Question

Aqueous hydrogen fluoride (hydrofluoric acid) is used to etch glass and to analyze minerals for their silicon content. Hydrogen fluoride will also react with sand (silicon dioxide).

(a) Write an equation of the reaction of solid silicon dioxide with hydrofluoric acid to yield gaseous silicon tetrafluoride and liquid water.

(b)The mineral fluorite (calcium fluoride) occurs extensively in Illinois. Solid calcium fluoride can also be prepared by the reaction of aqueous solution of calcium chloride and sodium fluoride, yielding aqueous sodium chloride as the other product. Write complete and net ionic equations for this reaction.

Short answer

1. \(Si{O_2}\left( s \right) + 4HF\left( {aq} \right) \to Si{F_4}\left( g \right) + 2{H_2}O\left( l \right).\)
2. \(CaC{l_2}\left( {aq} \right) + 2NaF\left( {aq} \right) \to Ca{F_2}\left( s \right) + 2NaCl\left( {aq} \right)\) (Molecular ionic equation).

\(C{a^{2 + }}\left( {aq} \right) + 2C{l^ - }\left( {aq} \right) + 2N{a^ + }\left( {aq} \right) + 2{F^ - }\left( {aq} \right) \to Ca{F_2}\left( s \right) + 2N{a^ + }\left( {aq} \right) + 2C{l^ - }\left( {aq} \right)\)(Complete ionic equation).

\(C{a^{2 + }}\left( {aq} \right) + 2{F^ - }\left( {aq} \right) \to Ca{F_2}\left( s \right)\)(Net ionic equation).

Step-by-step solution

Formation of silicon tetrafluoride

Hydrogen fluoride, a strong acid reacts with silicon dioxide leading to silicon tetrafluoride.

The equation is as shownbelow:

\(Si{O_2}\left( s \right) + 4HF\left( {aq} \right) \to Si{F_4}\left( g \right) + 2{H_2}O\left( l \right).\)

Molecular ionic equation is used to give complete ionic and net ionic equations

The complete ionic equation gives the complete formulas of all the dissolved ions of the ionic compounds.

\(C{a^{2 + }}\left( {aq} \right) + 2C{l^ - }\left( {aq} \right) + 2N{a^ + }\left( {aq} \right) + 2{F^ - }\left( {aq} \right) \to Ca{F_2}\left( s \right) + 2N{a^ + }\left( {aq} \right) + 2C{l^ - }\left( {aq} \right).\)

The spectator ions, sodium and chloride ions on both sides of the above equation gets cancelled out, giving the net ionic equation.

\(C{a^{2 + }}\left( {aq} \right) + 2{F^ - }\left( {aq} \right) \to Ca{F_2}\left( s \right).\)