Question

What is the difference between $\Delta$H and $\Delta$U? What is the sign of $\Delta$H for exothermic and endothermic reactions? Under what circumstances $\Delta$H = $\Delta$U?

Answer 1

$\Delta H=\Delta U+P\Delta V\therefore \Delta H-\Delta U=P\Delta V$

For exothermic reaction, $\Delta H<0$

For endothermic reaction, $\Delta H>0$

For solids and liquids, the volume change is negligible, i.e., $\Delta V=0$.

$\therefore \Delta H=\Delta U$