee / Dalda / Margarine).

Vegetable oils contain unsaturated fatty acids with C=C double bonds, making them liquid at room temperature. When hydrogen is passed through heated vegetable oil in the presence of a nickel catalyst, the C=C double bonds react with H₂ — the oil becomes progressively more saturated — its melting point rises — and it solidifies into Vanaspati ghee.

This solid fat is widely used in cooking, baking, and food manufacturing as a cheaper alternative to butter and animal ghee.

Other industrial applications: Petroleum refining (removing sulphur, hydrocracking), synthesis of ammonia (Haber Process: N₂ + 3H₂ → 2NH₃), production of cyclohexane (for nylon), and pharmaceutical manufacturing.

---

📌 Key Points Checklist

✅ Hydrogenation = addition of H₂ to unsaturated compound (C=C or C≡C bonds) ✅ Catalyst = Nickel (Ni) — essential; Pt and Pd also used ✅ Conditions = ~150–200°C, moderate pressure ✅ Reaction: —CH=CH— + H₂ →(Ni)→ —CH₂–CH₂— (double bond → single bond) ✅ Main application = vegetable oil (liquid, unsaturated) → Vanaspati/Dalda (solid, saturated) ✅ Why oil is liquid: C=C double bonds create kinks → chains can’t pack tightly → stays liquid ✅ Why Dalda is solid: H₂ fills the kinks → chains pack tightly → melting point rises → solid ✅ Nickel catalyst = lowers activation energy, provides surface for reaction, is NOT consumed ✅ Partial hydrogenation → trans fats → health risk (cardiovascular disease) ✅ After hydrogenation: product does NOT decolourise bromine water (no double bonds left) ✅ Other applications: Haber process (NH₃), petroleum refining, cyclohexane → nylon, pharma

---

📚 Want ALL of Class 10 Science Explained This Way? Every chapter. Every concept. Every NCERT question — with analogies, rhymes, mnemonics, and real-life examples.