According to the First Law of Thermodynamics (ΔU = q + w), what is the internal energy change (ΔU) of a system that absorbs 500 J of heat from its surroundings while performing 200 J of expansion work on the surroundings?
Answer and explanation
Correct answer: C. +300 J
- Heat absorbed by the system is positive: .
- Work done by the system on the surroundings is negative: .
- Work done by the system on the surroundings is negative: .
- Step 1: Apply standard IUPAC sign conventions:
- Step 2: Apply the First Law of Thermodynamics: .
- Step 3: Substitute values: .
Answer
+300 J
Why not the others: '+700 J' incorrectly treats work done by the system as positive (); '-300 J' inverts the sign of heat absorption; '-700 J' treats both terms as negative losses from the system.
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