Question

Drying agents such as Calcium oxide ($CaO$) and Phosphorus pentoxide ($P_2{O}_5$) are not used for drying Hydrogen chloride gas. Why?

Answer 1

• Drying agents are hygroscopic compounds that can be used to remove a trace amount of water from compounds.
• Calcium oxide ($\mathit{C}\mathit{a}\mathit{O}$) and Phosphorus pentoxide (${\mathit{P}}_{\mathbf{2}}{\mathit{O}}_{\mathbf{5}}$) are common drying agents that are used for removing water.
• But Hydrogen chloride ($\mathit{H}\mathit{C}\mathit{l}$) has a tendency to react with Calcium oxide and Phosphorous pentoxide to produce salt and water.
• Chemical equation showing the reaction of Hydrogen chloride with Calcium oxide.
  $$\begin{gathered} \mathrm{CaO}\left(\mathrm{s}\right)+\mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{CaCl}}_2\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ \mathrm{Calcium}\mathrm{Calcium} \\ \mathrm{oxide}\mathrm{chloride} \end{gathered}$$
• The reaction is that is happening when Phosphorous pentoxide reacts with Hydrogen chloride is
  $$\begin{gathered} 2{\mathrm{P}}_2{\mathrm{O}}_5\left(\mathrm{s}\right)+3\mathrm{HCl}\to {\mathrm{POCl}}_3\left(\mathrm{s}\right)+3{\mathrm{H}}_3{\mathrm{PO}}_3\left(\mathrm{l}\right) \\ \mathrm{Phosphorus}\mathrm{Phorsphorus}\mathrm{Phosphorous} \\ \mathrm{pentoxide}\mathrm{oxychloride}\mathrm{acid} \end{gathered}$$
• Hence, Calcium oxide and Phosphorus pentoxide cannot be used for drying Hydrogen chloride.