According to the First Law of Thermodynamics, what happens to the internal energy of a system as the temperature increases at constant pressure?

• A. The internal energy first increases, then decreases.
• B. The internal energy remains constant.
• C. The internal energy increases.
• D. The internal energy decreases.

Answer: (C)

Verified step by step guidance

1. Recall the First Law of Thermodynamics, which states that the change in internal energy (\( \Delta U \)) of a system is equal to the heat added to the system (\( q \)) plus the work done on the system (\( w \)): \[ \Delta U = q + w \]
2. Understand that at constant pressure, the system can do work by expanding or contracting, so the heat exchanged is related to enthalpy change (\( \Delta H \)), but internal energy change depends on both heat and work.
3. Recognize that temperature is a measure of the average kinetic energy of particles, so as temperature increases, the internal energy, which includes kinetic and potential energy of molecules, generally increases.
4. Since the problem states constant pressure, the system may do work on the surroundings, but the internal energy still increases because the temperature is rising, meaning the total energy inside the system is increasing.
5. Therefore, the internal energy of the system increases as the temperature increases at constant pressure.