Q.When a brown compound of manganese (A) is treated with HCl it gives a gas (B). The gas taken in excess, reacts with $NH_3$ to give an explosive compound (C). Identify compounds A, B and C.

Concept: Inorganic Synthesis – This is a reaction sequence involving a manganese compound, its reaction with HCl to produce a gas, and the subsequent reaction of that gas with ammonia to form an explosive product.

Step 1: The brown compound of manganese is likely manganese dioxide ($\text{MnO}_2$), which is a common brown solid.

Step 2: When $\text{MnO}_2$ is treated with concentrated HCl, it undergoes a redox reaction:

The gas produced (B) is chlorine ($\text{Cl}_2$). …

Why this approach works

The problem gives three clues: a brown manganese compound, a gas produced with HCl, and that gas reacting with ammonia to form an explosive. Each clue narrows the possibilities.

Manganese forms several oxides, but only one is brown: $\text{MnO}_2$ (manganese dioxide). It’s a common oxidizing agent. When treated with concentrated HCl, it oxidises chloride ions to chlorine gas — a classic lab preparation of chlorine. That gas (B) must be $\text{Cl}_2$.

Now, chlorine gas in excess reacts with ammonia. Most students recall the reaction where ammonia reduces chlorine to nitrogen (the “chlorine-ammonia fountain” or bleach formation). But that uses limited chlorine. Here, the problem says “the gas taken in excess” — that’s the key. Excess chlorine with ammonia produces nitrogen trichloride, $\text{NCl}_3$, a highly explosive oily liquid. That matches “explosive compound (C)”.

So the chain is: $\text{MnO}_2 \xrightarrow{\text{HCl}} \text{Cl}_2 \xrightarrow{\text{excess NH}_3} \text{NCl}_3$.

Step-by-step reasoning

1. Identify compound A — the brown manganese compound.

   Manganese dioxide ($\text{MnO}_2$) is a dark brown to black solid. It’s the most common brown oxide of manganese. Other manganese compounds (like $\text{Mn}_2\text{O}_3$ or $\text{Mn}_3\text{O}_4$) are black or dark grey, not typically described as brown. So A is $\text{MnO}_2$.

2. Reaction of A with HCl — formation of gas B.

   $\text{MnO}_2$ is a strong oxidising agent. With concentrated HCl, it oxidises $\text{Cl}^-$ to $\text{Cl}_2$ gas, while itself being reduced to $\text{Mn}^{2+}$ (as $\text{MnCl}_2$). The balanced equation:

The gas B is chlorine ($\text{Cl}_2$), greenish-yellow and pungent.

3. Reaction of excess B (chlorine) with ammonia — formation of explosive C. Chlorine reacts with ammonia in two different ways depending on the ratio:
   • With limited chlorine: $3\text{Cl}_2 + 2\text{NH}_3 \rightarrow \text{N}_2 + 6\text{HCl}$ (or further to $\text{NH}_4\text{Cl}$).
   • With excess chlorine: each N–H bond gets replaced by N–Cl, giving nitrogen trichloride: …

Concept: Redox Reactions & Chemical Properties of Manganese Compounds

The key idea here is that manganese in a high oxidation state acts as a strong oxidising agent. When treated with HCl, it oxidises chloride ions to chlorine gas. This chlorine gas then reacts with ammonia to form an explosive compound.

Method: Reaction Pathway Analysis

Step 1: Identify compound A (brown compound of manganese)

• The most common brown compound of manganese is manganese dioxide ($\text{MnO}_2$).
• In $\text{MnO}_2$, manganese is in the +4 oxidation state.

Step 2: Reaction of A with HCl → gas B

• $\text{MnO}_2$ oxidises HCl to chlorine gas.
• Reaction:

• Gas B is chlorine ($\text{Cl}_2$).

Step 3: Reaction of excess B with $\text{NH}_3$ → explosive compound C

• Chlorine reacts with ammonia. With excess chlorine, the product is nitrogen trichloride ($\text{NCl}_3$) — a highly explosive oily liquid.
• Reaction:

• Compound C is nitrogen trichloride ($\text{NCl}_3$).

Final Answer …

Here’s a breakdown of the common mistakes students make on this question, along with how to avoid each.

1. Mistake: Second-Guessing Compound A Because of the Colour Word

The Error:

Seeing "brown," some students abandon $MnO_2$ (manganese dioxide) and guess a different manganese oxide, since $MnO_2$ is more often described as black or dark grey.

Why It's a Trap:

$MnO_2$ is the correct compound A here. Powdered $MnO_2$ is commonly described as brown to black depending on particle size and hydration, and — more importantly — the reaction that follows (A + conc. HCl → $Cl_2$, the classic laboratory preparation of chlorine) only works for $MnO_2$ (Mn in the +4 state); $Mn_2O_3$ does not react with HCl this way.

How to Avoid:

• Let the reaction, not just the colour adjective, confirm the identity: only $MnO_2$ liberates $Cl_2$ from concentrated HCl in this classic manner.
• Don't over-correct on a single descriptive word — cross-check colour AND reactivity together.

2. Mistake: Confusing Gas B (Chlorine vs. Oxygen)

The Error:

Some students think the gas produced when $MnO_2$ reacts with HCl is oxygen ($O_2$) or hydrogen chloride ($HCl$) itself. In reality, the reaction is:

The gas B is chlorine ($Cl_2$).

Why It Happens:

• Students forget that $MnO_2$ (Mn in the +4 state) is a strong oxidising agent, oxidising $Cl^-$ to $Cl_2$ while itself being reduced to $Mn^{2+}$.

How to Avoid:

• Write the half-reactions:
  • $MnO_2 + 4H^+ + 2e^- \rightarrow Mn^{2+} + 2H_2O$ (reduction)
  • $2Cl^- \rightarrow Cl_2 + 2e^-$ (oxidation)
• This confirms $Cl_2$ is the only plausible gas.

3. Mistake: Getting Compound C Wrong (Explosive Product)

The Error:

Students often guess $NH_4Cl$ (ammonium chloride) or $N_2$ as the product when $Cl_2$ reacts with $NH_3$. The correct explosive compound is nitrogen trichloride ($NCl_3$).

Why It Happens:

• The reaction of excess chlorine with ammonia is less common in textbooks.
• Students recall the reaction: $3Cl_2 + NH_3 \rightarrow NCl_3 + 3HCl$, but forget that $NCl_3$ is highly explosive.

How to Avoid:

• Learn the specific reaction:

• Note the condition: excess chlorine is key. If ammonia is in excess, you get $N_2$ and $NH_4Cl$ instead.
• Memorise that $NCl_3$ is an oily, yellow, explosive liquid — a classic exam fact.

4. Mistake: Ignoring the “Explosive” Clue …