Which of the following best
describes the conversion of copper oxide to copper? A) Fusion, B) Precipitation, C)
Nuclear transformation, D) Combustion, or E) Reduction.
The question references copper
oxide, which could either be copper two oxide or copper one oxide. So we could be dealing with CuO,
copper two oxide, otherwise known as cupric oxide. Or we could be dealing with Cu₂O,
otherwise known as cuprous oxide or copper one oxide. As we’ll come to see, it doesn’t
make a difference for this question. So I’m going to write CuO
throughout. But bear in mind, it could equally
be Cu₂O. Either way, we’re looking at the
conversion of a copper oxide to metallic copper. The set-up for this conversion
might look like the following, where we have a Bunsen burner heating a sample of
copper oxide powder inside a tube. Hydrogen gas will be introduced
into the powder where it would react, forming water and copper metal.
The hydrogen might come from a
cylinder or, for a simple laptop demonstration, we might use a mixture of zinc and
hydrochloric acid to produce the hydrogen gas. And here, we have the balanced
equation for the reaction of copper two oxide with hydrogen forming copper and
water. Now that we’ve had a recap of the
whole process, let’s have a look at the descriptions to see which one best
applies. Remember, we’re looking for the
best description. So more than one may apply.
The word fusion isn’t used much
with chemical reactions. It’s more common to see it in terms
of nuclear reactions. For instance, the conversion of
four hydrogen nuclei to form a helium nucleus, as goes on in the sun. In fact, the sun consumes about 600
million tons of hydrogen every second. In the conversion of copper oxide
to copper, all the components preserve their elemental identity. We don’t see elements fusing
together, so it’s not a fusion reaction.
A precipitation reaction is where
we produce a solid from a solution. A good example of a precipitation
reaction is the reaction of lead nitrate with potassium iodide, which produces the
brilliant yellow precipitate lead iodide. We’re not dealing with anything in
solution. Only the conversion of solid copper
oxide, which is insoluble in water, to metallic copper, which is also insoluble.
A nuclear transformation process is
one where we see a change in the composition of one or more nuclei. For example, we see this kind of
transformation in nuclear decay processes. For instance, thorium 234
transforming into protactinium 234 with the release of a beta particle, among other
things. As with fusion, nuclear
transformation is not a good description of the conversion of copper oxide to copper
because no component changes its elemental identity. All the nuclei retain their
original composition and just rearrange themselves.
Meanwhile, combustion is the
reaction with oxygen. For instance, in the complete
combustion of methane, one molecule of methane reacts with two molecules of oxygen,
forming a molecule of carbon dioxide and two molecules of water. There’s no molecular oxygen present
in our reaction equation. So it’s not the combustion.
This leaves us with one option,
reduction. In a reduction reaction, we’ll see
a reduction in the oxidation state of one or more components. This usually means that they gain
electrons. The oxidation state of copper in
copper two oxide is plus two. This counterbalances the default
oxidation state for oxygen in oxide of negative two. Hydrogen, in its elemental form,
will have an oxidation state of zero, as will copper, our product. Hydrogen in water will have an
oxidation state of positive one. And oxygen will have an oxidation
state of negative two.
In this reaction, the stars of our
show are copper oxide and copper. So let’s have a look at the
oxidation state of copper and how it changes. The oxidation state of copper goes
from positive two to zero. So we see a reduction in its
oxidation state. So this reaction is most certainly
a reduction reaction. However, the sharp-eyed among you
might have noticed we’re also dealing with an oxidation of hydrogen, going from zero
to positive one. Whenever a reduction occurs, an
oxidation must also occur. So we have the term redox
reaction. However, it’s perfectly okay to
focus on only one component and describe its individual reaction. In which case, we can describe the
conversion of copper oxide to copper as a reduction reaction.