313_Chemistry_Eng_Lesson19.pdf). Content covers sections 19.1–19.6.Lesson 18 gave general p-block trends. This lesson develops important compounds of Groups 13, 14 and 15: boric acid, borax, diborane, BF₃, AlCl₃ and alums; carbon allotropes, CO/CO₂/SiO₂, CCl₄ vs SiCl₄, SiC, silicones, silicates and zeolites; nitrogen, ammonia (Haber), nitric acid (Ostwald), phosphorus allotropes and halides, and fertilizers. Groups 16–18 follow in L20.
Key themes: electron-deficient bonding (B, Al), catenation (C), empty d-orbitals (Si, P), industrial processes (Haber, Ostwald), and materials (diamond, graphite, silicones, fertilizers).
Group 13: B, Al, Ga, In, Tl. Valency three, but anhydrous compounds are largely covalent — removing three electrons costs huge energy (B never forms B³⁺). B is non-metallic; Al metallic; Ga–Tl weakly metallic. Inert pair makes Tl⁺ important (L18). Al is the most abundant metal in Earth’s crust; main ore bauxite. Boron occurs as borax and kernite.
Boric acid H₃BO₃ from borax + H₂SO₄: Na₂B₄O₇ + H₂SO₄ + 5H₂O → Na₂SO₄ + 4H₃BO₃. Weak Lewis acid: B(OH)₃ + 2H₂O → [B(OH)₄]⁻ + H₃O⁺. Structure: 2D sheets of B(OH)₃ units linked by H-bonds; layers held by van der Waals — flakes. Heat → metaboric acid → B₂O₃. Uses: antiseptic, food preservative, enamels and glass.
Borax Na₂B₄O₇·10H₂O (tincal; from colemanite + Na₂CO₃). Flux, buffer in dyeing, optical/borosilicate glass, glazes, preservative. Loses water of crystallisation on heating.
Prepared from BCl₃ + LiAlH₄ or BF₃ + LiH. Toxic, foul-smelling gas; burns with huge heat (ΔH = −1976 kJ mol⁻¹); hydrolyses to boric acid + H₂.
Structure: 4 terminal H and 2 B in a plane; 2 bridging H above and below. Only 12 valence electrons for eight B–H contacts → four normal 2c–2e B–H bonds + two 3c–2e (banana) B–H–B bridges. Classic electron-deficient bonding. Terminal B–H ~119 pm; bridge bonds longer (~134 pm) — weaker multi-centre links.
Al becomes passive after conc. HNO₃ because a thin Al₂O₃ film seals the surface — same idea as Be’s oxide film (L17). Always distinguish anhydrous AlCl₃ (covalent dimer, catalyst) from aqueous aluminium chemistry (hydrated ions).
BF₃ from B₂O₃ + HF. Incomplete octet → Lewis acid; complexes F₃B←NH₃, F₃B←OEt₂. Resonance shortens B–F. Catalyst in Friedel–Crafts and polymerisation. Hydrolyses: 4BF₃ + 3H₂O → H₃BO₃ + 3HBF₄.
AlCl₃ as Al₂Cl₆ dimer at room temperature (Cl bridges complete Al octets; roughly tetrahedral about each Al). Made from Al + Cl₂ or HCl. Sublimes; with water gives [Al(H₂O)₆]³⁺. Anhydrous form: Friedel–Crafts catalyst.
Double salts / alums: KAl(SO₄)₂·12H₂O (potash alum) crystallises as one solid but ions free in solution. General M⁺M′³⁺(SO₄)₂·12H₂O. Used as mordant and water purifier. Variants: chrome alum, ferric alum, ammonium alum.
Catenation is strong for carbon (strong C–C) but weak for silicon (weaker Si–Si) — hence vast organic chemistry vs few silanes.
Diamond: each C sp³-linked to four others in 3D; hardest natural substance; unreactive; burns above ~800°C to CO₂; jewellery and abrasives.
Graphite: sp² layers (hexagons); fourth e⁻ delocalised → electrical conductor; layers slip (lubricant); electrodes, pencils, crucibles. Fullerenes (e.g. C₆₀) — soccer-ball cages.
Oxides: CO neutral, toxic (Hb binding), industrial fuel and reductant (blast furnace Fe₂O₃ + 3CO). CO₂ acidic, dry ice (sublimes −78°C), carbonated drinks, soda manufacture. SiO₂ (silica, quartz): acidic macromolecule of SiO₄ tetrahedra sharing corners; high melting; glass, sand; reacts with HF/F₂ to SiF₄; alkalies → silicates.
Halides: CCl₄ and SiCl₄ both tetrahedral (sp³). CCl₄ not hydrolysed; SiCl₄ + 4H₂O → Si(OH)₄ + 4HCl because Si has empty 3d orbitals for H₂O attack. Si forms SiF₆²⁻; carbon cannot form CF₆²⁻.
SiC (carborundum): SiO₂ + 3C → SiC + 2CO; diamond-like Si–C network; abrasive.
Silicones: polymers with –Si–O–Si– backbone and organic R groups; from RCl + Si (Cu, 300°C) then hydrolysis/condensation; water-repellent, flexible, thermally stable.
Silicates & zeolites: based on SiO₄ tetrahedra sharing oxygen (isolated units, Si₂O₇, chains with two O shared, sheets, 3D frameworks). Zeolites: open frameworks used as molecular sieves, ion exchangers and catalysts — water softener and petrochemical cracking applications.
Why few silanes but millions of hydrocarbons? C–C bond energy is high and C forms strong multiple bonds; Si–Si is weaker and Si prefers bonding to O (Si–O very strong). Life’s backbone is carbon; Earth’s crust backbone is silica and silicates.
N, P, As, Sb, Bi. Mostly covalent compounds; N forms N³⁻ with reactive metals; Sb³⁺, Bi³⁺ via inert pair. N≡N triple bond makes N₂ inert at room temperature. N and P are essential in proteins, DNA and bones (phosphate).
Dinitrogen: 78% of air; laboratory from NH₄NO₂ heat or azides; industry by fractional distillation of liquid air. Forms nitrides with Li, Mg, Al; Haber NH₃; lightning gives NO (start of natural nitrogen fixation to nitrates).
Ammonia: lab from NH₄⁺ salt + base or Mg₃N₂ + H₂O. Colourless, pungent, highly soluble; weak base (NH₃ + H₂O ⇌ NH₄⁺ + OH⁻). Burns in O₂; catalytic oxidation to NO (Ostwald start); reduces CuO/PbO; forms ammonium salts; excess Cl₂ → N₂, excess Cl₂ conditions can give NCl₃; aqueous NH₃ precipitates metal hydroxides and forms ammine complexes [Cu(NH₃)₄]²⁺, [Ag(NH₃)₂]⁺.
Nitric acid (Ostwald): 4NH₃ + 5O₂ → 4NO + 6H₂O (Pt, hot) → 2NO + O₂ → 2NO₂ → 3NO₂ + H₂O → 2HNO₃ + NO. Lab: NaNO₃ + H₂SO₄. Strong acid and oxidant (C, S, P, I₂, Cu, Zn — products depend on concentration). Conc. HNO₃ passivates Al (oxide film). Uses: fertilizers, explosives, aqua regia, oxidant, rocket propellant (fuming).
Phosphorus allotropes: White — P₄ tetrahedra, 60° angles, strained, waxy, toxic, reactive; red — polymeric, from heating white, denser, safer; black — high pressure, layered, conducting, graphite-like.
PCl₃: P₄ + 6Cl₂ → 4PCl₃; hydrolyses to H₃PO₃ + HCl (d-orbitals allow expansion). PCl₅ also known for heavier congeners’ ability to form 5-covalent compounds (N cannot).
Fertilizers: nitrogenous (NH₃, urea, ammonium salts), phosphatic (superphosphate, etc.), mixed NPK — essential for agriculture after natural soil N is depleted. Industrial N₂ fixation (Haber) and natural fixation (lightning, legumes/bacteria) both feed the nitrogen cycle.
High-yield topics: boric acid as Lewis acid; diborane 3c–2e structure; BF₃/AlCl₃ as Lewis acids and catalysts; potash alum formula and uses; diamond vs graphite (hybridisation, properties, uses); CO toxicity; CCl₄ vs SiCl₄ hydrolysis; SiC; silicones; Haber conditions and equilibrium logic; Ostwald steps; white vs red P; fertilizer types.
Connect to L18 (inert pair, first-element anomalies, empty d-orbitals) and L13 (industrial catalysis). L20 continues with Groups 16–18 (O, S, halogens, noble gases).
Master structure sketches (B₂H₆, Al₂Cl₆, diamond/graphite, NH₃ pyramid) and process flow charts (Haber, Ostwald) — most NIOS marks on this chapter reward clear diagrams and balanced equations more than long prose.
Intext-style checks: formula of borax and boric acid; one preparation of B₂H₆; alum general formula; Al₂Cl₆ structure; diamond properties not shared by graphite; hybridisation in diamond/graphite; CCl₄ vs SiCl₄; CO vs CO₂ acidity; Haber and Ostwald outlines; white P structure; fertilizer classes. If you can answer these with equations, you are exam-ready for L19.
Most exam-important points from this chapter:
H₃BO₃ Lewis acid; B₂H₆ has 3c–2e bridges; BF₃/Al₂Cl₆ Lewis acids & catalysts; alum = double salt KAl(SO₄)₂·12H₂O.
Diamond sp³ hard insulator; graphite sp² soft conductor; C₆₀ fullerene. Structure → properties.
Catenation strong for C. CCl₄ no hydrolysis; SiCl₄ yes (3d). SiC abrasive; silicones & silicates/zeolites.
Haber NH₃ (high P, Fe). Ostwald HNO₃ via NO. NH₃ weak base; HNO₃ strong acid/oxidant.
White P₄ reactive; red polymeric; black layered. N/P/K fertilizers from NH₃ and phosphates.
Extracted from NIOS Chemistry (313) board exam papers in your PDF. Chapter L19 — p-Block Elements and Their Compounds-I only. Use Model Answer for marking points; Explanation for concept clarity.
9 question(s) · Sources: 313/MAY/205A, 313/MAY/205B, 313/MAY/205C
PYQ1. The number of oxygen atoms furnished by two molecules of permanganate in acidic medium is — (A) Aåbr¶ ‘mܶ‘ ‘
Model Answer
Answer using key concepts from L19 (definitions, equations, and one example where useful). Stay within the suggested word range for a 1-mark NIOS question.
Explanation
Cross-check with L19 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 · Q9 · 1 mark(s) · L19.
PYQ2. Complete the following choosing from the given options : ( 96 g, 3 g, 48 g, 34 g, 30 g, 28 g ) 4Fe(s) + 3O2(g)2Fe2O3(s) In this reaction, the mass of oxygen reacting is _____. N2(g) + 3H2(g)2NH3(g) The grams of NH3 produced in the above reaction are _____
Model Answer
Answer using key concepts from L19 (definitions, equations, and one example where useful). Stay within the suggested word range for a 2-mark NIOS question.
Explanation
Cross-check with L19 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 · Q17 · 2 mark(s) · L19.
PYQ3. Read the passage given below and answer the following questions : The binary compounds of oxygen with other elements (metals or non-metals) are called oxides. An understanding of the nature of an oxide provides a clue to the nature of the element which forms the oxide. These oxides may be acidic, basic, amphoteric or neutral depending on the element with which they are formed. Silicon dioxide is which type of oxide? Name two elements which form neutral oxides.
Model Answer
Answer using key concepts from L19 (definitions, equations, and one example where useful). Stay within the suggested word range for a 2-mark NIOS question.
Explanation
Cross-check with L19 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 · Q22 · 2 mark(s) · L19.
PYQ4. The number of oxygen atoms furnished by two molecules of permanganate in acidic medium is — (A) Aåbr¶ ‘mܶ‘ ‘
Model Answer
Answer using key concepts from L19 (definitions, equations, and one example where useful). Stay within the suggested word range for a 1-mark NIOS question.
Explanation
Cross-check with L19 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/205B · Q3 · 1 mark(s) · L19.
PYQ5. Read the passage given below and answer the following questions : The binary compounds of oxygen with other elements (metals or non-metals) are called oxides. An understanding of the nature of an oxide provides a clue to the nature of the element which forms the oxide. These oxides may be acidic, basic, amphoteric or neutral depending on the element with which they are formed. Silicon dioxide is which type of oxide? Name two elements which form neutral oxides.
Model Answer
Answer using key concepts from L19 (definitions, equations, and one example where useful). Stay within the suggested word range for a 2-mark NIOS question.
Explanation
Cross-check with L19 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 · Q25 · 2 mark(s) · L19.
PYQ6. The number of oxygen atoms furnished by two molecules of permanganate in acidic medium is — (A) Aåbr¶ ‘mܶ‘ ‘
Model Answer
Answer using key concepts from L19 (definitions, equations, and one example where useful). Stay within the suggested word range for a 1-mark NIOS question.
Explanation
Cross-check with L19 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 · Q13 · 1 mark(s) · L19.
PYQ7. Complete the following choosing from the given options : ( 34, 28, 17, H3O+, H2O, H2O4 ) 2NH (g) In the above reaction, _____ g of NH3 is produced. The empirical formula of H2O is _____
Model Answer
Answer using key concepts from L19 (definitions, equations, and one example where useful). Stay within the suggested word range for a 2-mark NIOS question.
Explanation
Cross-check with L19 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 · Q17 · 2 mark(s) · L19.
PYQ8. Read the passage given below and answer the following questions : The binary compounds of oxygen with other elements (metals or non-metals) are called oxides. An understanding of the nature of an oxide provides a clue to the nature of the element which forms the oxide. These oxides may be acidic, basic, amphoteric or neutral depending on the element with which they are formed. Silicon dioxide is which type of oxide? Name two elements which form neutral oxides.
Model Answer
Answer using key concepts from L19 (definitions, equations, and one example where useful). Stay within the suggested word range for a 2-mark NIOS question.
Explanation
Cross-check with L19 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 · Q19 · 2 mark(s) · L19.
PYQ9. Match the items in Column—I with Column—II : Column—I Column—II Groups with –I effect —CH3, —H, (CH3)3C— Groups with +I effect Presence of aluminium chloride/AlCl3 Dehydrating agent (iii) — NO , — CN, — OH Molecular rearrangements conc. sulphuric acid/H2SO4
Model Answer
Answer using key concepts from L19 (definitions, equations, and one example where useful). Stay within the suggested word range for a 2-mark NIOS question.
Explanation
Cross-check with L19 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 · Q24 · 2 mark(s) · L19.
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.
Why is N₂ chemically inert at room temperature?
Final answer: Strong triple bond
High bond dissociation enthalpy of N₂.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Hard to break N≡N, so air N₂ is unreactive.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Fixed industrially by Haber process.
Linked to chapter notes (L19). 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 shape and approximate bond angle of NH₃.
Final answer: Pyramidal ~107°
VSEPR: three bond pairs + one lone pair on nitrogen.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Lone pair pushes hydrogens down into a pyramid.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Basic due to lone pair.
Linked to chapter notes (L19). 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.
Outline Ostwald process for manufacture of HNO₃.
Final answer: NH₃ → NO → NO₂ → HNO₃
Industrial Ostwald route.
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 ammonia, make NO₂, dissolve.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Pt catalyst.
Linked to chapter notes (L19). 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.
Name two allotropes of phosphorus.
Final answer: White and red
White P is molecular P₄; red is polymeric.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
White is more reactive/toxic.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Store white under water.
Linked to chapter notes (L19). 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.
Why is O₂ paramagnetic?
Final answer: Unpaired e⁻ in MO diagram
MO theory places two electrons unpaired in π* orbitals.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Liquid oxygen is attracted to a magnet.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Classic evidence for MO theory.
Linked to chapter notes (L19). 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 one use of ozone.
Final answer: Disinfection / oxidant
O₃ is a strong oxidising allotrope of oxygen.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Kills microbes by oxidation.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Stratospheric ozone absorbs UV.
Linked to chapter notes (L19). 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.