Question

Under which of the following condition is the relation $\Delta H=\Delta U+P\Delta V$ valid for a closed system at

• A. constant pressure
• B. constant temperature
• C. constant temperature and pressure
• D. constant temperature, pressure and composition

Answer 1

Answer: (A)

constant pressure

Chemical reactions are generally carried out at constant pressure (atmospheric pressure) so it has been

found useful to define a new state function Enthalpy(H) as:

$H=U+PV$ (By definition)

$\Delta H=\Delta U+\Delta \left(PV\right)$

$\Delta H=\Delta U+P\Delta V$ (at constant pressure) combining with first law.

$\Delta H=q_p$