313_Chemistry_Eng_Lesson20.pdf). Content covers sections 20.1–20.9.L19 covered Groups 13–15. This lesson treats Groups 16, 17 and 18: oxygen and sulphur (oxides classification, ozone, sulphur allotropes, SO₂, Contact-process H₂SO₄, oxoacids of sulphur); halogens (hydrogen halides, chlorine oxides and oxoacids, CFCs, interhalogens); and noble gases (occurrence, inertness, xenon fluorides and oxides with VSEPR structures).
Industrial and environmental themes dominate: sulphuric acid as “king of chemicals,” ozone for purification and stratospheric protection, freons and ozone depletion, and the discovery that noble gases are not completely inert.
O, S non-metals; Se, Te semiconductors; Po metallic. Complete octet by gaining 2e⁻ (O²⁻, S²⁻) or two covalent bonds (H₂O, H₂S…). S, Se, Te expand octet using d-orbitals (SCl₄, SF₆); oxygen cannot. O₂ is diatomic with O=O; sulphur catenates as S₈ rings (rhombic/monoclinic).
Oxygen is ~21% of air (photosynthesis balance), ~50% of crust by weight, ~89% of water. Sulphur occurs free and as sulphides/sulphates; essential in proteins (cysteine).
Acidic oxides (non-metals / high OS metals): SO₂ + H₂O → H₂SO₃; SO₂ + 2NaOH → Na₂SO₃ + H₂O. SiO₂ does not form acid with water easily but reacts with alkalies → silicates. Basic oxides (metals): FeO + H₂SO₄ → FeSO₄ + H₂O; Group 1/2 oxides + water → alkalies. Amphoteric: ZnO and Al₂O₃ with both H₂SO₄/HCl and NaOH. Neutral: CO, NO, N₂O.
Allotrope of oxygen; formed near high-voltage equipment and in forests from organic decay. Siemens ozonizer: dry cold O₂ through silent electric discharge between tinfoil-coated coaxial tubes → 5–10% O₃. Avoid rubber/cork (attacked by O₃).
Structure: V-shaped, central O sp² hybridised; resonance hybrid of two structures; O–O 128 pm; angle 117°.
Properties: Pale blue gas; unstable; 2O₃ → 3O₂. Oxidations: PbS → PbSO₄ (black→white); Fe²⁺ → Fe³⁺; moist S → H₂SO₄; tails mercury (Hg₂O film); SnCl₂ → SnCl₄ without free O₂. Unsaturated organics form ozonides; hydrolysis gives carbonyls — ozonolysis locates double bonds.
Uses: sterilise water (no chlorinated by-products); purify air; refine oils; dry bleach (flour, sugar, wax); organic synthesis.
Rhombic (α) and monoclinic (β) sulphur both contain puckered S₈ rings; transition temperature ~369 K. Plastic sulphur is amorphous. SO₂ from burning S or roasting pyrites: colourless, pungent, acidic oxide, bleaching/reducing agent, precursor to H₂SO₄.
Contact process steps: (1) SO₂ from S or FeS₂; (2) purify (remove As etc.); (3) oxidise on V₂O₅; (4) SO₃ into conc. H₂SO₄ → H₂S₂O₇ (oleum); (5) dilute carefully with water. Never add water to conc. H₂SO₄ (violent heat) — add acid to water with stirring.
Hot conc. H₂SO₄ is an oxidant (Cu, C, S, HBr, HI, H₂S). Strong dehydrating agent: CuSO₄·5H₂O blue→white; sugar → carbon. Uses: fertilizers, paints, detergents, plastics, fibres — “king of chemicals.” Oxoacids of S include sulphurous, sulphuric, thiosulphuric, peroxydisulphuric (Caro’s, Marshall’s), polythionic acids.
Key Contact equations to memorise: S + O₂ → SO₂; 4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂; 2SO₂ + O₂ ⇌ 2SO₃; SO₃ + H₂SO₄ → H₂S₂O₇; H₂S₂O₇ + H₂O → 2H₂SO₄. Oxidising demos: Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O; C + 2H₂SO₄ → CO₂ + 2SO₂ + 2H₂O.
All non-metals (I shows slight metallic character). High EN; F most electronegative. F₂ strongest oxidant; reacts with almost all elements. Cl₂ industrial from electrolysis of brine (L13/L17). Lab Cl₂: MnO₂ + HCl or KMnO₄ + HCl.
Hydrogen halides: Acid strength HF < HCl < HBr < HI (H–X bond weakens down group). Bond energy and thermal stability reverse: HF strongest bond. HF is liquid at RT due to H-bonding; others gases. Industrial HF: CaF₂ + H₂SO₄. HCl: NaCl + H₂SO₄ (two stages) or H₂ + Cl₂. HBr/HI via phosphorus + halogen then hydrolysis (not conc. H₂SO₄ — oxidises HI/HBr).
Chlorine oxoacids: HOCl (+1), HClO₂ (+3), HClO₃ (+5), HClO₄ (+7). Acid strength increases with number of O atoms: HOCl < HClO₂ < HClO₃ < HClO₄ — more O pulls electron density from O–H. NaOCl is bleach. HClO₄ is one of the strongest acids. Oxides: Cl₂O, ClO₂ (explosive), Cl₂O₆, Cl₂O₇.
CFCs (freons): CCl₂F₂, CFCl₃, etc. Excellent refrigerants and aerosol propellants but destroy stratospheric ozone (ozone hole) — major environmental hazard; production restricted.
Interhalogens: XX′, XX₃′, XX₅′, XX₇′ (only IF₇). Prepared by direct combination under controlled T and F₂ ratio. Structures explained by VSEPR (ClF₃ T-shaped, IF₅ square pyramidal, IF₇ pentagonal bipyramidal).
He, Ne, Ar, Kr, Xe, Rn — closed shells (He 1s²; others ns²np⁶). High IE; historically “inert.” Present in air (Ar ~0.93%); He in natural gas; Rn radioactive from Ra decay. Not used as “inert atmosphere” label only — Xe chemistry is real.
Neil Bartlett (1962): first noble-gas compound (Xe + PtF₆). Xe reacts with F₂ under controlled conditions:
XeF₂: linear (3 lp). XeF₄: square planar (2 lp). XeF₆: distorted octahedral. XeO₃: pyramidal; XeO₄: tetrahedral; XeOF₄: square pyramidal. Hydrolysis: XeF₂ slow → Xe + O₂ + HF; XeF₄/XeF₆ → XeO₃. Partial hydrolysis of XeF₆ → XeOF₄, XeO₂F₂. Complexes with Lewis acids (e.g. [XeF]⁺[PF₆]⁻).
Must-know lists: oxide classes with equations; ozone structure and oxidations; Contact process steps and why oleum; HX and Cl-oxoacid strength orders; CFC–ozone link; interhalogen types; XeF₂/XeF₄/XeF₆ shapes and hydrolysis.
Links: L18 trends (EN, IE, first-element anomalies); L19 industrial chemistry; L13 electrolysis of brine for Cl₂; environmental chemistry of ozone and freons.
Module 6 p-block closes here. Practice drawing structures (O₃, S₈ idea, HX H-bonds, Cl oxoacids, Xe compounds) and writing Contact and ozone reactions — that combination covers the bulk of L20 exam credit.
Intext checkpoints: oxide classification (K₂O, SiO₂, Al₂O₃, ZnO…); ozonides and hydrolysis; O₃ with FeSO₄/SnCl₂; Contact catalyst and oleum reason; HX strength order; CFC and ozone hole; most electronegative halogen; XeF₂/XeF₄ shapes. Link L18 (EN, IE) and L19 (industrial N chemistry) — Group 16–18 complete the p-block story from boron to radon.
Most exam-important points from this chapter:
Classify acidic/basic/amphoteric/neutral with equations. O₂ gas (O=O); S solid (S₈). O₃ bent oxidant; ozonolysis.
Contact: V₂O₅, SO₃ into H₂SO₄ not water. Hot conc. oxidises; conc. dehydrates. Never water into acid.
Acid: HF < HCl < HBr < HI. HF liquid (H-bonds). Prepare HF/HCl with H₂SO₄; HBr/HI via P + X₂.
More O → stronger acid (HClO₄ strongest). CFCs destroy O₃. Interhalogens XXₙ′; only IF₇ is XX₇′.
Closed shell; Xe forms XeF₂ (linear), XeF₄ (sq. planar), XeF₆, XeO₃. Bartlett opened Xe chemistry.
Extracted from NIOS Chemistry (313) board exam papers in your PDF. Chapter L20 — p-Block Elements and Their Compounds-II only. Use Model Answer for marking points; Explanation for concept clarity.
2 question(s) · Sources: 313/TUS/105A
PYQ1. Write True (T) for correct statement and False (F) for incorrect statement (out of four attempt any two) : Sulphur shows two, four and six valencies in its compounds. Fluorine is prepared by electrolysis of hydrogen fluoride. Chlorofluorocarbons have very high capacity to retain heat. The oxidation state of chlorine in chlorous acid is +1. ghr
Model Answer
Answer using key concepts from L20 (definitions, equations, and one example where useful). Stay within the suggested word range for a 2-mark NIOS question.
Explanation
Cross-check with L20 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/TUS/105A · Q22 · 2 mark(s) · L20.
PYQ2. How are xenon oxofluorides obtained? Write the chemical equations involved. OrZm°Z Am°³gmoâbwAmoamBS> H¡$go àmá {H$E OmVo h¢? g§~Õ amgm¶{ZH$ g‘rH$aUm|
Model Answer
Xenon oxofluorides form by partial hydrolysis of xenon fluorides, e.g. XeF₆ + H₂O → XeOF₄ + 2HF; XeF₆ + 2H₂O → XeO₂F₂ + 4HF; XeF₆ + 3H₂O → XeO₃ + 6HF.
Explanation
Noble-gas compounds of Xe with F/O are classic p-block examples (L18/L20).
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 · Q35 · 2 mark(s) · L20.
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 F₂ the most reactive halogen?
Final answer: Weak F–F + high EN
Fluorine is the strongest elemental oxidising agent.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Grabs electrons extremely readily.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
HF shows strong H-bonding.
Linked to chapter notes (L20). 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.
Lab preparation of HCl from NaCl (outline).
Final answer: NaCl + H₂SO₄ → HCl
Less volatile acid displaces more volatile HCl.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Salt plus sulphuric acid frees HCl.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Dry carefully.
Linked to chapter notes (L20). 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 example of an interhalogen compound.
Final answer: e.g. ClF₃ or IF₇
Compounds between two different halogens.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Larger central halogen can bind more F.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Often more reactive than parents.
Linked to chapter notes (L20). 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 are noble gases largely inert?
Final answer: Full valence shell
High ionisation energy and complete octets (He duet).
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Little chemical drive to share or transfer electrons.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Xe compounds under special conditions.
Linked to chapter notes (L20). 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.
Formula of bleaching powder?
Final answer: Ca(OCl)Cl
From chlorine and slaked lime.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Source of chlorine for bleaching.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Also disinfectant.
Linked to chapter notes (L20). 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 the oxoacid of chlorine with OS +5.
Final answer: HClO₃
Series HClO, HClO₂, HClO₃, HClO₄.
Working formulas: (see solution steps). State the definition or law first (NIOS style), use SI units, and box the final numerical answer with unit.
Higher OS generally stronger acid.
Read once for the idea, once for the numbers. Write the formula, substitute, then simplify. Check whether you used moles, grams, or litres correctly.
Count O to find OS.
Linked to chapter notes (L20). 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.