When we heat lead nitrate [Pb(NO 3) 2] in a boiling tube, we observe the emission of brown fumes. Which one the following is the brown gas?
NO 2
Understanding the Thermal Decomposition of Lead Nitrate
When lead nitrate, with the chemical formula Pb(NO3)2, is heated strongly, it undergoes a chemical reaction known as thermal decomposition. This process breaks down the lead nitrate into simpler substances. One of the visible observations during this decomposition is the emission of brown fumes from the boiling tube.
Chemical Equation for Lead Nitrate Heating
The balanced chemical equation for the thermal decomposition of lead nitrate is:
\(\text{2Pb(NO}_3\text{)}_2\text{(s)} \xrightarrow{\text{Heat}} \text{2PbO(s)} + \text{4NO}_2\text{(g)} + \text{O}_2\text{(g)}\)
Let's break down what happens:
- Solid lead nitrate (Pb(NO3)2) is heated.
- It decomposes to form solid lead(II) oxide (PbO).
- It also produces nitrogen dioxide gas (NO2) and oxygen gas (O2).
Identifying the Brown Gas
In the products of the thermal decomposition of lead nitrate, nitrogen dioxide (\(\text{NO}_2\)) is a gas that is known for its distinctive reddish-brown color. Oxygen (\(\text{O}_2\)) is a colorless gas.
Therefore, the brown fumes observed when heating lead nitrate are due to the presence of nitrogen dioxide gas (\(\text{NO}_2\)).
Evaluating the Options
Let's look at the given options and determine which one corresponds to the brown gas observed:
- Option 1: NO (Nitric oxide) - This is a colorless gas, although it readily reacts with oxygen in the air to form brown \(\text{NO}_2\). However, the primary brown gas produced directly in the decomposition is not NO.
- Option 2: N2O2 (Dinitrogen dioxide) - While NO exists in equilibrium with its dimer \(\text{N}_2\text{O}_2\) at low temperatures, \(\text{N}_2\text{O}_2\) is also generally considered colorless. This is not the characteristic brown gas from lead nitrate decomposition.
- Option 3: NO3 (Nitrate radical or related species) - \(\text{NO}_3\) is an unstable species and not the brown gas observed.
- Option 4: NO2 (Nitrogen dioxide) - This is a reddish-brown gas that is a direct product of the thermal decomposition of lead nitrate.
Based on the known properties of the reaction products, nitrogen dioxide (\(\text{NO}_2\)) is the brown gas that is observed.
| Product | Chemical Formula | State at reaction temperature | Color |
|---|---|---|---|
| Lead(II) oxide | \(\text{PbO}\) | Solid | Yellow or red (depending on form) |
| Nitrogen dioxide | \(\text{NO}_2\) | Gas | Reddish-brown |
| Oxygen | \(\text{O}_2\) | Gas | Colorless |
Revision Table: Lead Nitrate Decomposition
| Reactant | Condition | Products | Observation (Brown Fumes) |
|---|---|---|---|
| Lead Nitrate (\(\text{Pb(NO}_3\text{)}_2\)) | Strong Heating | Lead(II) oxide (\(\text{PbO}\)), Nitrogen dioxide (\(\text{NO}_2\)), Oxygen (\(\text{O}_2\)) | Brown fumes observed due to the presence of \(\text{NO}_2\) gas. |
Additional Information: Properties and Reactions
Thermal Decomposition: This is a type of chemical reaction where a single compound breaks down into two or more simpler substances when heated. Many metal nitrates decompose upon heating, often producing metal oxides, nitrogen dioxide, and oxygen. The ease of decomposition depends on the reactivity of the metal.
Nitrogen Dioxide (\(\text{NO}_2\)):
- \(\text{NO}_2\) is a toxic gas.
- It has a pungent odor.
- It is an acidic gas, reacting with water to form nitric acid (\(\text{HNO}_3\)) and nitrous acid (\(\text{HNO}_2\)). This is why it contributes to acid rain.
- It is a major air pollutant.
Understanding thermal decomposition reactions like that of lead nitrate is important in studying the chemical properties of compounds and their behavior under different conditions.
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