What is the oxidation number of N in nitric acid (
)? a) 0 b) +3 c) +5 d) -5 e) -3
The Correct Answer and Explanation is:
Correct Answer: c) +5
To determine the oxidation number of nitrogen in nitric acid (HNO₃), we need to consider the known oxidation numbers of the other atoms and the overall charge of the molecule.
Step 1: Assign known oxidation numbers
- Hydrogen (H) usually has an oxidation number of +1.
- Oxygen (O) usually has an oxidation number of -2.
- Nitric acid (HNO₃) is a neutral compound, so the sum of all oxidation numbers must be zero.
Step 2: Let x be the oxidation number of nitrogen (N)
In HNO₃, there is:
- 1 hydrogen: +1
- 1 nitrogen: x
- 3 oxygens: 3 × (-2) = -6
Set up the equation:
(+1) + (x) + (-6) = 0
1 + x – 6 = 0
x – 5 = 0
x = +5
So, the oxidation number of nitrogen in HNO₃ is +5.
Explanation
The oxidation number, also known as the oxidation state, is a concept used to keep track of electrons in chemical reactions, especially redox (oxidation-reduction) reactions. It represents the hypothetical charge an atom would have if all bonds to atoms of different elements were 100 percent ionic.
In nitric acid (HNO₃), we have three elements: hydrogen, nitrogen, and oxygen. The molecule is neutral, meaning the total of all oxidation numbers must add up to zero.
Hydrogen almost always has an oxidation number of +1 when bonded to nonmetals, which it is here. Oxygen, being more electronegative, typically has an oxidation number of -2. With three oxygen atoms, their combined oxidation number is 3 × -2 = -6.
Letting x represent the unknown oxidation number of nitrogen, the total equation becomes:
+1 (from hydrogen) + x (from nitrogen) + -6 (from oxygen) = 0.
Solving this yields x = +5, which means nitrogen in HNO₃ is in the +5 oxidation state. This is consistent with nitrogen being highly oxidized in nitric acid, which is a strong oxidizing agent.
Understanding oxidation numbers is critical for analyzing chemical reactions, predicting the behavior of compounds, and balancing redox equations. Nitric acid’s strong oxidizing power comes from nitrogen being in its highest common oxidation state, allowing it to accept electrons in redox reactions.
