For which of the following elements, the standard enthalpy is not zero?

Options

A. C(Diamond)
B. C (Graphite)
C. Liquid mercury
D. Rhombic sulphur

Answer

To determine which of the following elements has a standard enthalpy that is not zero, we need to understand the concept of standard enthalpy of formation. The standard enthalpy of formation is defined as the change in enthalpy when one mole of a substance is formed from its pure elements in their standard states.

### Step-by-Step Solution:

1. **Understand Standard Enthalpy of Formation**:
- The standard enthalpy of formation for any element in its most stable form is defined to be zero. This is a reference point used in thermodynamics.

2. **Identify the Elements**:
- In the question, we are given different forms of carbon: graphite and diamond.

3. **Determine the Most Stable Form**:
- For carbon, the most stable form at standard conditions (1 atm pressure and 25°C) is graphite.

4. **Standard Enthalpy of Graphite**:
- Since graphite is the most stable form of carbon, its standard enthalpy of formation is zero.

5. **Standard Enthalpy of Diamond**:
- Diamond, while also a form of carbon, is not the most stable form. Therefore, its standard enthalpy of formation is not zero.

6. **Conclusion**:
- Among the options provided, diamond (the form of carbon that is not stable) has a standard enthalpy that is not zero. Thus, the answer is diamond.

### Final Answer:
The element for which the standard enthalpy is not zero is **diamond (carbon)**.

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