# How does HF dissolve glass?

By what mechanism does HF proceed in dissolving glass? Why is it the only acid that has this capability? Is it because of the small size and high electronegativity of fluorine?

• But what exactly do you mean by "only this capability"? – Crystal Lettuce Jan 23 '15 at 14:01
• @Matthew, it is why does only it have this capability or why is only HF capable of doing so – yawar Jan 23 '15 at 14:06

$$\ce{HF}$$ reacts with glass ($$\ce{SiO2}$$).[1]

$$\ce{SiO2 + 4HF -> SiF4 + 2H2O}$$

$$\ce{SiF4}$$ is not a solid that consists of vertex-connected tetrahedra like $$\ce{SiO2}$$ but is a gas at room temperature. Technically, $$\ce{HF}$$ is not a solvent since in this case it reacts with the glass vessel.

According to Spierings:[2]

$$\ce{HF2-}$$ ions are adsorbed on surface silanol groups, the $$\ce{HF}$$ molecules on vicinal silanol groups and $$\ce{H+}$$ on surface bridging oxygens in siloxane units. [...] These are transformed into surface groups such as $$\ce{\bond{#}Si-F}$$ and $$\ce{\bond{#}Si-O-SiF3}$$. The adsorption of $$\ce{HF}$$ and $$\ce{HF2-}$$ increases the electronic density on the bridging oxygen in the siloxane unit. This in turn makes these oxygens more basic, so more $$\ce{H+}$$ ions are adsorbed, which leads to more siloxane bonds being broken per time unit, i.e. a kind of catalytic effect. [...] The catalytic action of $$\ce{H+}$$ ions on breaking siloxane bonds also occurs in the dissolution of glasses in acidic and weakly alkaline solutions.

1. Hollemann, A. F.; Wiberg, E. Hollemann–Wiberg: Lehrbuch der Anorganischen Chemie, 34th ed.; de Gruyter: Berlin, 2007; Vol. 102.

2. Spierings, G. A. C. M. Wet chemical etching of silicate glasses in hydrofluoric acid based solutions. J. Mater. Sci. 1993, 28 (23), 6261–6273. DOI: 10.1007/BF01352182.

• I know the reaction, but my question is about mechanism how does this happen, does fluorine go interstities of the glass or h+ or the kinetic energy is responsible for it. Technically, HF doesn't dissolve glass since a solvent does not react with your compounds.; thats why I said does it disintegrate glass then because of what: Energy or size of ions or both – yawar Jan 23 '15 at 14:03
• What do you mean by the kinetic energy of the fluoride ion? – Dissenter Jan 23 '15 at 15:30
• @Dissenter, I am reffering to maxwell velocity distribution in liquids, which is related to collision theory of reactions – yawar Jan 28 '15 at 9:55
• I've no idea what "a solvent does not react with your compounds" means especially in this context. Hydrofluoric acid clearly does react with glass. And that is what matters since most other things do not. – matt_black Oct 26 '15 at 18:40
• Oh boy... thank you for pointing that out. I mixed things up. Now it should be correct. – Crystal Lettuce Oct 29 '15 at 13:32