Oxidation Number of Nitrogen and Oxygen Atoms in Nitric Acid (HNO₃)

Nitric acid, is an inorganic strong acid has one nitrogen atom, three oxygen atoms and one hydrogen atom. In this tutorial, we are going to learn how to find oxidation numbers of nitrogen and oxygen atoms in HNO₃ molecule.

Content

• Methods of finding oxidation numbers in HNO₃ acid
  • Find oxidation numbers of atoms of HNO₃ molecule using the lewis structure
  • Find oxidation numbers of atoms of HNO₃ using the algebraic equation
• Why oxidation numbers of Nitrogen atom in HNO₃ acid is important?
  • As an oxidizing acid

Methods of finding oxidation numbers in HNO₃ acid

There are two methods to determine oxidation numbers of atoms in molecules or ions.

1. Using lewis structure of molecule or ion
2. Using algebraic equation

Find oxidation numbers of atoms of HNO₃ molecule using the lewis structure

First, lewis structure of HNO₃ molecule should be drawn and the drawn structure is shown below.

Mark direction of electrons attracting of bonds due to electronegativity difference of atoms

Atoms which have higher electronegativity values, can attract electrons of bonds towards them. Now, we will see how this will affect to the HNO₃ molecule.

Electronegativity values of nitrogen, oxygen and hydrogen atoms - Pauling Scale

• Nitrogen: 3.04
• Oxygen: 3.44
• Hydrogen: 2.2

Now, we can look how electrons are attracted between nitrogen, oxygen and hydrogen atoms.

• Among nitrogen, oxygen and hydrogen atoms Oxygen is the most electrogetive element. Due to higher electronegativity, electrons of N-O and O-H bonds are attracted towards oxygen atoms as following image.

Oxidation number of nitrogen atom in HNO₃

• Oxidation number of nitrogen atom: According to the lewis structure of HNO₃, electrons of the N-O and N=O bonds tend to attract towards oxygen atom. Therefore, nitrogen partially loses it's 4 electrons (in C-O and C=O bonds).
• As due to the oxygen's effect, there is +1 charge on nitrogen atom due to a lack of an electron. So, it should be also counted for oxidation number calculation.
• Due to that excess four electron (partially lost) and +1 charge oxidation number of nitrogen atom is +5

Oxidation number of oxygen atoms in HNO₃

Each oxygen atom's oxidation number is -2.

Find oxidation numbers of atoms of HNO₃ using the algebraic equation

This method use an mathematical algebraic equation to find the oxidation number of an atom in molecules or ions. In this method, you can find the unknown oxidation number of an atom by using known oxidation numbers of other atoms in the molecule or ion.

This method is not 100% accurate for all molecules and ions, but can be used in lot of applications and we will try it now.

Oxidation numbers of oxygen

Oxygen has several oxidation numbers such as -2,-1, 0, +2. Because, oxygen is the most electronegative element, oxidation number of oxygen should be a negative one. As well, because oxygen's most common oxidation number is -2, we can substitute -2 in our following algebraic equation.

Oxidation number of hydrogen

Because hydrogen has the lowest electronegativity value in HNO₃<, hydrogen's oxidation number should be a positive and hydrogen's only positive oxidation number is +1.

Summation of individual oxidation numbers of each atom = overall charge of molecule or ion

For HNO₃ molecule, overall charge is zero and we can write following algebraic equation.

Oxidation number of nitrogen atom + oxidation number of oxygen atoms + oxygen number of hydrogen atom = overall charge of HNO₃ molecule

Because oxidation number of nitrogen atom is taken as unknown, take it as x.

Apply known values for the equation.

• x*1 + (-2)*3 + (+1)*1 = 0
• x = +5

We have found that, oxidation number of nitrogen atom in HNO₃ molecule is +5.

Why oxidation numbers of Nitrogen atom in HNO₃ acid is important?

Because Nitrogen is at +5 oxidation state, there are some important points to discuss about Nitric acid.

As an oxidizing acid

Because Nitrogen atom is at +5 oxidation state, it can be reduced to lower oxidation states. As an example, when concentrated nitric acid is reacted with copper, Nitric acid is reduced to Nitrogen dioxide gas. Oxidation number of Nitrogen in nitrogen dioxide is +4.

If dilute nitric acid used instead of concentrated nitric acid, nitric acid is reduced to Nitric dioxide.

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