Question

Hydrofluoric acid has the chemical formula HF. Unlike $\mathrm{HCl}$ you will find not find this acid in your school lab because it is a deadly chemical. It can eat through glass, ceramic, and metals and hence stored in plastic bottles. It is a contact poison and causes painless burns leading to permanent tissue death.

1. Is $\mathrm{HF}$ a weak acid or a strong acid?
2. Write the balanced reaction of $\mathrm{HF}$ with glass $\left({\mathrm{SiO}}_2\right).$
3. The discovery of $\mathrm{HF}$, Carl Wilhelm Scheele, had a nasty habit of sniffing and tasting the substance he made. As a result, he died at the young age of 43. What lesson should one learn from this regarding conduct in the lab?

Answer 1

a. Strength of an acid:

• Strong acid: An acid that completely dissociates in water or dissociates to a larger extent is called a strong acid. Examples, Hydrochloric acid $\left(\mathrm{HCl}\right)$, Sulfuric acid $\left({\mathrm{H}}_2{\mathrm{SO}}_4\right)$, etc.
• Weak acid: An acid that partially dissociates in an aqueous solution is called a weak acid. Examples, Acetic acid $\left({\mathrm{CH}}_3\mathrm{COOH}\right)$, Carbonic acid $\left({\mathrm{H}}_2{\mathrm{CO}}_3\right)$, etc.
• Hydrofluoric acid is a weak acid, as it fails to completely dissociate in water.
• Fluorine is a highly electronegative atom and it has a strong affinity toward Hydrogen. due to this reason, the bond energy of Hydrogen and Fluorine is high and an immense amount of energy is required to break this bond.

b. Dissolution of glass in Hydrofluoric acid.

• Though Hydrofluoric acid is considered a weak acid, it is highly corrosive in nature and capable of dissolving glass apparatus. The reason why it is kept in plastic bottles.
• The desired reaction would be$\underset{(\mathrm{Hydrogen}\mathrm{fluoride})}{\underbrace{4\mathrm{HF}\left(\mathrm{aq}\right)}}+\underset{(\mathrm{Silicon}\mathrm{dioxide})}{\underbrace{{\mathrm{SiO}}_2\left(\mathrm{s}\right)}}\to \underset{(\mathrm{Silicon}\mathrm{tetrafluoride})}{\underbrace{{\mathrm{SiF}}_4\left(\mathrm{aq}\right)}}+\underset{\left(\mathrm{Water}\right)}{\underbrace{2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}}$
• When Hydrogen fluoride reacts with Silicon dioxide(Glass) $\left({\mathrm{SiO}}_2\right)$, Silicon tetrafluoride $\left({\mathrm{SiF}}_4\right)$ and water are formed.

c. Health issues due to $\mathrm{HF}$

• Hydrogen fluoride gas when inhaled even at low concentrations could cause many health issues. This is the reason why Carl Wilhelm Scheele faced such a harsh situation.
• It could cause irritation in the eyes, and could also damage the respiratory tract when inhaled at a low concentration.
• And high concentration could cause irregular heartbeat, which can cause death.