Which statement is not correct about internal energy?

Options

A. Internal energy is the sum of kinetic energy and potential energy of the particles making up the system.
B. Internal energy of the system can be measured.
C. Internal energy is a state function.
D. `DeltaU` is the heat of reaction at constant volume.

Answer

To determine which statement is not correct about internal energy, we need to analyze the provided statements one by one.

### Step-by-Step Solution:

1. **Understanding Internal Energy**:
- Internal energy (U) is defined as the total energy contained within a system. It includes various forms of energy at the molecular level.

2. **Components of Internal Energy**:
- Internal energy is typically considered to be the sum of:
- Kinetic energy (due to the motion of particles)
- Potential energy (due to the position of particles)
- Other forms of energy such as rotational energy and vibrational energy.

3. **Evaluating the Statements**:
- **Statement 1**: "Internal energy is the sum of kinetic energy and potential energy of the particles making up the system."
- This statement is partially correct but incomplete, as it does not consider other forms of energy.

- **Statement 2**: "Internal energy is the sum of kinetic energy plus potential energy plus rotational energy plus internal energy."
- This statement is incorrect because it redundantly includes "internal energy" in its own definition. Internal energy cannot be defined as a sum that includes itself.

- **Statement 3**: "Internal energy of the system can be measured."
- This statement is generally correct. While internal energy itself cannot be measured directly, changes in internal energy can be measured through work and heat.

4. **Conclusion**:
- The statement that is not correct about internal energy is **Statement 2**, as it incorrectly defines internal energy by including itself in the definition.

### Final Answer:
The statement that is not correct about internal energy is: "Internal energy is the sum of kinetic energy plus potential energy plus rotational energy plus internal energy."