Why is HCl strong and HF weak? Why is $\mathrm{HClO_4}$ strong and HClO weak? The answers come from the bond to the acidic hydrogen and from how well the conjugate base holds its negative charge. You will use electronegativity, bond strength and resonance, and compare acids by Ka.
Key idea
An acid is stronger when the H–A bond is weaker (easier to break) and more polar, and when the anion left behind spreads or stabilizes its negative charge. Down a group, bond strength wins: $\mathrm{HF \ll HCl < HBr < HI}$. Across a period, polarity wins: $\mathrm{CH_4 < NH_3 < H_2O < HF}$. For oxyacids, more O atoms and a more electronegative central atom make a stronger acid.
Why it works
Binary acids: going down Group 17 the halogen grows, the H–X bond gets longer and much weaker (H–F 565, H–Cl 431, H–Br 366, H–I 299 kJ/mol), and the proton leaves more easily. Across a period the atoms are similar in size, so the deciding factor is electronegativity: a more electronegative atom polarizes the bond and holds the negative charge better. Oxyacids all lose a proton from an O–H group. Extra O atoms on the central atom pull electron density away from that O–H bond, and in the anion they share the negative charge by resonance, which steadies it.
Worked examples
Worked example: The chlorine oxyacids
Rank HClO, $\mathrm{HClO_2}$, $\mathrm{HClO_3}$ and $\mathrm{HClO_4}$ by strength, and explain.
- They differ only in the number of O atoms on Cl: 0, 1, 2 and 3 besides the OH.
- Each extra O pulls electron density from the O–H bond and gives the anion one more atom to share the charge. Measured: HClO $K_a = 2.9 \times 10^{-8}$, $\mathrm{HClO_2}$ $1.1 \times 10^{-2}$, and $\mathrm{HClO_3}$ and $\mathrm{HClO_4}$ are strong.
Answer: $\mathrm{HClO < HClO_2 < HClO_3 < HClO_4}$.
Worked example: Same shape, different halogen
Which is the stronger acid, HOCl or HOI?
- Both have one O–H with the halogen on the O. Chlorine is more electronegative than iodine.
- Cl pulls more electron density through the O, weakening the O–H bond and steadying $\mathrm{ClO^-}$. HOCl: $K_a = 2.9 \times 10^{-8}$; HOI: $2.3 \times 10^{-11}$.
Answer: HOCl.
Worked example: Chlorine atoms on acetic acid
$\mathrm{CH_3COOH}$ has $K_a = 1.8 \times 10^{-5}$ and $\mathrm{ClCH_2COOH}$ has $K_a = 1.4 \times 10^{-3}$. Explain the difference.
- The Cl atom, though not on the O–H, pulls electron density through the bonds (an inductive effect).
- That helps spread the negative charge of the anion, so the acid gives up its proton more readily: about 80 times stronger.
Answer: The electronegative Cl stabilizes the conjugate base.
Common mistake
Saying HF is the strongest hydrohalic acid because F is the most electronegative. Electronegativity decides across a period; down a group the bond strength dominates, and the H–F bond is by far the strongest. Check: HF has $K_a = 6.8 \times 10^{-4}$, a weak acid, while HCl, HBr and HI are all strong.