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: State the rule we will use.
Oxygen is taken as $-2$ in these species, and the sum of all oxidation numbers must equal the overall charge of the species.
Step 2: Work out nitrogen in $NO_3^-$.
Let it be $x$. Then $x + 3(-2) = -1$, the charge of the ion.
Step 3: Solve that equation.
$x - 6 = -1$, so $x = +5$. Nitrogen starts at $+5$.
Step 4: Work out nitrogen in $NO_2$.
Let it be $y$. The molecule is neutral, so $y + 2(-2) = 0$.
Step 5: Solve that equation.
$y - 4 = 0$, so $y = +4$. Nitrogen ends at $+4$.
Step 6: Describe the shift.
Nitrogen moves from $+5$ down to $+4$, a gain of one electron, so this is a reduction.
\[ \boxed{+5 \text{ to } +4 \text{ (option C)}} \]