First Law

[JamesAntonios 1E]

So delta U is 0 when it is an isothermal, reversible expansion. However, what if the expansion is irreversible? Is it still zero? Also, when is q in the first law zero?

[Chem_Mod]

Delta U = 0 for an isothermal process, because internal energy of an ideal gas only depends on the temperature. It doesn't have to be reversible for this to hold true.
Q is zero when we don't supply (or lose) any heat in the process we study.

[Sophie 1I]

q is zero when there is no heat lost or gained. In the isothermal system q is equal to -w

[Jessica Benitez 1K]

When we have an adiabatic system q=0 and delta U=w because in an adiabatic system there is no heat flow.

[Jaewoo Jo 2L]

delta U would also be zero in an isolated system or any system that doesn't transfer heat.

[AlyssaPeckham1A]

If no heat is transferred, then q will be equal to zero. This also means that delta U will now be equal to work.