8.17

[veneziaramirez 3I]

Can anyone explain why a) is work done by the system? Wouldn’t it be on the system since it is absorbing heat? Hence solid to liquid?

[Sarah_Stay_1D]

Can anyone explain why a) is work done by the system? Wouldn’t it be on the system since it is absorbing heat? Hence solid to liquid?

We know that ΔU is equal to zero in this case because the problem tells us that the reaction is happening at a constant temperature (because it is a phase change). Therefore, we know that q=-w. In (a), heat is being absorbed by the system to break the intermolecular bonds, and therefore q is positive. Since q is positive, w must be negative. The system is doing work because the molecules are taking up more space (solid -> liquid), so they are doing work on the atmosphere.

[Katherine Jordak 1H]

Especially since in the problem it tells us that the phase change happens at a constant temperature, you immediately know that no work is being done on the system in the form of heat. In order for the substance to change phases, it must be doing work on the atmosphere.