Which among the following reactions will be favoured at low pressure?

Options

A. `N_(2)(g)+O_(2)(g) hArr 2NO(g)`
B. `H_(2)(g)+I_(2)(g) hArr 2HI(g)`
C. `PCl_(5)(g) hArr PCl_(3)(g)+Cl_(2)(g)`
D. `N_(2)(g)+3H_(2)(g) hArr 2NH_(3)(g)`

Explanation

To determine which reaction will be favored at low pressure, we can apply Le Chatelier's principle. This principle states that if a system at equilibrium is subjected to a change in pressure, temperature, or concentration, the system will adjust to counteract that change and restore a new equilibrium.

### Step-by-Step Solution:

1. **Identify the Reactions**:
We have four reactions to analyze. Each reaction will have a different number of moles of reactants and products.

2. **Count the Moles**:
For each reaction, count the number of moles of reactants and products:
- **Reaction 1**: \( N_2 + O_2 \rightleftharpoons 2 NO \)
- Reactants: 1 (N2) + 1 (O2) = 2 moles
- Products: 2 (NO) = 2 moles
- **Reaction 2**: \( A + B \rightleftharpoons C + D \)
- Reactants: 1 (A) + 1 (B) = 2 moles
- Products: 1 (C) + 1 (D) = 2 moles
- **Reaction 3**: \( C \rightleftharpoons D + E \)
- Reactants: 1 (C) = 1 mole
- Products: 1 (D) + 1 (E) = 2 moles
- **Reaction 4**: \( F + G \rightleftharpoons H + I + J \)
- Reactants: 1 (F) + 1 (G) = 2 moles
- Products: 1 (H) + 1 (I) + 1 (J) = 3 moles

3. **Analyze the Effect of Pressure**:
According to Le Chatelier's principle, at low pressure, the equilibrium will shift towards the side with more moles of gas:
- **Reaction 1**: 2 moles (reactants) vs. 2 moles (products) → No effect
- **Reaction 2**: 2 moles (reactants) vs. 2 moles (products) → No effect
- **Reaction 3**: 1 mole (reactants) vs. 2 moles (products) → Favors forward direction (produces more moles)
- **Reaction 4**: 2 moles (reactants) vs. 3 moles (products) → Favors forward direction (produces more moles)

4. **Determine the Favored Reaction**:
The reactions that favor the forward direction at low pressure are those that produce more moles of gas. In this case, both Reaction 3 and Reaction 4 favor the forward direction.

5. **Conclusion**:
Since we are looking for the reaction that is favored at low pressure, we can conclude that **Reaction 3** and **Reaction 4** will be favored. However, if we need to select only one option, we should choose the one that has the highest increase in moles.

### Final Answer:
The reactions that will be favored at low pressure are **Reaction 3** and **Reaction 4**. If only one option is to be selected, choose **Reaction 4** as it has the highest increase in moles.

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