Video Transcript
A student uses a test tube to
collect the gas produced by a chemical reaction. After lighting a splint, they blow
out the flame and then insert the splint into the test tube. It reignites and glows
brightly. What is the probable identity of
the gas?
Before answering the question,
let’s have a quick review of the procedure. The student is performing a
chemical reaction of some sort. Some gas is being produced. And the gas is being collected
inside a test tube. They’re then taking a splint,
setting it alight, and then blowing out the flame, leaving embers behind on the end
of the splint. They then inserted that split into
the test tube. And the splint reignited. From this, we can deduce that the
gas inside the tube was reactive. A splint is made of wood which is
largely composed of lignin and cellulose. This means it’s mostly composed of
carbon, hydrogen, and oxygen.
For this question, we’re assuming
that the gas produced is a pure substance. So there’s one more piece of
information we need in order to make a good guess about the identity of the gas. The process occurring at the end of
the splint when the embers are burning is combustion. The fact the flame returns
indicates that the gas accelerates or is involved in combustion. Combustion of the carbon in the
splint will produce carbon dioxide. Combustion of the hydrogen will
produce water. This occurs normally by reaction
with oxygen in the air. Oxygen typically makes up 21
percent of the air we breathe. However, if the gas in the test
tube were pure oxygen, it would vastly accelerate the rate of combustion of the
splint. This would be consistent with our
findings.
So the probable identity of the gas
is oxygen. Just to be safe, I’m going to have
a look at the other common gases which you may have come across and see whether any
of them fit. Hydrogen is an explosive gas when
mixed with oxygen. However, a flame is needed in order
to trigger the detonation. Pure hydrogen in this circumstance
would quench the embers on the splint and would not cause reignition. It would block the access of oxygen
in the air to the splint and stop the embers reacting. Hydrogen is usually detected by
treatment with a lit splint because it generates a loud pop.
What about carbon dioxide? Carbon dioxide is a nonreactive
gas. So it too would put out the
embers. Carbon dioxide is usually detected
with limewater in which it forms a precipitate of calcium carbonate. Chlorine is another gas that might
be produced in the lab. However, it would react too slowly
and too incompletely with a splint in order to cause reignition and would quench the
embers like hydrogen and carbon dioxide. Chlorine is usually detected with
blue litmus paper. Damp blue litmus paper will turn
red upon contact with chlorine gas and then slowly turn white as it’s bleached.
And finally, of the common gases
that can be tested in a laboratory setting, we come to ammonia. Ammonia too would not support the
combustion of the splint and would quench the embers. Since ammonia is basic, it can be
detected with damp red litmus paper which turns blue upon exposure to ammonia
gas. Otherwise, it can be detected by
its distinctive smell. While there are many more gases we
could look at, these are the ones most likely to be produced by laboratory setup in
these quantities. So the probable identity of the gas
is most certainly oxygen.
Why does the splint glow more
brightly than it did when burning in air?
Let’s consider the scenario where
the splint is burning in air. Oxygen from the air is supporting
combustion of the splint. But the majority of the air is made
up of nitrogen, while, in the test tube, there’s 100 percent oxygen. This means that the combustion of
the splint is much much faster. This produces a lot more heat and
much more light. So the reason the splint glows much
more brightly when in 100 percent oxygen than it does in air is because it is
burning faster. There is a misconception that
oxygen itself can burn. But that’s not true. Oxygen is necessary to support
combustion. Oxygen is also consumed in the
process. So it’s not acting as a
catalyst.
Which of the following reactions
could’ve produced the gas? A) Zinc and sulfuric acid, B)
silver oxide and hydrochloric acid, C) barium carbonate plus nitric acid, D)
hydrogen peroxide with manganese oxide catalyst, or E) aluminum chloride plus sodium
hydroxide.
Just to recap, the gas the student
produced was oxygen, O₂. So we’re looking for a reaction
that would generate oxygen. Let’s start with option A, zinc
plus sulfuric acid. This is an example of a metal plus
acid reaction. The expected outcome is a metal
salt plus hydrogen gas. In this case, the salt will be zinc
sulphate. As this reaction does not generate
oxygen, it is not a correct answer.
What about option B? Silver oxide is a metal oxide. And most metal oxides are
bases. So what we have is an acid base
reaction. So the products are metal salt and
water. In this case, the salt is silver
chloride. Again, this answer does not produce
oxygen as a product. So it cannot be a correct
answer. Barium carbonate is an example of a
metal carbonate. When treated with acid, we expect
the products to be a metal salt, carbon dioxide, and water. In this case, the salt is barium
nitrate, but unfortunately, no oxygen.
So onto option D, hydrogen peroxide
is well-known to slowly decompose into water and oxygen. Left on its own, this reaction is
too slow. So a catalyst is introduced when
large quantities of oxygen are needed. In this case, manganese four oxide
is being used with symbol MnO₂ to break down the hydrogen peroxide. Crucially, this reaction generates
the oxygen we need. So this is the correct answer. But just to be safe, let’s have a
look at option E.
This reaction you might not be so
familiar with. But it’s a reaction of a metal
chloride with sodium hydroxide. In many cases, the outcome is a
metal hydroxide plus sodium chloride. Here’s the chemical equation,
aluminum chloride plus three equivalents of sodium hydroxide react to form aluminum
hydroxide plus three equivalents of sodium chloride. If excess hydroxide is present,
aluminum hydroxide can react to form sodium aluminate. You need not know all the details
of this chemistry, only that the reaction does not produce oxygen. So this option is not a correct
answer. Of the five options given, only one
produces hydrogen gas. And that is hydrogen peroxide with
manganese oxide catalyst.
