A black compound of manganese reacts with a halogen acid to give greenish yellow gas. When excess of this gas reacts with `NH_(3) an unstable trihalide is formed. In this process the oxidation state of nitrogen changes from . . . . . . . . . .

Options

A. `-3" to "+3 `
B. `-3" to" 0`
C. `- 3" to "+ 5`
D. `0" to "- 3`

Explanation

To solve the problem step by step, we will analyze the information provided about the black compound of manganese, its reaction with a halogen acid, and the subsequent reaction with ammonia (NH₃).

### Step 1: Identify the Black Compound of Manganese
The question states that there is a black compound of manganese. The most common black compound of manganese is manganese dioxide (MnO₂).

**Hint:** Look for common manganese compounds and their colors.

### Step 2: Reaction with Halogen Acid
The black compound (MnO₂) reacts with a halogen acid (HX). The question specifies that the product is a greenish-yellow gas. The only halogen acid that produces a greenish-yellow gas when reacting with MnO₂ is hydrochloric acid (HCl), which produces chlorine gas (Cl₂).

**Chemical Reaction:**
\[ \text{MnO}_2 + 4\text{HCl} \rightarrow \text{MnCl}_2 + 2\text{Cl}_2 + 2\text{H}_2\text{O} \]

**Hint:** Identify the halogen acid that produces the specified gas.

### Step 3: Excess Chlorine Gas Reaction with Ammonia
The greenish-yellow gas (Cl₂) reacts with excess ammonia (NH₃) to form an unstable trihalide. The reaction can be represented as follows:

**Chemical Reaction:**
\[ 3\text{NH}_3 + 3\text{Cl}_2 \rightarrow 3\text{NCl}_3 + 3\text{HCl} \]

**Hint:** Recognize that chlorine reacts with ammonia to form nitrogen trichloride (NCl₃).

### Step 4: Determine the Oxidation States of Nitrogen
Now, we need to determine the change in the oxidation state of nitrogen during this reaction.

1. **In Ammonia (NH₃):**
- Let the oxidation state of nitrogen be \( x \).
- The oxidation state of hydrogen is +1.
- The overall charge of NH₃ is 0:
\[ x + 3(+1) = 0 \implies x + 3 = 0 \implies x = -3 \]

2. **In Nitrogen Trichloride (NCl₃):**
- Let the oxidation state of nitrogen be \( y \).
- The oxidation state of chlorine is -1.
- The overall charge of NCl₃ is 0:
\[ y + 3(-1) = 0 \implies y - 3 = 0 \implies y = +3 \]

### Step 5: Calculate the Change in Oxidation State
The oxidation state of nitrogen changes from -3 (in NH₃) to +3 (in NCl₃).

**Final Answer:**
The oxidation state of nitrogen changes from -3 to +3.

### Summary of Steps:
1. Identify the black compound of manganese as MnO₂.
2. Recognize that MnO₂ reacts with HCl to produce Cl₂ gas.
3. Understand that Cl₂ reacts with NH₃ to form NCl₃.
4. Calculate the oxidation states of nitrogen in NH₃ and NCl₃.
5. Conclude that the oxidation state of nitrogen changes from -3 to +3.