Which of the following reactions yield elementary gases like `N_(2), H_(2), O_(2)`  as the byproducts ?
I) `CaO+NH_(3)rarr`
II) `2NH_(3)+2Na overset(Fe)underset(300-400^(@)C)rarr`
III) `8NH_(3)+3Cl_(2)rarr`
IV) `2Pb(NO_(3))_(2)overset(Delta)rarr` 

Options

A. I and II only
B. II, III and IV only
C. I, II and III only
D. All of these

Answer

To determine which of the given reactions yield elementary gases like \(N_2\), \(H_2\), and \(O_2\) as byproducts, we will analyze each reaction step by step.

### Step-by-Step Solution:

**I) \(CaO + NH_3 \rightarrow ?\)**

1. **Identify the reactants**: Calcium oxide (\(CaO\)) and ammonia (\(NH_3\)).
2. **Write the reaction**: The reaction between calcium oxide and ammonia typically produces calcium nitride and water.
\[ CaO + 2NH_3 \rightarrow Ca + N_2 + 3H_2O \]
3. **Identify the byproducts**: The byproducts include nitrogen gas (\(N_2\)) and water. Here, \(N_2\) is an elementary gas.

**Conclusion for I**: This reaction yields \(N_2\) as a byproduct.

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**II) \(2NH_3 + 2Na \xrightarrow{Fe, 300-400^\circ C} ?\)**

1. **Identify the reactants**: Ammonia (\(NH_3\)) and sodium (\(Na\)).
2. **Write the reaction**: The reaction of sodium with ammonia in the presence of iron produces sodamide and hydrogen gas.
\[ 2NH_3 + 2Na \rightarrow 2NaNH_2 + H_2 \]
3. **Identify the byproducts**: The byproduct here is hydrogen gas (\(H_2\)), which is an elementary gas.

**Conclusion for II**: This reaction yields \(H_2\) as a byproduct.

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**III) \(8NH_3 + 3Cl_2 \rightarrow ?\)**

1. **Identify the reactants**: Ammonia (\(NH_3\)) and chlorine (\(Cl_2\)).
2. **Write the reaction**: The reaction produces ammonium chloride and nitrogen gas.
\[ 8NH_3 + 3Cl_2 \rightarrow 6NH_4Cl + N_2 \]
3. **Identify the byproducts**: The byproduct here is nitrogen gas (\(N_2\)), which is an elementary gas.

**Conclusion for III**: This reaction yields \(N_2\) as a byproduct.

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**IV) \(2Pb(NO_3)_2 \xrightarrow{\Delta} ?\)**

1. **Identify the reactants**: Lead(II) nitrate (\(Pb(NO_3)_2\)).
2. **Write the reaction**: Upon heating, lead(II) nitrate decomposes to produce lead(II) oxide, nitrogen dioxide, and oxygen.
\[ 2Pb(NO_3)_2 \rightarrow 2PbO + 4NO_2 + O_2 \]
3. **Identify the byproducts**: The byproducts include oxygen gas (\(O_2\)), which is an elementary gas.

**Conclusion for IV**: This reaction yields \(O_2\) as a byproduct.

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### Final Conclusion:
All four reactions yield elementary gases as byproducts:
- I) \(N_2\)
- II) \(H_2\)
- III) \(N_2\)
- IV) \(O_2\)

Thus, the correct answer is that all of these reactions yield elementary gases.

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