If we talk about hydrolysis F- will be more heavily hydrolysed so it should be more stable in aqueous state. But I know that HI is a stronger acid than HF. So the stability of conjugate acid ie I- should be more than F-. As dissociation takes place in water.

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    $\begingroup$ If a question is asked on Chemistry SE site, then, in contrary to sites like Quora, it is expected from the author to elaborate the topic of the question by doing at least basic own topic review, writing what he/she has found and understood, and what is the stumble stone. The quick questions without explicitly expressed particular effort are not very welcome, and may be closed. $\endgroup$ – Poutnik Jun 25 '20 at 6:36
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    $\begingroup$ Your logic is all upside down. A thing that is easier to destroy is less stable. $\endgroup$ – Ivan Neretin Jun 25 '20 at 6:50

If we consider $\ce{HX}$ as the common formula for hydrogen halogenides, then there is an equilibrium

$$\ce{HX(aq) + H2O <=> X-(aq) + H3O+(aq)}$$

that can be expressed also as:

$$\ce{acid1 + base2 <=> base1 + acid2}$$

The equilibrium is competition of 2 acids, $\ce{HX}$ ansd $\ce{H3O+}$. $\ce{H3O+}$ is the strongest acid stable in aquaeous environment. Acids we consider as strong acids ($\ce{HCl}$, $\ce{H2SO4}$, $\ce{HNO3}$ ) are much stronger acids than $\ce{H3O+}$. Therefore for $\ce{HCl}$, $\ce{HBr}$, $\ce{HI}$, the equilibrium is practicaly fully pushed toward right:

$$\ce{HX(aq) + H2O -> X-(aq) + H3O+(aq)}$$

On the other hand, $\ce{HF(aq)}$ is a weak acid, weaker than $\ce{H3O+}$, with dissociation constant $\mathrm{p}K_\ce{a}=3.17$, for which the equilibrium is rather on the left side:

$$\ce{HF(aq) + H2O <<=> F-(aq) + H3O+(aq)}$$

Highly concentrated $\ce{HF(aq)}$ is rather a strong acid:

$$\ce{2 HF(aq) + H2O <=>> HF2-(aq) + H3O+(aq)}$$

and liquid HF is very strong acid ( H0 = −11 ) :

$$\ce{3 HF <=>> HF2- + H2F+}$$


Fluorine ion and chlorine ion are very corrosive in nature, so In water they easily form the hydrohaloacid as order of acidic strength in water is $hi>hbr>hcl>hf$ hence the stability order will be $F^{-} >Cl^{-} >Br^{-} >I^{-}$ which also shows stability of their acids.

  • $\begingroup$ The only halogenides forming acid in water are fluorides: $\ce{F- + H2O <=> HF + OH-}$, with the resulting pH alkaline, not acidic. $\endgroup$ – Poutnik Jun 26 '20 at 10:22
  • $\begingroup$ @poutnik sorry I did mistake in hurry . is it fine now? $\endgroup$ – Jack Rod Jun 26 '20 at 10:27
  • $\begingroup$ How does NaCl easily form HCl ? Or rather H3O+, as H3O+ is the strongest acid stable in water. $\endgroup$ – Poutnik Jun 26 '20 at 10:30
  • $\begingroup$ From where the NaCl arrives?@poutnik $\endgroup$ – Jack Rod Jun 26 '20 at 10:31
  • $\begingroup$ It is not relevant, @Yuvraj. NaCl does not undergo hydrolysis. Its corrosiveness is not due acidic properties, but because of electrochemical impact. $\endgroup$ – Poutnik Jun 26 '20 at 10:35

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