The change in enthalpy of a substqance when one mole of the substance is completely burnt in excess of air is called

Options

A. Heat of sublimation
B. Heat of combustion
C. Heat of fusion
D. Both (1) & (2)

Answer

To solve the question, we need to identify the term that describes the change in enthalpy when one mole of a substance is completely burnt in excess of air. Let's analyze the options step by step:

### Step 1: Understand the Question
The question asks for the change in enthalpy when one mole of a substance is completely burnt in excess of air. This process involves a chemical reaction where the substance reacts with oxygen.

**Hint:** Look for terms related to combustion or burning processes.

### Step 2: Analyze the Options
1. **Heat of Sublimation**: This refers to the energy required to convert one mole of a solid directly to a gas at constant pressure. This does not relate to combustion, so we can eliminate this option.

**Hint:** Recall that sublimation involves a phase change from solid to gas, not combustion.

2. **Heat of Combustion**: This is defined as the energy released when one mole of a substance is completely burned in the presence of excess oxygen. This matches the description given in the question.

**Hint:** Focus on the definition of combustion and energy release during burning.

3. **Enthalpy of Fusion**: This refers to the energy required to convert one mole of a solid into a liquid at constant pressure. This is also unrelated to combustion, so we can eliminate this option.

**Hint:** Remember that fusion involves melting, not burning.

4. **Option 1 and 2**: This option suggests that both heat of sublimation and heat of combustion are correct. Since we have already eliminated heat of sublimation, this option is incorrect.

**Hint:** Check if both terms can be correct simultaneously based on their definitions.

### Step 3: Conclusion
After analyzing all the options, the correct answer is **Heat of Combustion**. This term specifically refers to the change in enthalpy when one mole of a substance is completely burnt in excess of air.

**Final Answer:** The change in enthalpy of a substance when one mole of the substance is completely burnt in excess of air is called **Heat of Combustion**.