Question

What is the change in oxidation number of nitrogen in following conversion?
$$\mathrm{NO_3^{-} \longrightarrow NO_2}$$

• A. $+4$ to $+5$
• B. $+3$ to $+5$
• C. $+5$ to $+4$
• D. $-3$ to $+5$

Hint:

To balance redox parameters mentally, remember that each oxygen atom contributes a net demand of $-2$. For neutral $\mathrm{NO_2}$, two oxygens demand $-4$, forcing $\mathrm{N}$ to be $+4$. For $\mathrm{NO_3^-}$, three oxygens demand $-6$, but with a net $-1$ remaining charge, the central $\mathrm{N}$ must supply $+5$.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question requires us to determine the initial and final oxidation states of the nitrogen atom during the chemical reduction of a nitrate ion ($\mathrm{NO_3^{-}}$) into nitrogen dioxide gas ($\mathrm{NO_2}$).

Step 2: Key Formula or Approach:
We assign oxidation numbers using standard rules: oxygen is consistently assigned an oxidation number of $-2$ in these oxo-species. The algebraic sum of the oxidation numbers of all atoms in a chemical species must equal its net structural charge.

Step 3: Detailed Explanation:
Let's find the oxidation state of nitrogen in both species:

• In Nitrate ion ($\mathrm{NO_3^{-}}$): Let $x$ be the oxidation number of nitrogen. $$x + 3(-2) = -1$$ $$x - 6 = -1 \implies x = +5$$ Thus, nitrogen starts in the $+5$ oxidation state.

• In Nitrogen Dioxide ($\mathrm{NO_2}$): Let $y$ be the oxidation number of nitrogen in this neutral molecule. $$y + 2(-2) = 0$$ $$y - 4 = 0 \implies y = +4$$ Thus, nitrogen ends in the $+4$ oxidation state.
The oxidation number of nitrogen shifts from $+5$ to $+4$.

Step 4: Final Answer:
The change in the oxidation number of nitrogen is from $+5$ to $+4$, which matches option (C).