HBrO3 vs HClO3 in acidity. (think, then open)
Same number of oxygens (q=2 each), so Pauling's rule predicts nearly equal. The small difference comes from electronegativity: Cl is more electronegative than Br, so HClO3 is slightly stronger. The dominant factor is oxygen count, electronegativity is the tie-breaker.
Recall Forecast: Which is stronger,
H2SO4 or H2SO3?
H2SO4: q=2, H2SO3: q=1. More terminal O ⇒ H2SO4 stronger. Same logic as halogen oxoacids. ✔
HClO4 beat HClO in acidity?
ClO4− delocalises its −1 charge over 4 equivalent O atoms (−41 each) → very stable anion; ClO− has the charge stuck on 1 O → unstable. Stable conjugate base = strong acid.
Recall Where is the acidic H located in
HClO3?
On an O–H group (the structure is (O)2Cl–O–H), never directly on Cl.
Recall State Pauling's rule and use it for
HClO2.
pKa≈8−5q. For HClO2, q=1⇒pKa≈3. Matches experiment (~2).
Order of acid strength of HClO, HClO₂, HClO₃, HClO₄
HClO < HClO₂ < HClO₃ < HClO₄ (increases with number of oxygens)
Why does acidity increase along this series
More terminal O atoms delocalise & withdraw the anion's negative charge → conjugate base more 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, where q = number of terminal (non-OH) oxygens
Predicted pKa of HClO₄ by Pauling's rule
q=3 → pKa ≈ 8 − 15 = −7 (a very strong acid)
Trend of oxidising power: HClO vs HClO₄
HClO is the stronger oxidiser; oxidising power DECREASES as O increases (opposite to acidity)
Where is the acidic proton in these acids
On an O–H bond (Cl–O–H), not directly bonded to Cl
Which is a stronger acid, HClO₃ or HBrO₃, and why
HClO₃, slightly — same O count, but Cl is more electronegative; oxygen count dominates, electronegativity is the tie-breaker
Conjugate base of HClO₄ and its stability reason
ClO₄⁻; charge spread equally over 4 O atoms (−¼ each) → very stable
Recall Feynman: explain to a 12-year-old
Imagine four kids each holding one heavy water balloon (the negative charge) — that's the leftover ion after the acid lets its H go. A kid holding the balloon alone gets tired fast (unstable) — that's ClO−, so HClO doesn't like to let its H go (weak acid). But if four friends share one balloon by all touching it, it feels light for everyone (very stable) — that's ClO4−, so HClO4 happily throws its H away (strong acid). More oxygen-friends to share the load = the acid lets go of its H more easily = stronger acid.
Dekho yaar, chaar acids hain — HClO, HClO₂, HClO₃, HClO₄ — aur sabme ek hi acidic hydrogen hota hai jo oxygen pe baitha hota hai (Cl–O–H), Cl pe nahi. Question yeh hai ki yeh H₊ chhodne mein kaun zyada eager hai? Iska jawab depend karta hai ki H chhodne ke baad jo anion banta hai (ClOₓ⁻), woh kitna stable hai. Jitna stable anion, utna strong acid.
Ab trick yeh hai: jitne zyada oxygen chlorine ke around honge, utna woh negative charge zyada O atoms pe spread ho jaata hai. ClO⁻ mein poora −1 charge ek hi O pe atka rehta hai (unstable), par ClO₄⁻ mein −1 charge 4 oxygen pe baant jaata hai (−¼ har ek pe) — bahut stable. Isiliye acidity ka order banta hai: HClO < HClO₂ < HClO₃ < HClO₄. More oxygen = more sharing = stronger acid. Pauling ka shortcut bhi yaad rakho: pKa≈8−5q, jisme q = terminal (non-OH) oxygens. Jaise HClO₄ ke liye q=3, toh pKa ≈ −7, matlab super strong.
Ek important trap: oxidising power ka order ulta hai — HClO sabse strong oxidiser hai, HClO₄ sabse weak. Weak acid hone ka matlab weak oxidiser nahi hota! Low oxidation state (+1 in HClO) electrons grab karne ko zyada ready hota hai. Toh exam mein yaad rakho: acidity badhti hai O ke saath, par oxidising power aur thermal stability ghatti hai. Bas yeh ek line aapko bahut saare MCQ bacha legi.
