Question Video: Ordering the Reactions of Magnesium Metal with Varying Concentrations of Nitric Acid Chemistry

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. (1) Mg + 4 HNO₃ ⟶ Mg(NO₃)₂ + 2 NO₂ + 2 H₂O, (2) Mg + 2 HNO₃ ⟶ Mg(NO₃)₂ + H₂, (3) 3 Mg + 8 HNO₃ ⟶ 3 Mg(NO₃)₂ + 2 NO + 4 H₂O

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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.

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