Question Video: Identifying the Oxidation Number of Nitrogen in Nitric Acid Chemistry

Fill in the blank: The oxidation number of nitrogen is zero in nitrogen molecules and _ in nitric acid.

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Video Transcript

Fill in the blank. The oxidation number of nitrogen is zero in nitrogen molecules and blank in nitric acid. (A) Plus six, (B) plus five, (C) plus three, (D) minus five, (E) minus six.

This problem is asking us to find the oxidation number of nitrogen in nitric acid. In order to solve this problem, let us first discuss oxidation numbers. Oxidation numbers show the degree of oxidation of an atom in isolation or in a compound in terms of counting electrons. Oxidation numbers tell us how many electrons an element will gain or lose to form a bond or an ion.

Since we need to know the oxidation number of nitrogen in nitric acid, let us write the formula for nitric acid. Nitric acid is composed of the hydrogen ion, which has a charge of one plus, and the nitrate ion, NO3, with a charge of one minus. We will crisscross the charge numbers and drop the positive and negative signs of the ions. This gives us the formula of HNO3 for nitric acid.

This question reminds us that nitrogen has an oxidation number of zero in nitrogen molecules. This is because the nitrogen molecule is solely composed of only one type of element, nitrogen. But we need to find the oxidation number of nitrogen in HNO3, which does not have an oxidation number of zero since the nitrogen atom is within a compound composed of more than only one type of element. When determining the oxidation number of an atom in a molecule, the molecule will always have a net charge of zero.

Now, let us discuss some of the rules for determining the oxidation numbers of elements within a compound that will help us to answer this question. The first rule we will discuss is the oxidation number for hydrogen in a compound is plus one. Therefore, the oxidation number of hydrogen in nitric acid is plus one. The other rule that will assist us is the oxidation number for oxygen in a compound is minus two. So, since there are three atoms of oxygen in nitric acid and each oxygen atom has an oxidation number of minus two, the overall oxidation number is minus six for oxygen.

We now know that the oxidation numbers of hydrogen, nitrogen, and oxygen must equal zero. We can set up an equation and substitute the known oxidation numbers, where 𝑁 is the oxidation number for nitrogen. Therefore, one plus 𝑁 minus six equals zero. When we solve for nitrogen’s oxidation number in nitric acid, it is equal to plus five. Therefore, answer choices (A) plus six, (C) plus three, (D) minus five, and (E) minus six are all incorrect. And answer choice (B) plus five is the correct answer choice for the oxidation number of nitrogen in nitric acid.

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