Oxoacids of halogens — HClO, HClO₂, HClO₃, HClO₄ — acidity trend

3.2.10 · Chemistry › p-Block

YE acids KYA hain?

Definition Chlorine ke chaar oxoacids

Acid	Formula	Cl oxidation state	Name
Hypochlorous	$H C l O$ (HOCl)	+1	hypochlorous
Chlorous	$H C l O_{2}$	+3	chlorous
Chloric	$H C l O_{3}$	+5	chloric
Perchloric	$H C l O_{4}$	+7	perchloric

Har ek mein ek acidic H hota hai, aur wo hamesha oxygen se attached hota hai, seedha chlorine se nahi. General structure hai $(O)_{n} C l - O - H$ , jahan $n$ = extra (non-OH) oxygens ki sankhya = 0,1,2,3.

Acidity zyada oxygens ke saath KYUN badhti hai?

Acid ionise hoti hai: $H C l O_{x} ⇌ H^{+} + C l O_{x}^{-}$

Acid strength = ye kitna right side ki taraf shift hai = anion $C l O_{x}^{-}$ kitna stable hai.

Figure — Oxoacids of halogens — HClO, HClO₂, HClO₃, HClO₄ — acidity trend

Effect kitna bada hai? (numbers)

Worked example Pauling's rule chaaon acids pe apply karo

$q$ = terminal oxygens (total O minus woh ek jo O–H mein hai):

Acid	terminal O $(q)$	$p K_{a} \approx 8 - 5 q$	actual $p K_{a}$
$H C l O$	0	$8$	$7.5$
$H C l O_{2}$	1	$3$	$2.0$
$H C l O_{3}$	2	$- 2$	$\sim - 1$ (strong)
$H C l O_{4}$	3	$- 7$	$\sim - 10$ (super strong)

Ye step kyun? Kam $p K_{a}$ ⇒ bada $K_{a}$ ⇒ stronger acid. Numbers neeche jaate hain, jo confirm karta hai $H C l O < H C l O_{2} < H C l O_{3} < H C l O_{4}$ .

Forecast-then-Verify

Recall Forecast:

$H B r O_{3}$ vs $H C l O_{3}$ ki acidity rank karo. (sochlo, phir kholo) Oxygens ki sankhya same hai (dono mein $q = 2$ ), toh Pauling's rule predict karta hai lagbhag equal. Chhota sa difference electronegativity se aata hai: Cl, Br se zyada electronegative hai, toh $H C l O_{3}$ thodi si stronger hai. Dominant factor oxygen count hai, electronegativity sirf tie-breaker hai.

Recall Forecast:

$H_{2} S O_{4}$ ya $H_{2} S O_{3}$ mein kaun stronger hai? $H_{2} S O_{4}$ : $q = 2$ , $H_{2} S O_{3}$ : $q = 1$ . Zyada terminal O ⇒ $H_{2} S O_{4}$ stronger. Halogen oxoacids jaisi hi logic. ✔

Common mistakes (Steel-man + fix)

Active recall

Recall

$H C l O_{4}$ , $H C l O$ ko acidity mein kyun beat karta hai? $C l O_{4}^{-}$ apna $- 1$ charge 4 equivalent O atoms pe delocalize karta hai (har ek pe $- \frac{1}{4}$ ) → bahut stable anion; $C l O^{-}$ mein charge 1 O pe hi atka rehta hai → unstable. Stable conjugate base = strong acid.

Recall

$H C l O_{3}$ mein acidic H kahan located hai? O–H group pe (structure hai $(O)_{2} C l - O - H$ ), kabhi seedha Cl pe nahi.

Recall Pauling's rule batao aur

$H C l O_{2}$ ke liye use karo. $p K_{a} \approx 8 - 5 q$ . $H C l O_{2}$ ke liye, $q = 1 \Rightarrow p K_{a} \approx 3$ . Experiment se match karta hai (~2).

Order of acid strength of HClO, HClO₂, HClO₃, HClO₄

HClO < HClO₂ < HClO₃ < HClO₄ (oxygens ki sankhya ke saath badhta hai)

Why does acidity increase along this series

Zyada terminal O atoms anion ka negative charge delocalize aur withdraw karte hain → conjugate base zyada stable → stronger acid

Oxidation state of Cl in HClO, HClO₂, HClO₃, HClO₄

+1, +3, +5, +7

Pauling's rule for oxoacid pKa

pKa ≈ 8 − 5q, jahan q = terminal (non-OH) oxygens ki sankhya

Predicted pKa of HClO₄ by Pauling's rule

q=3 → pKa ≈ 8 − 15 = −7 (bahut strong acid)

Trend of oxidising power: HClO vs HClO₄

HClO zyada strong oxidiser hai; oxidising power GHATTI hai jab O badhta hai (acidity ke opposite)

Where is the acidic proton in these acids

O–H bond pe (Cl–O–H), seedha Cl se bonded nahi

Which is a stronger acid, HClO₃ or HBrO₃, and why

HClO₃, thodi si — same O count, lekin Cl zyada electronegative hai; oxygen count dominant hai, electronegativity tie-breaker hai

Conjugate base of HClO₄ and its stability reason

ClO₄⁻; charge 4 O atoms pe equally spread (har ek pe −¼) → bahut stable

Recall Feynman: ek 12-saal ke bacche ko explain karo

Socho chaar bacche, har ek akela ek heavy water balloon (negative charge) pakde hua hai — ye woh leftover ion hai jab acid apna H jaane deta hai. Akele balloon pakdne wala baccha jaldi thak jaata hai (unstable) — ye hai $C l O^{-}$ , toh $H C l O$ apna H jaane dena pasand nahi karta (weak acid). Lekin agar chaar dost ek balloon ko milke pakdein — sabke haath lage hue — toh har kisi ke liye halka lagta hai (bahut stable) — ye hai $C l O_{4}^{-}$ , toh $H C l O_{4}$ khushi se apna H phenk deta hai (strong acid). Zyada oxygen-dost load share karne ke liye = acid apna H zyada aasaani se jaane deta hai = stronger acid.

Connections

p-Block — group 17 halogen chemistry
Inductive effect — electron withdrawal se anions stabilise hote hain
Resonance and charge delocalisation — kyun charge spread karne se energy kam hoti hai
Conjugate acid–base pairs — strength = base stability
Oxidising power of oxoacids — opposite trend
Pauling rules for oxoacids — $p K_{a} \approx 8 - 5 q$ shortcut
Oxoacids of sulfur and nitrogen — same logic ( $H_{2} S O_{4} > H_{2} S O_{3}$ )