Question

For gaseous reactions, if $\Delta H$ is the change in enthalpy & $\Delta U$ that in internal energy, then

• A. is always greater than
• B. is always less than
• C. only if the number of mole of product is less than that of reactants
• D. only if the number of mole of the reactants is less than that of the products.

Answer 1

The correct option is C

only if the number of mole of product is less than that of reactants

$\Delta H=\Delta U+\Delta ngRT$

If number of moles of product $<$ number of moles of reactant

$\Delta ng<0$

$\therefore \Delta H=\Delta U-\Delta ngRT$

$\therefore U=\Delta H+\Delta ngRT$

$\therefore \Delta U>\Delta H$