If a system loses 250 kJ of heat at the same time that it is doing 500 kJ of work, what is the change in the internal energy of the system?
Correct answer: D. -750 kJ
- A. +250 kJ
- B. -250 kJ
- C. +750 kJ
- D. -750 kJ
Explanation
According to the first law of thermodynamics, the change in internal energy (ΔU) is equal to the heat added to the system (q) minus the work done by the system (w), expressed as ΔU = q - w. In this case, the system loses 250 kJ of heat, so q = -250 kJ, and the system does 500 kJ of work, so w = 500 kJ. Therefore, ΔU = (-250 kJ) - (500 kJ) = -750 kJ. The internal energy decreases by 750 kJ. The other options incorrectly interpret the signs or the contributions of heat and work.
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About Thermochemistry and Energetics of Chemical Reactions
Thermochemistry measures energy changes in chemical reactions and distinguishes exothermic from endothermic processes. Work covers systems, surroundings and state functions, internal energy, the first law of thermodynamics, enthalpy and Hess's law, including the sign conventions used when heat enters or leaves a system.
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