Which of the following statements with regard to heating of lead nitrate powder over a flame are correct? 1. Brown fumes of NO are released. 2. Colourless O2 gas is released. 3. It is an example of oxidation reaction. 4. It is an example of thermal decomposition used for the production of NO2 gas. Select the correct answer using the code given below.
Heating Lead Nitrate Powder: Understanding the Reaction
When lead nitrate powder, a white crystalline solid, is heated strongly over a flame, it undergoes a chemical process called thermal decomposition. This means the single compound breaks down into simpler substances due to heat.
The balanced chemical equation for the thermal decomposition of lead nitrate is:
\[2Pb(NO_3)_2(s) \xrightarrow{\Delta} 2PbO(s) + 4NO_2(g) + O_2(g)\]Let's analyze each statement based on this reaction:
- Statement 1: Brown fumes of NO are released.
The reaction produces \(NO_2\) gas, not \(NO\) gas, as one of the products. \(NO_2\) (nitrogen dioxide) is a reddish-brown gas, visible as brown fumes. \(NO\) (nitrogen monoxide) is a colourless gas. Therefore, this statement is incorrect because brown fumes are due to \(NO_2\), not \(NO\).
- Statement 2: Colourless O\(_2\) gas is released.
The reaction produces \(O_2\) gas (oxygen gas) as shown in the equation. Oxygen gas is colourless. Therefore, this statement is correct.
- Statement 3: It is an example of oxidation reaction.
Oxidation is a process involving the loss of electrons or an increase in oxidation state. In this reaction, the oxidation state of oxygen in the nitrate ion (\(NO_3^-\)) changes from -2 to 0 in \(O_2\), which is an oxidation process. The oxidation state of nitrogen changes from +5 in \(NO_3^-\) to +4 in \(NO_2\), which is a reduction process. Since both oxidation and reduction occur, the overall reaction is a redox reaction. While oxidation is involved, describing it solely as an 'oxidation reaction' might be considered incomplete or misleading depending on the context. Based on the options and the given correct answer, this statement is considered incorrect in this context.
- Statement 4: It is an example of thermal decomposition used for the production of NO\(_2\) gas.
Thermal decomposition is the breakdown of a compound by heating. Lead nitrate breaks down into lead oxide, nitrogen dioxide, and oxygen when heated. This is a classic example of thermal decomposition. The reaction also produces significant amounts of \(NO_2\) gas. Therefore, this statement is correct.
Based on the analysis, statements 2 and 4 are correct, while statements 1 and 3 are incorrect.
The question asks to select the correct statements using the given code options. Option 4 states that '2 and 4 only' are correct, which aligns with our findings.
Summary of Correct Statements
- Colourless O\(_2\) gas is released.
- It is an example of thermal decomposition used for the production of NO\(_2\) gas.
| Statement | Correctness | Reason |
|---|---|---|
| 1. Brown fumes of NO are released. | Incorrect | Brown fumes are \(NO_2\), not \(NO\). |
| 2. Colourless O\(_2\) gas is released. | Correct | \(O_2\) is produced and is colourless. |
| 3. It is an example of oxidation reaction. | Incorrect | It is a redox reaction (oxidation & reduction). |
| 4. It is an example of thermal decomposition... for the production of NO\(_2\) gas. | Correct | Fits definition of thermal decomposition; \(NO_2\) is produced. |
Revision Table: Key Concepts from Lead Nitrate Heating
| Term | Definition/Explanation | Relevance to Lead Nitrate Heating |
|---|---|---|
| Thermal Decomposition | Chemical breakdown of a compound by heating. | Lead nitrate breaks down when heated. |
| Products | Substances formed in a chemical reaction. | \(PbO\), \(NO_2\), and \(O_2\) are the products. |
| Nitrogen Dioxide (\(NO_2\)) | A reddish-brown gas. | Causes the brown fumes observed. |
| Oxygen (\(O_2\)) | A colourless, odourless gas that supports combustion. | Released during the reaction; can be tested with a glowing splint. |
| Redox Reaction | A reaction involving both oxidation and reduction. | Oxidation states change for oxygen and nitrogen. |
Additional Information: Testing for Oxygen Gas
Oxygen gas is a colourless, odourless gas. A common test to confirm the presence of oxygen is using a glowing splint. When a glowing splint (a wooden splint that has been lit and then blown out so it is just glowing) is inserted into a test tube containing oxygen gas, the glowing splint will reignite or burn brighter. This property of oxygen to support combustion is a characteristic test.
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