Video Transcript
Magnesium can react with different
concentrations of nitric acid to produce different products. Put the following reactions in
order from least to most concentrated nitric acid used in the reaction. Equation one, Mg plus four HNO3
produces Mg(NO3)2 plus two NO2 plus two H2O. Equation two, Mg plus two HNO3
produces Mg(NO3)2 plus H2. Equation three, three Mg plus eight
HNO3 produces three Mg(NO3)2 plus two NO plus four H2O.
The three equations in this
question each use nitric acid as a reactant. However, each reaction uses a
different concentration of nitric acid. This question is asking us to
determine the relative concentration of nitric acid in these three reactions and to
put them in order from least to most concentrated. Essentially, we need to know how
nitric acid behaves at different concentrations in order to pair these reactions
with those concentration levels. The question gives us a clue that
these different concentrations will result in different products. Since each reaction begins with
magnesium and nitric acid, we need to look at the products to see how they
differ.
Thankfully, one of the reactions
here looks like a stereotypical reaction between a metal and a dilute acid, which
produces some form of salt and hydrogen gas. Since the second equation fits this
model so well, we can assume that it uses dilute nitric acid. Dilute means low concentration. So we can start by saying that
equation number two has the least concentrated nitric acid. Equation one and equation three
both produce magnesium nitrate and water, but it is the middle product that differs
between the two. To understand this difference, we
can follow a simple rule.
At higher concentrations, nitric
acid becomes a stronger oxidizing agent. Comparing the nitrogen dioxide of
reaction one with the nitric oxide of reaction three, we can say that nitrogen
dioxide is more oxidized, since it has more oxygen atoms. In reaction number one, nitric
acid, the oxidizing agent, was able to transfer more oxygen atoms to create nitrogen
dioxide as a product. Therefore, it must have a higher
concentration. Reaction number three has an
oxidized product, but it is less oxidized than the product of reaction number one,
so the concentration of nitric acid in reaction number three is in the middle.
It’s worth noting that in this case
oxidizing involves adding oxygen atoms. However, oxidation is defined not
as the addition of oxygen atoms, but rather as the loss of electrons. So oxidation can still occur in the
absence of oxygen atoms. Comparing the concentrations of
nitric acid in the three reactions, we can now put them in order from least to most:
two, three, one. This is the correct answer. The reactions in order from least
to most concentrated nitric acid used is reaction two followed by reaction three
followed by reaction one.
