Question
If $$\Delta H$$ is the change in enthalpy and $$\Delta E,$$ the change in internal energy accompaning a gaseous reaction, then
A.
$$\Delta H$$ is always greater than $$\Delta E$$
B.
$$\Delta H < \Delta E$$ only if the number of moles of products is greater than the number of moles of the reactants
C.
$$\Delta H$$ is always less than $$\Delta E$$
D.
$$\Delta H < \Delta E$$ only if the number of moles of products is less than the number of moles of the reactants
Answer :
$$\Delta H < \Delta E$$ only if the number of moles of products is less than the number of moles of the reactants
Solution :
Reactions in which there is a decrease in the number of moles of the gaseous components, i.e. $$\Delta {n_g}$$ is negative, the enthalpy change $$\left( {\Delta H} \right)$$ is lesser than the internal energy change $$\left( {\Delta E} \right).$$
Reaction in which there is a increase in the number of moles of gaseous components i.e. $$\Delta ng$$ is positive, the enthalpy change is greater than the internal energy change.
$$\Delta H = \Delta E + \Delta {n_g}RT$$