# Reaction of Silicon Dioxide with Hydrofluoric Acid [duplicate]

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?

• – Nilay Ghosh Jul 20 '20 at 7:16
• Thank you Nilay ghosh :) – R. Anusha Jul 20 '20 at 12:31
• 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. – Andrew Jul 20 '20 at 16:04

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.
• @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: $$\ce{SiO2 + 4 HF -> SiF4 + 2 H2O}\tag{1}$$ $$\ce{SiO2 + 4 HF -> SiF4 + 2 H2O}\tag{1}$$ – andselisk Jul 20 '20 at 9:52
• 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. – andselisk Jul 20 '20 at 9:55