JEE Main & Advanced Chemistry Thermodynamics / रासायनिक उष्मागतिकी Enthalpy And Enthalpy Change

Heat content of a system at constant pressure is called enthalpy denoted by ‘H’.

From first law of thermodynamics, \[q=E+PV\]        ……….(i)

Heat change at constant pressure can be given as

\[\Delta q=\Delta E+P\Delta V\]                                                ……….(ii)

At constant pressure heat can be replaced at enthalpy.

\[\Delta H=\Delta E+P\Delta V\]                                                                ………(iii)

\[\therefore \,\Delta H=\] Heat change or heat of reaction (in chemical process) at constant pressure

\[\Delta E=\] Heat change or heat of reaction at constant volume.

In case of solids and liquids participating in a reaction,

\[\Delta H=\Delta E\,(P\Delta V\approx 0)\]

Difference between \[\Delta H\] and \[\Delta E\] is significant when gases are involved in chemical reaction.

\[\Delta H=\Delta E+P\Delta V\]

\[\Delta H=\Delta E+\Delta nRT\]

\[P\Delta V=\Delta nRT\]

Here, \[\Delta n=\] n_P – n_R