Question

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

• A. $\Delta H$ is always greater than $\Delta U$
• B. $\Delta H$ is always less than $\Delta U$
• C. $\Delta H>\Delta U$ only if the number of mole of the products is less than that of the reactants.
• D. $\Delta U<\Delta H$ only if the number of mole of the reactants is less than that of the products.

Answer 1

Answer: (C), (D)

The correct options are
C $\Delta U<\Delta H$ only if the number of mole of the reactants is less than that of the products.
D $\Delta H>\Delta U$ only if the number of mole of the products is less than that of the reactants.

As we know,
$\Delta H=\Delta U+\Delta nRT$
$\Delta n=n_P-n_R$
so
$\Delta U<\Delta H$ only if the number of mole of the reactants is less than that of the products.
$\Delta H>\Delta U$ only if the number of mole of the products is less than that of the reactants.