When to use internal energy vs enthalpy equations?

[ipocoyo]

What is the difference between Q=m(u2-u1) + W & Q=m(h2-h1)?

Basically I am trying to figure out 2 different sets of questions and apparently using these separate equations yield different answers, and I don't know which equation to use. From my understanding, both of them are used in closed systems and also when there is a constant pressure but a change in volume? So when exactly do I use which one?

 

[Chestermiller]

What is the difference between Q=m(u2-u1) + W & Q=m(h2-h1)?

Basically I am trying to figure out 2 different sets of questions and apparently using these separate equations yield different answers, and I don't know which equation to use. From my understanding, both of them are used in closed systems and also when there is a constant pressure but a change in volume? So when exactly do I use which one?

The first equation is always true. The second equation applies when the pressure is constant during the process, and equal to the initial pressure of the gas.