For which of the following reaction is product formation favoured by low pressure and high temperature?

Options

A. `H_(2)(g)+I_(2)(g)hArr2HI(g),DeltaH^(@)=-9.4KJ`
B. `CO_(2)(g)+C(s)hArr2CO(g),DeltaH^(@)=172.5KJ`
C. `CO(g)+2H_(2)(g)hArrCH_(3)OH,DeltaH^(@)=-21.7KJ`
D. `3O_(2)(g)hArr2O_(3)(g),DeltaH^(@)=285KJ`

Answer

To determine which reaction favors product formation under low pressure and high temperature, we can apply Le Chatelier's principle. This principle states that if a system at equilibrium is subjected to a change in conditions (such as temperature or pressure), the equilibrium will shift in a direction that counteracts the change.

### Step-by-Step Solution:

1. **Identify the Reactions**: We have four reactions to analyze. We need to evaluate each one based on its enthalpy change (ΔH) and the number of moles of gases on both sides of the equation.

2. **Understand the Effects of Temperature**:
- For **exothermic reactions** (ΔH < 0), increasing temperature shifts the equilibrium to the left (favoring reactants).
- For **endothermic reactions** (ΔH > 0), increasing temperature shifts the equilibrium to the right (favoring products).

3. **Understand the Effects of Pressure**:
- Decreasing pressure favors the side of the reaction with more moles of gas. We need to count the number of gaseous moles on both sides of each reaction.

4. **Analyze Each Reaction**:
- **Reaction A**: H2 + I2 ⇌ 2HI (ΔH < 0)
- Exothermic reaction. Increasing temperature does not favor product formation. (Incorrect)

- **Reaction B**: CO2(g) + C(s) ⇌ 2CO(g) (ΔH > 0)
- Endothermic reaction. Increasing temperature favors product formation.
- On the reactant side, there is 1 mole of gas (CO2), and on the product side, there are 2 moles of gas (2CO). Decreasing pressure favors the products. (Correct)

- **Reaction C**: CO(g) + 2H2O(g) ⇌ CH3OH(g) (ΔH < 0)
- Exothermic reaction. Increasing temperature does not favor product formation. (Incorrect)

- **Reaction D**: 3O2(g) ⇌ 2O3(g) (ΔH > 0)
- Endothermic reaction. Increasing temperature favors product formation.
- On the reactant side, there are 3 moles of gas, and on the product side, there are 2 moles of gas. Decreasing pressure favors the reactants. (Incorrect)

5. **Conclusion**: The only reaction that favors product formation with low pressure and high temperature is **Reaction B**: CO2(g) + C(s) ⇌ 2CO(g).

### Final Answer:
**Reaction B: CO2(g) + C(s) ⇌ 2CO(g)** is favored by low pressure and high temperature.