Video Transcript
Barium metal can be obtained
through electrolysis of its molten salt. Which of the following equations
shows the reaction occurring at the negative electrode? (A) Barium plus two electrons
produces barium two plus. (B) Barium produces a barium two
plus ion plus two electrons. (C) A barium two plus ion plus two
electrons produces barium. (D) Barium two plus ion plus two
electrons produces two barium atoms. Or (E) a barium two plus ion
produces barium and two electrons.
This question is asking about the
electrolysis of a molten salt. This process occurs when we dip two
electrodes connected to a battery into the liquid form of a salt. The liquid salt is made of
free-floating positive and negative ions. In this question, the positive ions
or cations will be barium ions. The identity of the negative ions
isn’t important to answering the question, so let’s just use chloride as an
example. When the circuit is turned on, the
ions will be attracted to the electrodes of the opposite charge. When the ions reach each electrode,
a reaction will occur.
This question is asking, what
happens to the barium ions on the surface of the electrode shown here on the
left? Each choice is essentially the same
three pieces of information rearranged. In order to answer this question,
we need to ask, when are barium ions present? When are barium atoms present? And are electrons absorbed or
released during this process?
Let’s look at the wording of the
question for some clues about what’s going on. As we’ve just explained, a molten
salt involves ions. And the electrolysis of a molten
salt starts with ions. Similarly, if we are trying to
obtain barium metal, that must mean that the atomic form of barium is a product of
the reaction. That means we can eliminate choice
(A) and (B) from consideration. We want the ion to be on the left
side of the equation as a reactant and the atom to be on the right side of the
equation as a product. We know that barium metal will form
as a product of the reaction. It will do so by plating on the
electrode.
Our next question is, what is going
on with the electrons to make this happen? In electrolysis, electrons flow
from the anode to the cathode. They are taken from the ions at the
anode and given to the ions at the cathode. In our example here, the electrons
at the cathode are donated to the barium two plus ions. The two plus charge of the barium
ion and the combined two minus charge of the two electrons balance out. As a result, barium metal atoms
form. We can simplify this process by
saying that the barium ion has gained electrons to form the atom. Choice (E), where electrons are a
product of the reaction, is the opposite situation, where electrons are released
from the ion. We want the electrons to combine
with the ion to form the atom like they do in choice (C) and (D).
The last thing we need to consider
is whether the combination of a barium ion and two electrons would produce one
barium atom or two barium atoms. Simply put, the correct answer is
choice (C). One ion combines with electrons to
form one atom. This reaction describes what occurs
at the negative electrode when we electrolyze a molten salt containing barium. The barium ion will gain two
electrons to form a barium metal atom. When an atom or ion gains
electrons, we call that a reduction.
Electrolysis is one way to isolate
pure metals. In fact, barium was first isolated
by British chemist Sir Humphry Davy in 1808 when he electrolyzed molten barium
oxide. So when we obtain barium metal
through electrolysis of its molten salt, the equation that shows the reaction
occurring at the negative electrode is choice (C), a barium ion plus two electrons
produces a barium atom.
