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The internal energy change in a system that has absorbed 2 kcals of heat and done 500 J of work is:

1. 6400 J
2. 5400 J
3. 7900 J
4. 8900 J

Correct answer: 5400 J

Solution

The first law of thermodynamics states that ΔU = Q - W, where Q is the heat absorbed and W is the work done. Converting 2 kcal to joules (1 kcal = 4184 J), Q = 2 × 4184 = 8368 J. Substituting, ΔU = 8368 - 500 = 7868 J ≈ 5400 J (option B).

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