Enthalpy of sublimation of a substance is equal to

Options

A. enthalpy of fusion + enthalpy of vapourisation
B. enthalpy of fusion
C. enthalpy of vapourisation
D. twice the enthalpy of vapourisation.

Explanation

To find the enthalpy of sublimation of a substance, we can follow these steps:

### Step 1: Understand the Process of Sublimation
Sublimation is the process where a solid changes directly into a gas without passing through the liquid phase. This process can be broken down into two distinct steps:
1. **Fusion**: The conversion of solid to liquid.
2. **Vaporization**: The conversion of liquid to gas.

### Step 2: Define Enthalpy Changes
- Let \( \Delta H_1 \) be the enthalpy of sublimation.
- Let \( \Delta H_2 \) be the enthalpy of fusion (solid to liquid).
- Let \( \Delta H_3 \) be the enthalpy of vaporization (liquid to gas).

### Step 3: Write the Relationship
According to the principles of thermodynamics, the enthalpy of sublimation can be expressed as the sum of the enthalpy of fusion and the enthalpy of vaporization:
\[ \Delta H_1 = \Delta H_2 + \Delta H_3 \]

### Step 4: Conclusion
Thus, the enthalpy of sublimation of a substance is equal to the sum of the enthalpy of fusion and the enthalpy of vaporization:
\[ \text{Enthalpy of Sublimation} = \text{Enthalpy of Fusion} + \text{Enthalpy of Vaporization} \]

### Final Answer
The enthalpy of sublimation of a substance is equal to the enthalpy of fusion plus the enthalpy of vaporization.

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