Question

Enthalpy is an extensive property. In general, if enthalpy of an overall reaction A → B along one route is $\triangle _f H$ and $\triangle _f H_{1}, \triangle _f H_{2}, \triangle _f H_{3}...$ which represents enthalpies of intermediate reactions leading to product B. What will be the relation between $\triangle _f H$ for overall reaction and $\triangle _f H_{1}, \triangle _f H_{2}...$ etc. for intermediate reactions.

Answer 1

“Enthalpy”:

It is defined as the sum of “internal energy” and product of “pressure and volume”.

$H\quad =\quad U\quad +\quad PV$

Relation between internal energy and enthalpy, $\Delta H\quad =\quad \Delta U\quad +\quad \Delta PV$

$\Delta n\quad =\quad Number\quad of\quad gaseous\quad products\quad -\quad Number\quad of\quad gaseous\quad reactants$

$If\quad \Delta n\quad =\quad +ve,\quad \Delta H\quad >\quad \Delta U$

$If\quad \Delta n\quad =\quad -ve,\quad \Delta H\quad <\quad \Delta U$

$If\quad \Delta n\quad =\quad 0,\quad \Delta H\quad =\quad \Delta U$

From the given data,

$A\rightarrow B$

${ \Delta }_{ r }H, { \Delta }_{ r }{ H }_{ 2 },{ \Delta }_{ r }{ H }_{ 3 }$ are different enthalpies.

According to the Hess law,

${ \Delta }_{ r }H\quad =\quad { \Delta }_{ r }H\quad +\quad { \Delta }_{ r }{ H }_{ 2 }\quad +{ \Delta }_{ r }{ H }_{ 3 }$