Question

For gaseous reaction, if $△$H is the change in enthalpy and $△$U that in internal energy then:

• A. $△H$ is always greater than $△U$
• B. $△H$ is always less than $△U$
• C. $△H<△U$ only if the number of moles of the products is less than that of the reactants
• D. $△U<△H$ only if the number of moles of the reactants is less than that of the products

Answer 1

Answer: (C)

$△H<△U$ only if the number of moles of the products is less than that of the reactants

$△H=△V+△n_{gas}RT$

where $△n_{gas}$= (moles of gases in produce)-(moles of gases in reactant )

$\therefore △H<△U$

if $△n_{gas}$ is

$\therefore$ moles of gas in product < moles of gas in reactant