Question

Identify reductant in following reaction.
$\mathrm{H_2S} + \mathrm{NO_2} \rightarrow \mathrm{H_2O} + \mathrm{NO} + \mathrm{S}$

• A. $\mathrm{H_2S}$
• B. $\mathrm{NO_2}$
• C. $\mathrm{NO}$
• D. $\mathrm{S}$

Hint:

A reductant must always be chosen from the reactant side of the equation, which instantly rules out products like $\mathrm{NO}$ and $\mathrm{S}$. Since sulfur goes from a negative oxidation state ($-2$) up to zero ($0$) by shedding electrons, $\mathrm{H_2S}$ is clearly the oxidized agent!

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to identify the reductant (reducing agent) in the given redox reaction: $\mathrm{H_2S} + \mathrm{NO_2} \rightarrow \mathrm{H_2O} + \mathrm{NO} + \mathrm{S}$.

Step 2: Key Formula or Approach:
In a oxidation-reduction (redox) chemical reaction: The reductant (reducing agent) is the substance that undergoes oxidation. Oxidation is defined as an increase in the oxidation state of an element due to the loss of electrons.

Step 3: Detailed Explanation:
Let's determine the oxidation states of the key elements on both sides of the chemical equation: 1. For Sulfur ($\mathrm{S}$): In the reactant hydrogen sulfide ($\mathrm{H_2S}$), hydrogen has a standard oxidation state of $+1$. Therefore, the oxidation state of sulfur is $-2$. In the product side, sulfur exists as an uncombined element ($\mathrm{S}$), so its oxidation state is $0$. The oxidation state of sulfur changes from $-2$ to $0$. Since this value increases, $\mathrm{H_2S}$ is oxidized and acts as the reductant. 2. For Nitrogen ($\mathrm{N}$): In the reactant nitrogen dioxide ($\mathrm{NO_2}$), oxygen has an oxidation state of $-2$, meaning nitrogen has an oxidation state of $+4$. In the product nitric oxide ($\mathrm{NO}$), oxygen is $-2$, meaning nitrogen has dropped to an oxidation state of $+2$. Because its oxidation state decreases, $\mathrm{NO_2}$ is reduced and acts as the oxidant.

Step 4: Final Answer:
The reductant in this reaction is $\mathrm{H_2S}$, which corresponds to option (A).