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I learnt that $\ce{SiO2}$(Silicon dioxide) doesn't react with any acid except $\ce{HF}$. So what is special about $\ce{HF}$?

Why does only $\ce{HF}$ reacts with $\ce{SiO2}$ even though $\ce{HF}$ has a very high bond energy compared to other acids in that group?

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    $\begingroup$ Also: chemistry.stackexchange.com/questions/51223/… $\endgroup$
    – Nilay Ghosh
    Commented Jul 20, 2020 at 7:16
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    $\begingroup$ If you are using comparative bond strengths to determine if a reaction is favorable, you need to consider the bonds in the products as well as those in the reactants. Although H-F has a stronger bond than H-Cl and other halogen acids, the Si-F bond is one of the strongest single bonds known, so forming four of them in the product makes the reaction thermodynamically favorable. Si-Cl bonds are not nearly as strong, and the difference is greater than 4x the difference between HF and HCl. $\endgroup$
    – Andrew
    Commented Jul 20, 2020 at 16:04

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It is surprising that Silicon halides behave differently with water according to the choice of the halogen fluor or chlorine. The following reactions are known to happen : $$\ce{SiO_2 + 4 HF -> SiF_4 + 2 H_2O} ......... (1)$$ $$\ce{SiCl_4 + 2 H_2O -> SiO_2 + 4 HCl}........ (2)$$ These equations are exactly the opposite from one another. Why does reaction ($1$) not proceed in the other way like ($2$) ? I don't know, but I would like present a personal interpretation. Maybe it is due to the fact that reaction ($1$) proceeds in two steps, with the intermediate formation of $\ce{H_2SiF_6}$. Maybe ($1$) proceeds like that $$\ce{SiO_2 + 6 HF -> H_2SiF_6 + 2 H_2O}$$ $$\ce{H_2SiF_6 <=> SiF_4(g) + 2 HF } $$$\ce{H_2SiF_6}$ is well known to be formed if $\ce{HF}$ is in excess. And of course HCl cannot form $\ce{H_2SiCl_6}$, because the chlorine atom is too big to act as a ligand this octahedral structure. $\ce{H_2SiCl_6}$ does not exist.

This is a personal idea. I would be pleased to hear about any comment.

Sorry for presenting these lines as an Answer. Alas the text is much too long for a comment...

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    $\begingroup$ @Maurice I also think this is more like a comment, but I wanted to point out a formatting tip. Instead of using multiple dots like ........ for spacing out reaction numbers, please use \tag{<number>} macro. This way the numbers are perfectly aligned to the right margin and there is no distraction. Example: $$\ce{SiO2 + 4 HF -> SiF4 + 2 H2O}\tag{1}$$ $$\ce{SiO2 + 4 HF -> SiF4 + 2 H2O}\tag{1}$$ $\endgroup$
    – andselisk ♦
    Commented Jul 20, 2020 at 9:52
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    $\begingroup$ And if you add \label{<unique id>}, you can refer to the equation from the text with \eqref{<unique id>}. LaTeX writers usually use the following prefixes for <unique id> eqn: for equation, rxn: for reactions, tbl: for tables. Example: $$\ce{SiO2 + 4 HF -> SiF4 + 2 H2O}\label{rxn:sio2hf}\tag{1}$$ and later in text \eqref{rxn:sio2hf} for the dynamic reference. $\endgroup$
    – andselisk ♦
    Commented Jul 20, 2020 at 9:55
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    $\begingroup$ @R.Anusha The reactions of HF are not about acid strength. They are more about the specific ability of fluoride to attack silicon dioxide. Acid strength has nothing to do with this reaction. $\endgroup$
    – matt_black
    Commented Jul 20, 2020 at 10:42
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    $\begingroup$ @Safdar There is MathJax basic tutorial and quick reference and TeX.SE. Just as pretty much always, one learns by doing. If you want to go further down the rabbit hole, find blogs and sites of interest that are using MathJax, and read books about LaTeX and literate programming. Just remember that MathJax ≠ LaTeX, they just share some markup commands, macro names and default fonts. Under the hood they are vastly different entities. $\endgroup$
    – andselisk ♦
    Commented Jul 20, 2020 at 11:33
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    $\begingroup$ @maurice Your reasoning can be summed up as there being a kinetic barrier to formation of SiCl4, but a quick check of thermodynamic data shows that SiF4 is thermodynamically favored over SiO2, while SiCl4 is not. The key difference is the strength of the Si-F bonds compared to Si=O and Si-Cl. Much greater than the difference in bond strength of HF vs HCl $\endgroup$
    – Andrew
    Commented Jul 20, 2020 at 12:18

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