Which of the following substances
cannot be readily electrolyzed? A) AlCl₃ liquid, B) NaCl solid, C)
BaBr₂ aqueous D) MgF₂ liquid, or E) LiCl aqueous.
Electrolysis is the chemical
decomposition of a substance through the passage of an electric current. “Electro” refers to electric and
“lysis” to breakdown. This is what a basic
electrochemical setup might look like. What the question is asking is what
would happen for each of these substances if they are placed in the electrochemical
setup. What we’re looking for is four
samples that can be electrolyzed and one that can’t.
One of the basic principles of
electrolysis is that we need a complete circuit. This means that we need something
that will respond to the positive and negative charges of the electrode. This means we need some form of
charged particle that is mobile.
So a simple test we can apply to
all the candidates is to ask whether there are mobile ions present and what they
are. All the substances are ionic. So they contain ions. But their state will dictate
whether they are mobile or not.
AlCl₃ liquid is molten aluminum
chloride. So the ions aluminum 3+ and
chloride are mobile. If we did electrolyze aluminum
chloride in its molten state, our products will be aluminum in its solid form and
chlorine gas. This is, therefore, not the correct
What about NaCl solid? NaCl or sodium chloride when solid
does not contain mobile ions. The Na+ and Cl- ions of which it is
composed are locked in place. Therefore, sodium chloride solid
cannot be electrolyzed. This is, therefore, the correct
answer. However, if it were sodium chloride
liquid or sodium chloride aqueous, it would be electrolyzable.
Just to be safe, let’s have a look
at options C, D, and E. In solution, barium bromide, BaBr₂,
forms mobile ions Ba2+ and Br-. In solution, these ions are
completely mobile and can be electrolyzed. The products from the electrolysis
of barium are hydrogen gas, bromine gas, and barium hydroxide. Barium solid is not formed because
barium is more reactive than hydrogen. So it is H+ that is reduced to the
electrode and not Ba2+. Therefore, BaBr₂ is not the correct
Molten magnesium fluoride MgF₂ will
have mobile ions Mg2+ and F-. These mobile ions will be
electrolyzed to form magnesium solid and fluorine gas. Therefore, magnesium fluoride
liquid is not a correct answer.
Finally, we come to lithium
chloride aqueous. The mobile ions in solution will be
Li+ and Cl-. And as with barium, because we’re
dealing with a more active metal than hydrogen we produce hydrogen gas, chlorine
gas, and lithium hydroxide solution. So this too is not the correct
So of the five substances given,
the only one that cannot be readily electrolyzed is sodium chloride solid.