Animated Solution for Chemistry - s and p-Block Elements: On heating compound (A) gives a gas (B) which is a constituent of air. This gas when treated with ${\text{H}}_2$ in the presence of a catalyst gives another gas (C) which is basic in nature. (A) should not be

Imagine you are a chemical detective, and you've just been handed a mystery. You have an unknown compound, let's call it Compound (A). When you heat it, it releases a gas, Gas (B). Now, Gas (B) isn't just any gas; it's a major constituent of the air we breathe. But the clues don't stop there. If you take Gas (B) and react it with hydrogen gas ($H_2$) in the presence of a catalyst, it transforms into another gas, Gas (C), which happens to be basic in nature.

Your mission? To find out what Compound (A) should not be from a given list of suspects.

Let's start by unmasking Gas (B) and Gas (C). The problem tells us that Gas (B) is a constituent of air. The air around us is primarily composed of two gases: Nitrogen ($N_2$), making up about 78%, and Oxygen ($O_2$), making up about 21%. So, Gas (B) is likely one of these two.

Next, we are told that Gas (B) reacts with hydrogen ($H_2$) to form a basic gas, Gas (C). Let's test our two candidates. If Gas (B) were oxygen, its reaction with hydrogen would produce water ($H_{2}O$). While water is essential for life, it is neutral, not basic. So, oxygen is out.

What if Gas (B) is nitrogen? When nitrogen reacts with hydrogen in the presence of an iron catalyst at high temperature and pressure, it forms ammonia ($N{H}_3$). This is the famous Haber-Bosch process, a cornerstone of industrial chemistry!

Ammonia is a well-known basic gas. It turns red litmus blue and reacts with acids to form ammonium salts. Therefore, Gas (B) is definitely Nitrogen ($N_2$), and Gas (C) is Ammonia ($N{H}_3$).

Now that we know Gas (B) is nitrogen, we can rephrase the first part of the problem: "On heating, Compound (A) gives nitrogen gas."

But here is where many students make a silly mistake. The question asks: "(A) should not be". This means we are looking for the one compound among the options that does not produce nitrogen gas when heated. We need to examine the thermal decomposition of each option.

Thermal Decomposition

Let's put our suspects under the heat and see how they react.

Suspect 1: Ammonium Nitrite ($N{H}_{4}N{O}_2$) When ammonium salts containing an oxidizing anion (like nitrite, nitrate, or dichromate) are heated, they undergo an intramolecular redox reaction. The ammonium ion ($N{H}_4^{+}$) acts as a reducing agent, and the nitrite ion ($N{O}_2^{-}$) acts as an oxidizing agent. Upon gentle heating, ammonium nitrite decomposes to yield nitrogen gas and water vapor.

Since it produces nitrogen, it fits the description of Compound (A). So, this is not our answer.

Suspect 2: Ammonium Dichromate ($(N{H}_4{)}_{2}C{r}_2{O}_7$) This compound is famous for the "chemical volcano" demonstration. It's a bright orange solid. When ignited, it decomposes vigorously, sparking and spewing out a voluminous green powder of chromium(III) oxide ($C{r}_2{O}_3$), along with steam and a large amount of nitrogen gas.

Again, nitrogen gas is produced. This suspect is cleared.

Suspect 3: Sodium Azide ($Na{N}_3$) Sodium azide is a fascinating compound. It is highly stable at room temperature but decomposes rapidly when heated to around 300°C. This rapid decomposition produces sodium metal and a massive burst of pure nitrogen gas. This exact reaction is utilized in automobile airbags to inflate them instantly during a collision!

Because it produces nitrogen gas, it also fits the profile of Compound (A).

Suspect 4: Lead(II) Nitrate ($Pb(N{O}_3{)}_2$) Now we come to our final suspect. Lead(II) nitrate is a heavy metal nitrate. The thermal decomposition of heavy metal nitrates follows a different pathway. Instead of releasing nitrogen gas, they decompose to form the corresponding metal oxide, nitrogen dioxide gas ($N{O}_2$), and oxygen gas ($O_2$).

When you heat white lead(II) nitrate crystals, you will hear a crackling sound (decrepitation) and see the emission of dense, reddish-brown fumes. These fumes are nitrogen dioxide. The residue left behind is lead(II) oxide ($PbO$), which is reddish-brown when hot and yellow when cold.

Notice the products carefully. It produces nitrogen dioxide ($N{O}_2$) and oxygen ($O_2$), but no nitrogen gas ($N_2$).

We were looking for the compound that does not produce nitrogen gas upon heating. Lead(II) nitrate is the only compound among the choices that fails to produce $N_2$. Therefore, Compound (A) cannot be lead(II) nitrate.

This problem is a beautiful amalgamation of qualitative analysis, industrial processes, and thermal decomposition reactions. It tests your ability to connect the dots between the properties of gases and the behavior of different salts under heat. Always read the question carefully—that little word "not" changes the entire objective of the investigation!