Phosphorous pentachloride when heated in a sealed tube at `700 K` it undergoes decomposition as
`PCl_(5)(g) hArr PCl_(3)(g)+Cl_(2)(g), K_(p)=38` atm
Vapour density of the mixture is `74.25`.
The reaction is

Options

A. Endothermic
B. Exothermic
C. May be endothermic or exothermic
D. Unpredictable

Answer

To determine the nature of the reaction of phosphorus pentachloride (PCl₅) when heated, we can follow these steps:

### Step 1: Understand the Reaction
The decomposition of phosphorus pentachloride is represented by the equation:
\[ PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g) \]

### Step 2: Identify the Conditions
The reaction occurs at a temperature of 700 K in a sealed tube, and the equilibrium constant \( K_p \) is given as 38 atm.

### Step 3: Analyze the Reaction Type
To determine whether the reaction is endothermic or exothermic, we need to consider the following:
- **Endothermic Reaction**: Absorbs heat from the surroundings (ΔH > 0).
- **Exothermic Reaction**: Releases heat to the surroundings (ΔH < 0).

### Step 4: Consider the Heating of the Reaction
Since the reaction is initiated by heating, it suggests that energy is required to break the bonds in PCl₅. This is characteristic of endothermic reactions, where heat is absorbed to facilitate the decomposition.

### Step 5: General Knowledge of Decomposition Reactions
Decomposition reactions, in general, require energy input to break down the compound into its constituent parts. Therefore, it is reasonable to conclude that this reaction is endothermic.

### Conclusion
Based on the analysis, the decomposition of phosphorus pentachloride (PCl₅) when heated is an **endothermic reaction**.

### Final Answer
The reaction is endothermic.

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