Suppose you add 45 J of heat to a system, let it do 10. J of expansion work, and then return the system to its initial state by cooling and compression. Which statement is true for this process?

Question

Quizplus · Accepted Answer

The process is a closed cycle, meaning the system returns to its initial state, so the change in internal energy must be zero according to the first law of thermodynamics (ΔU = Q - W). The enthalpy change (ΔH) can be calculated as ΔH = ΔU + PΔV, where PΔV is the pressure-volume work done by the system. Since the system does 10 J of expansion work, and assuming no other work is done, then PΔV = -10 J. Therefore, ΔH = ΔU - 10 J. Because ΔU is zero, ΔH = -10 J, which means the enthalpy of the system decreases when 45 J of heat is added and then the system is cooled and compressed. Choice A is incorrect because ΔH = ΔU + PΔV, meaning ΔH cannot be less than ΔU. Choice B is incorrect because ΔH was calculated to be -10 J, not 70 J. Choice C is incorrect because the work done in compressing the system does not necessarily have to equal the work done by the system in the expansion step.

Suppose You Add 45 J of Heat to a System