Concepts about Internal Energy and Entropy

[Yoon Lee 2A]

Hello!

I was hoping someone could clarify what we learned in lecture today about how internal energy has to be zero in order to figure out what makes a reaction favorable (entropy or enthalpy) and so that q equals -w. Why does internal energy have to be zero?

And I am also a bit confused about what makes a reaction reversible while others are not. Are all isothermic reactions reversible, or anything with a really slow change in temperature/pressure/volume?

Thank you in advanced!

[Chem_Mod]

Not sure about internal energy being zero having to do with how favorable it is, but any isothermal process has U=0 and therefore q=-w. How favorable a reaction is is determined by its total effect on entropy, or equivalently, its deltaG.

Any of the processes: isochoric isobaric isothermal or adiabatic, can each be irreversible or reversible just depending on whether or not it is slow and in equilibrium along the path.