What is the oxidation number of nitrogen in nitric acid HNO3?a) +2 b) +3 c) +4 d) +4
The Correct Answer and Explanation is:
Correct answer: c) +5
To determine the oxidation number of nitrogen in nitric acid (HNO₃), we need to apply the standard rules of assigning oxidation numbers:
Step-by-step approach:
- Oxidation number of hydrogen (H) is always +1 when bonded to nonmetals.
- Oxygen (O) almost always has an oxidation number of -2.
- The sum of oxidation numbers in a neutral compound is zero.
Now apply these rules to HNO₃:
- H = +1
- Let N = x (we are solving for nitrogen’s oxidation number)
- Each O = -2, and there are 3 oxygen atoms, so total for oxygen = 3 × (-2) = -6
Now write the equation for the total oxidation number:
(+1) + (x) + (-6) = 0
Simplify:
1 + x – 6 = 0
x – 5 = 0
x = +5
Final Answer:
The oxidation number of nitrogen in nitric acid (HNO₃) is +5.
Explanation:
Oxidation numbers are a bookkeeping system in chemistry to keep track of electron transfer in redox reactions. Even though nitric acid is a covalent compound (not ionic), we assign oxidation numbers to atoms as if the electrons in each bond were assigned to the more electronegative atom. In HNO₃, oxygen is more electronegative than both nitrogen and hydrogen, so it “takes” the bonding electrons in our calculation model.
Knowing these numbers helps chemists understand which atoms are being oxidized or reduced in chemical reactions. In nitric acid, nitrogen is in a high oxidation state (+5), meaning it is highly oxidized. This also explains why nitric acid is such a powerful oxidizing agent: nitrogen in +5 wants to stay there, so it forces other substances to give up electrons (get oxidized) in redox reactions.
