Which one of the following molten
salts would not produce sodium metal when electrolyzed? A) Na₂O, B) NaCl, C) NaBr, D) NaF,
or E) MgCl₂.
Let’s imagine we have an
electrochemical cell with a molten salt. In the molten salt, ions are going
to be mobile. The anion is going to be attracted
to the positive electrode and will be oxidized, often to form a gas. The cation, typically a metal, will
be attracted to the negative electrode, where it will be reduced and form a solid
metal. Since there’s the cation in any
molten salt that will form a solid metal when electrolyzed, we only need to identify
the cation in each substance.
Na₂O is sodium oxide. The cation is sodium plus. NaCl is sodium chloride. Again, the cation is Na⁺. The same is true for NaBr, which is
sodium bromide, and NaF, sodium fluoride. However, MgCl is magnesium
chloride. The cation in this case is
Mg²⁺. The cations in option A, B, C, and
D are all Na⁺. Upon reduction, they will all form
sodium metal. The only molten salt that will not
produce sodium metal is MgCl₂. The cation for MgCl₂, Mg²⁺, will be
reduced to magnesium metal.
So of the five molten salts given,
the only one that would not produce sodium metal when electrolyzed is MgCl₂.