Which of the following defines the first law of thermodynamics?

• A. The internal energy of a system is always constant.
• B. Energy cannot be created or destroyed, only transformed from one form to another.
• C. The entropy of the universe always increases in a spontaneous process.
• D. The enthalpy change of a reaction is equal to the heat absorbed at constant pressure.

Answer: (B)

Verified step by step guidance

1. Understand that the first law of thermodynamics is a statement about the conservation of energy in a system.
2. Recall that the first law states that energy cannot be created or destroyed, only transformed or transferred between the system and its surroundings.
3. Recognize that the internal energy of a system can change, so it is not always constant, which rules out the first option.
4. Note that the entropy of the universe increasing relates to the second law of thermodynamics, not the first law.
5. Understand that enthalpy change at constant pressure relates to heat transfer but is a specific case, not the general statement of the first law.