313_Chemistry_Eng_Lesson24.pdf). Content covers sections 24.1–24.4.Hydrocarbons contain only carbon and hydrogen. They are fuels, lubricants, solvents and starting materials for almost all organic chemistry. Sources: petroleum (fractional distillation) and coal (destructive distillation). This lesson covers alkanes, alkenes, alkynes and aromatic hydrocarbons (benzene) — preparation, physical and chemical properties, tests, aromaticity and directive effects.
Saturated hydrocarbons CnH2n+2 — little reactivity (“paraffins”).
Preparation: (1) Haloalkanes — Zn/HCl, HI/red P, H₂/Pt; Grignard RMgX + H₂O/ROH → RH; Wurtz 2RX + 2Na → R–R. (2) Hydrogenation of alkenes/alkynes (Ni/Pt/Pd). (3) Alcohols/aldehydes/ketones + HI/red P. (4) Carboxylic acids: soda lime decarboxylation (RCOONa + NaOH/CaO → RH); reduction with HI; Kolbe electrolysis of RCOO⁻ → R–R.
Physical: C₁–₄ gases, C₅–₁₇ liquids, higher solids. b.p. rises with mass; branching lowers b.p. (smaller surface). Density < water. Melting points irregular (packing of odd/even chains).
Chemical: Free-radical halogenation (Cl₂, light) — initiation, propagation, termination; F₂ > Cl₂ > Br₂ > I₂. Combustion (complete → CO₂; limited O₂ → CO). Cracking at high T. Isomerisation with AlCl₃/HCl. Uses: fuels, LPG (propane/butane), solvents, methane for synthesis.
Unsaturated CnH2n with C=C (“olefins”). Prep: dehydrohalogenation of RX with alc. KOH (Saytzeff — more substituted alkene major); dehydration of alcohols (H₂SO₄ or Al₂O₃).
Addition: H₂/Ni; X₂ (Br₂ colour test); HX (Markovnikov; HBr + peroxide = anti-Markovnikov); H₂O/H⁺; H₂SO₄; polymerisation → polyethene.
Oxidation: cold alk. KMnO₄ (Baeyer) → diol, purple discharged; hot KMnO₄ cleaves C=C; O₂/Ag → ethylene oxide; ozonolysis → carbonyls; combustion.
Uses: plastics (polyethene), ethanol, ethanal, antifreeze (ethanediol).
CnH2n−2 with C≡C. Ethyne: CaC₂ + 2H₂O → HC≡CH; dihaloalkanes + alc. KOH; RC≡CNa + R′X for higher alkynes.
Physical: gases then liquids/solids with size; slightly polar (higher b.p. than alkanes); ethyne garlic odour; slightly soluble in water, soluble in acetone.
Chemical: sequential addition of H₂, X₂, HX; H₂O/Hg²⁺/H₂SO₄ → ethanal (via enol); oxidation with KMnO₄; combustion (oxyacetylene flame ~2800°C); ozonolysis to dicarbonyls without chain break.
Acidic nature: sp carbon 50% s-character → more electronegative → terminal ≡C–H acidic; Na or NaNH₂ → acetylides; ammoniacal AgNO₃ (white) or Cu₂Cl₂ (red) ppt for terminal alkynes only.
Distinction table: Br₂ and Baeyer — positive for alkenes and alkynes, not alkanes. Ammoniacal AgNO₃/Cu₂Cl₂ — only terminal alkynes.
Uses: welding, fruit ripening, synthesis of ethanal, polymers, Orlon.
From coal tar (destructive distillation). Formula C₆H₆ — unsaturated by addition of H₂/Cl₂, but prefers electrophilic substitution; does not decolourise Br₂ water or Baeyer’s reagent under mild conditions.
Structure: Kekulé alternate double bonds inadequate (all C–C 139 pm; one ortho product). Resonance hybrid of two forms; circle notation; resonance energy ~150 kJ mol⁻¹ (heat of hydrogenation lower than expected for three double bonds). MO: six sp² carbons, delocalised π cloud above and below ring.
Electrophilic aromatic substitution (EAS): halogenation (FeX₃); nitration (NO₂⁺ from HNO₃/H₂SO₄); sulphonation (oleum); Friedel–Crafts alkylation/acylation (AlCl₃).
Directive influence: o/p directors (–OH, –CH₃, –NH₂, –OR); m directors (–NO₂, –COOH, –SO₃H, –CN).
Physical: colourless liquids with characteristic odour; immiscible with water; dissolve fats. b.p. rises with size (benzene 353 K, toluene 383 K).
Carcinogenicity: some polycyclic aromatics (coal tar fractions) are toxic/carcinogenic. Uses: solvents, dyes, drugs, plastics precursors.
High-yield: alkane prep (Wurtz, Kolbe, soda lime); free-radical chlorination mechanism; ethane conformations; alkene prep and Saytzeff; Markovnikov + mechanism + peroxide exception; Baeyer and Br₂ tests; CaC₂ ethyne; acidic alkyne + acetylide tests; distinction table; benzene structure/resonance/Hückel; EAS reagents; o/p vs m directors.
Builds on L23 (naming, mechanisms, isomerism). Next lessons add functional groups (haloalkanes, alcohols, carbonyls). Practice writing mechanisms for HX addition and free-radical halogenation — they are exam staples.
Intext drills: Grignard and active hydrogen; isomers of pentane and b.p. order; chlorination steps; Saytzeff major product; conditions for hydrogenation; CaC₂ reaction; % s-character ethane/ethene/ethyne; distinction tests; Hückel check for benzene; nitration electrophile; o/p vs m directors. Coal tar fractions: light oil (benzene), middle oil (phenol), heavy oil (naphthalene), green oil (anthracene), pitch.
Most exam-important points from this chapter:
Prep: Wurtz, Grignard, Kolbe, H₂/Ni, soda lime. Free-radical X₂. Staggered ethane preferred. Fuels & LPG.
Prep by elimination (Saytzeff). Additions: Markovnikov; HBr/peroxide anti-M. Baeyer & Br₂ tests. Polymers.
CaC₂ → ethyne. Acidic ≡CH (sp). Ag⁺/Cu⁺ ppt for terminal. Distinction table vs alkanes/alkenes.
Resonance hybrid; Hückel 6π. Substitution not addition. EAS: X₂, NO₂, SO₃H, R, COR.
o/p: activating groups (OH, CH₃). m: deactivating (NO₂, COOH, SO₃H).
Extracted from NIOS Chemistry (313) board exam papers in your PDF. Chapter L24 — Hydrocarbons only. Use Model Answer for marking points; Explanation for concept clarity.
13 question(s) · Sources: 313/MAY/205A, 313/MAY/205B, 313/MAY/205C, 313/TUS/105A
PYQ1. Alkanes undergo pyrolysis — (A) at a very high pressure and in the presence of air (B) at a very low temperature and in the absence of air (C) at a very high temperature and in the absence of air (D) at a very high temperature and in the presence of air Eoë
Model Answer
Answer: (B) at a very low temperature and in the absence of air
Explanation
Pyrolysis is thermal decomposition of alkanes at high temperature.
Map to L24 syllabus. Paper 313/MAY/205A, Q13 (1 mark).
PYQ2. The correct order of stability of 1°, 2° and 3° carbocations is — (A) 1°, 2° Am¡a 3°
Model Answer
Answer using key concepts from L24 (definitions, equations, and one example where useful). Stay within the suggested word range for a 1-mark NIOS question.
Explanation
Cross-check with L24 notes. Structure: definition/law → working → conclusion. Partial marks for correct equations even if explanation is short.
How to write for NIOS: Use 1 mark — concise correct choice/fact. Open with definition/equation, then reason, end with conclusion. Paper 313/MAY/205A · Q14 · 1 mark(s) · L24.
PYQ3. Write True (T) for correct statement and False (F) for incorrect statement : Acetylene has a bond order of 3. The lone pair-bond pair repulsion is intermediate between lone pair- lone pair and bond pair-bond pair repulsion. ghr
Model Answer
Answer using key concepts from L24 (definitions, equations, and one example where useful). Stay within the suggested word range for a 2-mark NIOS question.
Explanation
Cross-check with L24 notes. Structure: definition/law → working → conclusion. Partial marks for correct equations even if explanation is short.
How to write for NIOS: Use 30–50 words (VSA) or short objective. Open with definition/equation, then reason, end with conclusion. Paper 313/MAY/205A · Q19 · 2 mark(s) · L24.
PYQ4. Complete and balance the following reactions : CaC2 + 2H2O 40% H SO 1% HgSO
Model Answer
CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂ (ethyne). Used industrially for acetylene generation.
Explanation
Ionic carbide reacts with water; product is the simplest alkyne (L24).
How to write for NIOS: Use 30–50 words (VSA) or short objective. Open with definition/equation, then reason, end with conclusion. Paper 313/MAY/205A · Q25 · 2 mark(s) · L24.
PYQ5. Alkanes undergo pyrolysis — (A) at a very high pressure and in the presence of air (B) at a very low temperature and in the absence of air (C) at a very high temperature and in the absence of air (D) at a very high temperature and in the presence of air Eoë
Model Answer
Answer: (B) at a very low temperature and in the absence of air
Explanation
Pyrolysis is thermal decomposition of alkanes at high temperature.
Map to L24 syllabus. Paper 313/MAY/205B, Q5 (1 mark).
PYQ6. The correct order of stability of 1°, 2° and 3° carbocations is — (A) 1°, 2° Am¡a 3° (B) Note : Question Nos. 17 to 28 are objective type questions of 2 marks each. {ZX}e : à0 g§0 17 go 2
Model Answer
Answer: (A) 1°, 2° Am¡a 3°
Explanation
Carbocation stability order: 3° > 2° > 1°.
Map to L24 syllabus. Paper 313/MAY/205B, Q16 (1 mark).
PYQ7. Write True (T) for correct statement and False (F) for incorrect statement : Acetylene has a bond order of 3. The lone pair-bond pair repulsion is intermediate between lone pair- lone pair and bond pair-bond pair repulsion. ghr
Model Answer
Answer using key concepts from L24 (definitions, equations, and one example where useful). Stay within the suggested word range for a 2-mark NIOS question.
Explanation
Cross-check with L24 notes. Structure: definition/law → working → conclusion. Partial marks for correct equations even if explanation is short.
How to write for NIOS: Use 30–50 words (VSA) or short objective. Open with definition/equation, then reason, end with conclusion. Paper 313/MAY/205B · Q22 · 2 mark(s) · L24.
PYQ8. Complete and balance the following reactions : CaC2 + 2H2O 40% H SO 1% HgSO
Model Answer
CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂ (ethyne). Used industrially for acetylene generation.
Explanation
Ionic carbide reacts with water; product is the simplest alkyne (L24).
How to write for NIOS: Use 30–50 words (VSA) or short objective. Open with definition/equation, then reason, end with conclusion. Paper 313/MAY/205B · Q26 · 2 mark(s) · L24.
PYQ9. The correct order of stability of 1°, 2° and 3° carbocations is — (A) 1°, 2° Am¡a 3°
Model Answer
Answer using key concepts from L24 (definitions, equations, and one example where useful). Stay within the suggested word range for a 1-mark NIOS question.
Explanation
Cross-check with L24 notes. Structure: definition/law → working → conclusion. Partial marks for correct equations even if explanation is short.
How to write for NIOS: Use 1 mark — concise correct choice/fact. Open with definition/equation, then reason, end with conclusion. Paper 313/MAY/205C · Q5 · 1 mark(s) · L24.
PYQ10. Alkanes undergo pyrolysis — (A) at a very high pressure and in the presence of air (B) at a very low temperature and in the absence of air (C) at a very high temperature and in the absence of air (D) at a very high temperature and in the presence of air Eoë
Model Answer
Answer: (B) at a very low temperature and in the absence of air
Explanation
Pyrolysis is thermal decomposition of alkanes at high temperature.
Map to L24 syllabus. Paper 313/MAY/205C, Q15 (1 mark).
PYQ11. Complete and balance the following reactions : CaC2 + 2H2O 40% H SO 1% HgSO
Model Answer
CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂ (ethyne). Used industrially for acetylene generation.
Explanation
Ionic carbide reacts with water; product is the simplest alkyne (L24).
How to write for NIOS: Use 30–50 words (VSA) or short objective. Open with definition/equation, then reason, end with conclusion. Paper 313/MAY/205C · Q22 · 2 mark(s) · L24.
PYQ12. Write True (T) for correct statement and False (F) for incorrect statement : Acetylene has a bond order of 3. The lone pair-bond pair repulsion is intermediate between lone pair- lone pair and bond pair-bond pair repulsion. ghr
Model Answer
Answer using key concepts from L24 (definitions, equations, and one example where useful). Stay within the suggested word range for a 2-mark NIOS question.
Explanation
Cross-check with L24 notes. Structure: definition/law → working → conclusion. Partial marks for correct equations even if explanation is short.
How to write for NIOS: Use 30–50 words (VSA) or short objective. Open with definition/equation, then reason, end with conclusion. Paper 313/MAY/205C · Q28 · 2 mark(s) · L24.
PYQ13. Read the passage given below and answer the following questions (out of four attempt any two) : Nitro compounds are those derivatives of hydrocarbons in which a hydrogen atom is replaced by a nitro group. These may be aliphatic or aromatic. Nitroalkanes are divided into primary, secondary or tertiary depending upon the attachment of nitro group to primary, secondary or tertiary carbon atom respectively. Name the following compound according to the IUPAC nomenclature : CH —CH—CH—CH —CH What happens when propane reacts with nitric acid at 680 K? Give the chemical equation. Write the reduction of nitrobenzene in alkaline medium. Compare the boiling points nitro compounds with the corresponding alkanes. Justify your answer.
Model Answer
Present a clear comparison in 2–3 points (definition / structure / property / example). Use a table style in prose: A vs B for each criterion.
Explanation
Comparison answers need parallel points. Award marks for each distinct difference.
How to write for NIOS: Use 30–50 words (VSA) or short objective. Open with definition/equation, then reason, end with conclusion. Paper 313/TUS/105A · Q26 · 2 mark(s) · L24.
Six problems spanning this chapter’s NIOS syllabus. Every question is built from the notes and formula sheet: solve with equations first, then read the formal textbook-style write-up, the easy explanation, and the topic in depth (formulas, meaning, exam tips). If the question says draw, a labelled pencil sketch is provided. Explanations open by default.
Draw/label alkane, alkene, alkyne bonding. Give one example of each.
Final answer: Ethane / ethene / ethyne
Hydrocarbons contain only C and H; classified by bonds.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Single, double, triple carbon–carbon bonds define the family.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Aromatic hydrocarbons contain benzene-type rings.
Linked to chapter notes (L24). Remember: (see solution steps). Most exam errors are unit mix-ups (g vs mol, mL vs L) or wrong mole ratios from the equation.
Write (see solution steps) before substituting. Keep three significant figures until the end when data allow.
State Markownikov’s rule for HBr addition to propene.
Final answer: 2-bromopropane major
Electrophilic addition via more stable carbocation.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
H goes where there are already more H’s; Br goes to the busier carbon.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Peroxide effect gives anti-Markownikov with HBr.
Linked to chapter notes (L24). Remember: (see solution steps). Most exam errors are unit mix-ups (g vs mol, mL vs L) or wrong mole ratios from the equation.
Write (see solution steps) before substituting. Keep three significant figures until the end when data allow.
State Hückel’s rule briefly.
Final answer: (4n+2) π electrons
Explains special stability of benzene.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Magic electron counts 6,10,14… for aromatic rings.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Benzene undergoes substitution more than addition.
Linked to chapter notes (L24). Remember: (see solution steps). Most exam errors are unit mix-ups (g vs mol, mL vs L) or wrong mole ratios from the equation.
Write (see solution steps) before substituting. Keep three significant figures until the end when data allow.
Write balanced combustion of CH₄.
Final answer: CH₄ + 2O₂ → CO₂ + 2H₂O
Complete combustion yields CO₂ and H₂O.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Burn methane fully: carbon dioxide and water.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Incomplete combustion can give CO/soot.
Linked to chapter notes (L24). Remember: (see solution steps). Most exam errors are unit mix-ups (g vs mol, mL vs L) or wrong mole ratios from the equation.
Write (see solution steps) before substituting. Keep three significant figures until the end when data allow.
Give chain isomers of C₄H₁₀.
Final answer: n-butane & isobutane
Same formula, different carbon skeleton.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Straight chain vs branched.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Affects boiling points.
Linked to chapter notes (L24). Remember: (see solution steps). Most exam errors are unit mix-ups (g vs mol, mL vs L) or wrong mole ratios from the equation.
Write (see solution steps) before substituting. Keep three significant figures until the end when data allow.
How is ethene prepared from ethanol in lab (outline)?
Final answer: Dehydration of ethanol
Elimination of water forms the double bond.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Heat ethanol with strong acid—lose water, get ethene gas.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Industrial: cracking of alkanes.
Linked to chapter notes (L24). Remember: (see solution steps). Most exam errors are unit mix-ups (g vs mol, mL vs L) or wrong mole ratios from the equation.
Write (see solution steps) before substituting. Keep three significant figures until the end when data allow.