Lead nitrate on heating gives
PbO and NO 2
Understanding Lead Nitrate Decomposition Upon Heating
Let's explore what happens when lead nitrate is subjected to heat. This process is known as thermal decomposition, where a substance breaks down into simpler substances when heated.
Thermal Decomposition of Lead Nitrate (\( \text{Pb(NO}_3)_2 \))
Lead nitrate is a white crystalline solid. When it is heated strongly, it decomposes to produce lead(II) oxide, nitrogen dioxide gas, and oxygen gas. This is a characteristic reaction for the nitrates of many heavy metals.
The balanced chemical equation for the thermal decomposition of lead nitrate is:
\[ 2\text{Pb(NO}_3)_2\text{(s)} \xrightarrow{\Delta} 2\text{PbO(s)} + 4\text{NO}_2\text{(g)} + \text{O}_2\text{(g)} \]Let's identify the products from the equation:
- Lead(II) oxide (\( \text{PbO} \)): This is a solid residue left after heating. It can be yellow (litharge) or red (massicot) depending on the cooling conditions.
- Nitrogen dioxide (\( \text{NO}_2 \)): This is a reddish-brown gas with a pungent smell. The evolution of this gas is a clear visual indicator of the decomposition reaction.
- Oxygen (\( \text{O}_2 \)): This is a colorless gas that supports combustion. While produced, it is often not the most visually noticeable product compared to \( \text{NO}_2 \).
Analyzing the Given Options
The question asks what lead nitrate gives on heating, and the options list pairs of products. Let's compare the products we identified with the options:
- Option 1: \( \text{PbO}_2 \) and \( \text{NO}_2 \). This option lists lead(IV) oxide (\( \text{PbO}_2 \)) instead of lead(II) oxide (\( \text{PbO} \)) as the solid product. Thermal decomposition of lead nitrate primarily yields \( \text{PbO} \).
- Option 2: \( \text{PbO} \) and \( \text{NO}_2 \). This option lists lead(II) oxide and nitrogen dioxide, which are two of the primary products observed during the decomposition of lead nitrate. While oxygen is also produced, this option correctly identifies the main solid product and the characteristic gaseous product.
- Option 3: \( \text{PbO} \) and \( \text{NO} \). This option lists nitrogen monoxide (\( \text{NO} \)) instead of nitrogen dioxide (\( \text{NO}_2 \)). \( \text{NO}_2 \) is the nitrogen oxide primarily formed under these conditions.
- Option 4: \( \text{PbO}_2 \) and \( \text{NO} \). This option lists incorrect solid and incorrect gaseous nitrogen oxide products.
Based on the chemical reaction and the products formed, Option 2, listing \( \text{PbO} \) and \( \text{NO}_2 \), is the most accurate representation of the main products among the choices provided.
Step-by-Step Summary
- Start with solid lead nitrate (\( \text{Pb(NO}_3)_2 \)).
- Apply heat (\( \Delta \)).
- The compound breaks down (decomposes).
- Solid lead(II) oxide (\( \text{PbO} \)) is formed as a residue.
- Reddish-brown nitrogen dioxide gas (\( \text{NO}_2 \)) is released.
- Colorless oxygen gas (\( \text{O}_2 \)) is also released.
- The question asks for the products given. Option 2 lists \( \text{PbO} \) and \( \text{NO}_2 \), which are two key products.
Revision Table: Key Aspects of Lead Nitrate Heating
| Reactant | Conditions | Products | Observations |
|---|---|---|---|
| Lead(II) nitrate (\( \text{Pb(NO}_3)_2 \)) - white solid | Heating (\( \Delta \)) | Lead(II) oxide (\( \text{PbO} \)) - yellow/red solid Nitrogen dioxide (\( \text{NO}_2 \)) - brown gas Oxygen (\( \text{O}_2 \)) - colorless gas |
White solid turns yellow/red. Brown gas evolves. |
Additional Information: Thermal Decomposition Reactions
Thermal decomposition is a type of chemical reaction where a single compound breaks down into two or more simpler substances when heated. The products of thermal decomposition vary depending on the nature of the compound being heated.
- Metal Carbonates: Most metal carbonates decompose on heating to give a metal oxide and carbon dioxide. For example, calcium carbonate (\( \text{CaCO}_3 \)) gives calcium oxide (\( \text{CaO} \)) and carbon dioxide (\( \text{CO}_2 \)).
- Metal Hydroxides: Many metal hydroxides decompose on heating to form a metal oxide and water. For example, calcium hydroxide (\( \text{Ca(OH)}_2 \)) gives calcium oxide (\( \text{CaO} \)) and water (\( \text{H}_2\text{O} \)).
- Metal Nitrates: As seen with lead nitrate, the decomposition of metal nitrates can produce metal oxide, nitrogen dioxide, and oxygen. However, nitrates of very reactive metals like sodium or potassium decompose differently, usually forming the metal nitrite and oxygen (\( 2\text{NaNO}_3 \xrightarrow{\Delta} 2\text{NaNO}_2 + \text{O}_2 \)).
The decomposition of lead nitrate is a classic example often demonstrated in laboratories due to the visible evolution of brown nitrogen dioxide gas.
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