Question

Under what conditions does q, the heat evolved or absorbed by the system in a physical or chemical process, equal the change in enthalpy of the system? 1. When q ≥ 0. 2. When w = 0. 3. When w = −PΔV.

Answer 1

Answer: When $w=-P\Delta V$

Explanation:

According to first law of thermodynamics, energy can neither be created nor be destroyed. It can only be transformed from one form to another.

$\Delta H=\Delta E+P\Delta V$

$\Delta E=q+w$

$\Delta H$ = change in enthalpy

P = pressure

$\Delta V$ = change in volume

$\Delta E$=Change in internal energy

q = heat absorbed or released

w = work done on or by the system

$\Delta H=q+w+P\Delta V$

Thus for $\Delta E=q+w$ , $w=-P\Delta V$

The heat evolved or absorbed by the system in a physical or chemical process, equal the change in enthalpy of the system when $w=-P\Delta V$