Which of the following metals would
react with acid and has an oxide that can be reduced by carbon? (A) Magnesium, (B) iron, (C)
copper, (D) silver, or (E) platinum.
This question is asking us to find
the metal that both would react with acid and has an oxide that can be reduced by
carbon. Where can we find this
information? Well, to answer this question, we
need to look at the reactivity series. The reactivity series is a list of
metals with the most reactive metals at the top and the least reactive metals at the
bottom. The reactivity series tells us
which metals are more or less reactive than other metals. And we should also know that more
reactive metals can displace less reactive metals from a compound or solution. Displace simply means to replace in
the compound or solution.
Let’s take a look at the reactions
described in the question and see what information we can glean about them from the
reactivity series. One example of a reaction between a
metal and an acid is the reaction of iron and hydrochloric acid to produce iron
chloride and hydrogen gas. If we look at how the chloride
compound changes from the beginning to the end of the reaction, we can see that the
hydrogen is displaced by the metal ion to form iron chloride. In order for a medal to react with
an acid, the metal must displace the hydrogen from the acid.
Following the rule that more
reactive metals can displace less reactive metals, it follows that the metals that
can displace hydrogen are the ones above it on the reactivity series. Only these metals will be able to
displace hydrogen and react with acids. Copper, silver, and platinum are
all below hydrogen on the reactivity series, so they will not be able to displace
hydrogen to react with acid.
Now let’s take a look at the other
reaction described in the question. One example of an oxide being
reduced by carbon is the combination of zinc oxide and carbon to form zinc and
carbon monoxide. Being reduced is the opposite of
being oxidized. So, in this reaction, zinc oxide
being reduced simply means that it loses its oxygen, although in other reactions
without oxygen, being reduced simply means gaining electrons from the beginning to
the end of the reaction. If we look at what happens to the
oxide in this reaction, we can see that the carbon displaces the zinc from the oxide
to form carbon monoxide. If we again follow our rule that
more reactive metals can displace less reactive metals, we can look at the
reactivity series to see which metals carbon will be able to displace.
Carbon can displace metals that are
less reactive than it. So, it’s the metals below carbon on
the reactivity series that can be reduced by carbon. Carbon is not able to displace the
more reactive metals above it on the reactivity series. So, those metal oxides will not be
able to be reduced by carbon. Now that we know when these
reactions can and can’t occur, we can ask this question in a simpler way. We can ask, which of the following
metals is more reactive than hydrogen and is less reactive than carbon? Magnesium is more reactive than
carbon, so the second half of the question is not true for magnesium.
Iron is the only one of the five
choices in the sweet spot, more reactive than hydrogen and less reactive than
carbon. Choice (B) iron is the correct
answer. Both of the reactions we’ve looked
at here, the reaction between a metal and an acid and a metal oxide and carbon,
belonged to a group of reactions, called single displacement reactions. Single displacement reactions occur
when a lone element, like iron, swaps places with one of the elements from a
compound or solution, like the hydrogen from hydrogen chloride. The second reaction here where
carbon displaces a metal from a metal oxide is a commonly used method to isolate
pure metals. In the example, we could use this
process to isolate pure zinc.
As we determined while answering
the question, a similar reaction can reduce iron three oxide to extract pure
iron. For metals too reactive to be
reduced by carbon, electrolysis must be used to extract them instead. So, which of the following metals
would react with acid and has an oxide that can be reduced by carbon? That’s choice (B) iron.