In which one of the following reactions, the yield of the products decreases by in creasing the pressure ?

Options

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

Answer

To solve the question, we need to analyze the given reactions and determine how the yield of products is affected by an increase in pressure. Here's a step-by-step solution:

### Step 1: Understand the Effect of Pressure on Equilibrium
According to Le Chatelier's Principle, if the pressure of a system at equilibrium is increased, the equilibrium will shift towards the side with fewer moles of gas. Conversely, if the pressure is decreased, the equilibrium will shift towards the side with more moles of gas.

### Step 2: Analyze Each Reaction
We need to look at the number of moles of reactants and products in each reaction to see how the yield of products is affected by increasing pressure.

1. **Reaction 1**: A + B ⇌ C + D
- Moles of Reactants = 2 (1 A + 1 B)
- Moles of Products = 2 (1 C + 1 D)
- Since the number of moles of reactants equals the number of moles of products, increasing pressure will have no effect on the yield.

2. **Reaction 2**: A + B + C ⇌ D + E + F
- Moles of Reactants = 3 (1 A + 1 B + 1 C)
- Moles of Products = 3 (1 D + 1 E + 1 F)
- Again, the number of moles of reactants equals the number of moles of products, so increasing pressure will have no effect.

3. **Reaction 3**: A + B ⇌ C
- Moles of Reactants = 2 (1 A + 1 B)
- Moles of Products = 1 (1 C)
- Here, the number of moles of products is less than the number of moles of reactants. Therefore, increasing pressure will shift the equilibrium to the left, decreasing the yield of products.

4. **Reaction 4**: A ⇌ B + C
- Moles of Reactants = 1 (1 A)
- Moles of Products = 2 (1 B + 1 C)
- In this case, the number of moles of products is greater than the number of moles of reactants. Increasing pressure will shift the equilibrium to the left, decreasing the yield of products.

### Step 3: Conclusion
From the analysis, we find that in **Reaction 3**, the yield of products decreases by increasing the pressure because the number of moles of products is less than the number of moles of reactants.

### Final Answer
The reaction in which the yield of products decreases by increasing the pressure is **Reaction 3**: A + B ⇌ C.

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