Question

Reaction of silica with mineral acids may be given as:
$Si{O}_2+4HF⟶Si{F}_4+2H_{2}O;\Delta H=-10.17kcal$
$Si{O}_2+4HCl⟶Si{Cl}_4+2H_{2}O;\Delta H=+36.7kcalHCl$
Which among the following is correct?

• A. $HF$ and $HCl$ both will react with silica
• B. Only $HF$ will react with silica
• C. Only $HCl$ will react with silica
• D. Neither $HF$ nor $HCl$ will react with silica

Answer 1

The correct option is B Only $HF$ will react with silica

When silica reacts with $HCl$ then the reaction is endothermic and heat is absorbed by the system, when silica reacts with $HF$ we get $\Delta H=-ve$, i.e., exothermic reaction and hence the reaction become feasible,

so, we can say that only $HF$ reacts with silica.