Which of the following is/are correct for spontaneous isothermal chemical reaction?
(A) $\Delta H = 0$, because $\Delta T = 0$
(B) $\Delta S = 0$
(C) $\Delta U = 0$, because $\Delta T = 0$
(D) $\Delta G < 0$
My answer: $\mathrm{A, C, D}$
Reasoning:
$U$ is a state function and only depends on the initial and final states and their temperature. Since $\Delta T = 0$, $\Delta U = 0$
$\Delta H = \Delta U + \Delta(PV)$, but $PV = \text{constant}$, so $\Delta H = 0$
$\Delta G <0$ - the requirement for any reaction to be spontaneous.
But answer given is only option D.
I think the flaw in my $\Delta H = 0$ argument is that reactants can be solids and liquids as well and we cannot apply $PV = \text{constant}$ to them. Am I right?
But I still can't figure out why $\mathrm C$ is wrong.