The dissociation of phosgene which occurs according to the reaction `COCI_2 (g) hArr CO(g) +CI_2(g)` is an endothermic process. Which of the following factors will increase the degree of dissociation of `COCI_2` ?

Options

A. Adding `CI_2` to the system
B. Adding helium to the system at constant pressure
C. Decreasing the temperature of the system
D. Reducing the total pressure

Answer

To solve the problem regarding the dissociation of phosgene (COCl₂) and the factors that can increase its degree of dissociation, we will analyze the reaction and apply Le Chatelier's principle.

### Step-by-Step Solution:

1. **Write the Reaction**:
The dissociation of phosgene can be represented as:
\[ \text{COCl}_2 (g) \rightleftharpoons \text{CO} (g) + \text{Cl}_2 (g) \]

2. **Identify the Nature of the Reaction**:
The problem states that this reaction is endothermic. This means that heat is absorbed during the reaction.

3. **Understand Le Chatelier's Principle**:
Le Chatelier's principle states that if a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the system will adjust itself to counteract that change and restore a new equilibrium.

4. **Analyze Each Option**:
- **Option A: Adding Cl₂ to the system**:
Adding more Cl₂ would shift the equilibrium to the left (towards reactants) to reduce the concentration of Cl₂. This would **decrease** the degree of dissociation.

- **Option B: Adding Helium to the system at constant pressure**:
Helium is an inert gas and does not participate in the reaction. Adding helium at constant pressure does not change the concentrations of the reactants or products, hence it has no effect on the equilibrium. Thus, it does not increase the degree of dissociation.

- **Option C: Decreasing the temperature**:
Since the reaction is endothermic, decreasing the temperature would shift the equilibrium to the left (towards reactants) to counteract the decrease in heat. However, if we consider the reverse process (the formation of reactants), it would favor the dissociation of COCl₂. Thus, this option would **increase** the degree of dissociation.

- **Option D: Reducing the total pressure**:
Reducing the total pressure would favor the side with more moles of gas. In this case, there are 2 moles of products (CO and Cl₂) for every 1 mole of reactant (COCl₂). However, since the total pressure reduction does not significantly impact the equilibrium in this case, it would not effectively increase the degree of dissociation.

5. **Conclusion**:
The factor that will increase the degree of dissociation of COCl₂ is:
\[ \text{C: Decreasing the temperature} \]