Video Transcript
Metal X is more reactive than
hydrogen. If an aqueous salt solution of
metal X with the formula XCl2 was electrolyzed, what substance would we expect to be
produced at the cathode? (A) Steam, (B) hydrogen gas, (C)
oxygen gas, (D) chlorine gas, (E) metal X.
Our question asks us to predict the
product of an electrolysis reaction involving an aqueous salt solution. We can do this with the help of a
reactivity series. The reactivity series is a ranking
of common metals and some nonmetals based on their reactivity. Typically, hydrogen is a valuable
reference in this series in comparison to the reactivity of other metals. Metals above hydrogen in the series
are typically highly reactive, while metals below hydrogen are typically quite
unreactive.
In a typical electrolysis reaction,
a pure form of a particular element is the desired product. But for an electrolysis reaction to
work, the solution that the elements are in must be able to conduct electricity and
allow ions to freely move about in solution. That is why we often use an aqueous
salt solution to ensure ion mobility.
This solution will often complicate
the potential reaction products by introducing other ions into the electrolyte
mixture. However, we can use a reactivity
series to help quickly determine which products we will liberate from different salt
solutions. Typically, any metal that is above
hydrogen in a series will produce hydrogen gas at the cathode of a reaction. Any element below hydrogen will
instead produce the pure elemental form at the cathode.
For example, in our question, we
use an unknown element X with the formula XCl2. We only need to know the reactivity
in relation to hydrogen to determine the typical product that is formed at the
cathode. Since our question tells us that
our unknown metal is more reactive than hydrogen, we know that we can expect
hydrogen gas to form as a product at the cathode within our reaction. Real-world examples of this could
be magnesium chloride or zinc chloride, which would liberate hydrogen gas at the
cathode as both of these metals are more reactive than hydrogen.
If we look at our answer choices,
we can see that answer choice (B) lists hydrogen gas. So we can say that this answer
choice is correct. However, let’s look at the other
answer choices just to be sure.
Answer choice (A) mentions
steam. And as steam is not a typical
product at any electrode within an electrolysis reaction, we can eliminate it from
consideration.
Answer choice (C) mentions oxygen
gas. However, since oxygen gas is not a
metal and is generated in electrolysis reactions from negative hydroxide ions, we
would not expect this gas to be generated at the negative cathode.
Answer choice (D) mentions chlorine
gas as a product. However, as chlorine is the anion
in our aqueous salt solution, it will not migrate towards the cathode but instead
the anode and therefore cannot be a product at the cathode.
Answer choice (E) mentions our
unknown metal X as a product at the cathode. However, since we know that metal X
is more reactive than hydrogen, we know that our cation will not form a pure
elemental product at the cathode.
Hydrogen gas is the correct
answer.
