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`