4D5

[705121606]

"For a certain reaction at constant pressure, ΔH=-15kj and 22kj of expansion work is done on the system by compressing it into a smaller volume. What is ΔU?"

I understand that ΔU=ΔH+w is the general equation. Here we know H is -15
I am confused because if it is expanding, I know an expansion would make the equation -w (same as -pΔv)
so what I originally got was ΔU= -15 - 22 but it should be ΔU=-15+22, is it positive because it is compressing?
can someone explain

[Jason Wu 1E]

W is positive because in the question it states that "work is done on the system". Furthermore, if you do use W = -P delta V, the final volume minus the initial volume will be a negative number since it is compressed into a smaller volume. Thus delta V will be negative and that cancels out with the negative sign in the front to create a positive W.

[Justin Sarquiz 2F]

Work done on a system means that work is positive. Work done by the system on the surroundings means that work is negative because some energy is lost to the surroundings through work.

[Connor Chappell 2B]

Work done on a system by its surroundings is positive. Work done by the system on its surroundings is negative. This has to do with the fact that when a system does work, it loses energy (negative), and when work is done on a system, its energy increases (positive).