3.2.7
Intuition Badi picture (YE kyun important hai)Oxygen do molecular outfits mein aata hai: aam dioxygen (O₂) jo hum breathe karte hain, aur ozone (O₃) — ek pale-blue, reactive, smelly gas. Same element, alag bonding ⇒ alag properties. Yahi hai allotropy . Stratosphere mein upar ozone hamara sunscreen hai; ground level par ye ek pollutant hai. Ye samajhna ki ozone reactive kyun hai , ye kaise banta aur decompose hota hai , aur ye hume kaise shield karta hai — ye is subtopic ka 80% hai.
Allotropy ek element ka do ya zyada alag physical forms mein same physical state mein exist karna hai, jisme atoms ke bond/arrangement ka tarika alag hota hai. Oxygen ke allotropes hain O₂ (dioxygen) aur O₃ (ozone) .
YE alag kyun hain: same atoms, lekin atoms per molecule ki number alag aur bond arrangement alag → alag bond order, energy, colour, reactivity.
Intuition O₂ paramagnetic kyun hai — ye surprise haiSimple Lewis structure O = O paramagnetic hai (magnet se chipakta hai). Sirf Molecular Orbital Theory hi ye explain karti hai: last do electrons unpaired hoke do degenerate antibonding orbitals π 2 p ∗
O₂ ke liye MOT electron filling (16 electrons):
σ 1 s 2 σ 1 s ∗ 2 σ 2 s 2 σ 2 s ∗ 2 σ 2 p z 2 π 2 p x 2 π 2 p y 2 π 2 p x ∗ 1 π 2 p y ∗ 1
Ozone oxygen ka ek triatomic, bent, V-shaped allotrope hai, pale blue gas, teez smell, O₂ ko silent electric discharge se pass karke banaya jata hai.
Lab mein YE kaise banta hai:
3 O 2 silent electric discharge 2 O 3 Δ H = + 142 kJ mol − 1 ( endothermic )
Intuition Ozone bent kyun hai aur resonance kyun haiCentral O mein 2 bond pairs + 1 lone pair hain ⇒ ~sp² ⇒ bent , angle ≈ 117° . Ek σ + ek delocalised π dono O–O bonds mein spread hai, isliye dono bonds identical hain jinka bond order 1.5 hai — ek resonance hybrid:
O=O–O ↔ O–O=O
Intuition Ozone itna reactive kyun hai (core idea!)Ozone thermodynamically unstable hai (+ve ΔHf): ye extra energy store karta hai. Ye aasani se decompose ho jata hai:
O 3 → O 2 + [ O ] ( nascent oxygen ) nascent oxygen [O] ek bhooka oxidising species hai → isliye ozone ek powerful oxidising agent hai.
Sab O 3 → O 2 + [ O ]
(a) Lead sulphide ko oxidise karta hai → lead sulphate ([O] symbol ki zaroorat nahi):
P b S + 4 O 3 → P b S O 4 + 4 O 2
(b) Iodide ko oxidise karta hai → iodine (estimation ka basis):
2 K I + O 3 + H 2 O → 2 K O H + I 2 + O 2 Ye kyun important hai: nikla hua I 2 quantitative estimation .
(c) Moist potassium iodide / mercury ko oxidise karta hai (meniscus kho deta hai — "tailing of mercury"):
H g + O 3 →
(d) Bleach & germicide ki tarah kaam karta hai [O] ki wajah se.
Worked example Worked: KI titration se O₃ ke moles estimate karoOzonised stream ke 25.0 cm³ se nikla I₂ ko 0.020 mol Na₂S₂O₃ chahiye. O₃ ke mol nikalo.
Reactions: O 3 + 2 K I + H 2 O → I 2 + 2 K O H + O 2 I 2 + 2 S 2 O 3 2 − → 2 I − + S 4 O 6 2 − Ye step kyun? 1 O₃ → 1 I₂; 1 I₂ → 2 thiosulphate.
To mol I₂ = 0.020/2 = 0.010 Kyun? thiosulphate I₂ se double hai.
mol O₃ = = 0.010 mol Kyun? O₃:I₂ ka stoichiometry 1:1 hai.
Worked example Worked: bond order & length predictionQ: O₃ ki bond O₂ se lambi hai ya choti? Kyun?
O₂ B.O. = 2; O₃ B.O. = 1.5. Kam bond order ⇒ kamzor, lambi bond. Ye step kyun? bond order ∝ bond strength ∝ 1/length. Isliye O₃ bond (128 pm) > O₂ (121 pm). ✔
Intuition Natural Chapman cycle (stratosphere mein ozone KAISE banta hai)High-energy UV, O₂ ko tod deta hai; atoms, O₂ se milke O₃ banate hain, jise UV phir tod deta hai — ek steady dynamic equilibrium jo harmful UV-B/UV-C absorb karta hai:
O 2 U V O + O O + O 2 → O 3 O 3 U V O 2 + O heat mein convert hoti hai, surface ko DNA-damaging radiation se shield karti hai.
Intuition CFCs ozone ko KAISE destroy karte hain (catalytic chain — thoda sa itna nuksaan kyun karta hai)Chlorofluorocarbons (jaise C F 2 C l 2 Cl radicals release karta hai:
C F 2 C l 2 U V C l ˙ + C ˙ F 2 C l C l ˙ + O 3 → C l O ⋅ + O 2 C l O ⋅ + O → C l ˙ + O 2 Net: O 3 + O → 2 O 2 Cl regenerate ho jata hai → ek Cl atom hazaron O₃ molecules ko destroy karta hai. Isse layer patli hoti hai → ozone hole (Antarctica ke upar). Cl ek catalyst hai, consume nahi hota — isliye CFC bans (Montreal Protocol) itne important the.
Common mistake "O₂ diamagnetic hai kyunkiO = O Ye sahi kyun lagta hai: Lewis double-bond drawing sab kuch pair kar deta hai. Fix: Lewis antibonding orbitals nahi dikha sakta. MOT 2 unpaired e⁻ ko π 2 p ∗ paramagnetic . Experiment + MOT par trust karo.
Common mistake "Ozone ke do O–O bonds alag hain (ek single, ek double)."Ye sahi kyun lagta hai: resonance picture mein draw hota hai ek single + ek double. Fix: real molecule resonance hybrid hai — dono bonds equal hain, B.O. = 1.5.
Common mistake "Ground-level ozone accha hai kyunki ye hume protect karta hai."Ye sahi kyun lagta hai: ozone = sunscreen association. Fix: Stratospheric ozone shield karta hai; tropospheric (ground) ozone smog mein ek toxic pollutant/oxidant hai.
Common mistake "CFCs ozone destroy karte karte khatam ho jaate hain, isliye jaldi ruk jaate hain."Ye sahi kyun lagta hai: reactions mein reactants consume hote hain. Fix: Cl catalytically kaam karta hai aur regenerate hota hai — isliye CFCs itne dangerous aur persistent hain.
Recall Quick self-test (answers cover karo)
O₂ ka bond order? → 2 , paramagnetic.
O₃ mein har O–O ka bond order? → 1.5 .
O₃ ki shape & angle? → bent, ~117° .
O₃ itna strong oxidant kyun hai? → nascent [O] release karta hai.
Net CFC catalytic equation? → O 3 + O → 2 O 2
Recall Feynman: ek 12-saal ke bacche ko explain karoOxygen LEGO jaisi hai. Do blocks saath = woh hawa jo tum saans lete ho (O₂). Teen blocks ek saath chipke = ozone (O₃), jo thoda grumpy hai aur aasani se toot jaata hai, ek "gusse wala" oxygen block phenkta hai jo doosri cheezein pakad leta hai (bleach karta hai aur germs maarta hai). Waayi upar aasman mein teen-block pieces ka ek blanket Sun ki haanikaarak invisible rays ko kha jaata hai taaki hum jal na jayein. Purane fridges aur sprays se kuch chemicals wahan tak pahunch jaate hain aur blanket ko barbad karte hain — ek troublemaker chemical ek ke baad ek piece ko todta rehta hai, jaise ek baccha jo kabhi thakta nahi towers girane mein.
Mnemonic Numbers yaad karo"Ozone hai TEEN, Bent, aur Dedh (One-and-a-half)."
3 atoms · Bent ≈117° · B.O. 1.5 · pale B lue.
CFC chain: "Cl wapas aata hai" = catalyst.
Allotropy kya hai? Ek element ka same physical state mein do ya zyada forms mein exist karna (jaise O₂ aur O₃).
Lab mein ozone kaise prepare kiya jaata hai? O₂ ko silent electric discharge se pass karke: 3O₂ → 2O₃ (endothermic).
MOT se O₂ ka bond order aur magnetism? Bond order 2; paramagnetic (π*₂p mein 2 unpaired e⁻).
O₃ ki shape, angle aur bond order? Bent V-shape, ~117°, har O–O bond order 1.5 (resonance hybrid).
Ozone ek powerful oxidising agent kyun hai? Ye decompose hota hai O₃ → O₂ + [O], reactive nascent oxygen release karta hai.
O₃ ka KI se reaction? 2KI + O₃ + H₂O → 2KOH + I₂ + O₂ (ozone estimate karne mein use hota hai).
Ozone formation ka ΔH positive kyun hai? Formation endothermic hai; ozone thermodynamically unstable hai (energy store karta hai).
Stratospheric ozone banane ke Chapman reactions? O₂ →(UV) 2O; O + O₂ → O₃; O₃ →(UV) O₂ + O.
CFCs ozone ko kaise destroy karte hain? UV, Cl• release karta hai; Cl•+O₃→ClO•+O₂; ClO•+O→Cl•+O₂; Cl regenerated (catalytic).
Net CFC ozone-destruction equation? O₃ + O → 2O₂ jisme Cl catalyst ki tarah kaam karta hai.
Stratospheric aur tropospheric ozone mein fark? Stratospheric = protective UV shield; tropospheric = haanikaarak pollutant.
O₃ ke dono bonds equal length kyun hain? Ye ek resonance hybrid hai, koi ek fixed structure nahi, isliye dono bonds identical hain (~128 pm).
Molecular Orbital Theory — O₂ paramagnetism & bond order explain karta haiResonance and Delocalisation — O₃ structure ka basisOxidation and Reducing Agents — [O] ke zariye ozone as oxidantFree Radical Chain Reactions — CFC–ozone catalytic cycleEnvironmental Chemistry — Air Pollution — ozone hole, smogGroup 16 — Oxides and Oxoacids — wider oxygen family chemistry
Silent electric discharge
Thermodynamically unstable
