If one mole of ammonia and one mole of hydrogen chloride are mixed in a closed container to form ammonium chloride gas, then

• A

  $\Delta \mathrm{H}>\Delta \mathrm{U}$
• B

  $\Delta \mathrm{H}=\Delta \mathrm{U}$
• Correct answer
  C

  $\Delta \mathrm{H} < \Delta \mathrm{U}$
• D

  there is no relationship

Solution

$$ \begin{aligned} &\Delta \mathrm{H}=\Delta \mathrm{U}+\mathrm{n}_{\mathrm{g}} \mathrm{RT}\\ &\mathrm{n}_{\mathrm{g}}=\text { Number of moles of product }\\ &\text { - number of moles of reactant }\\ &=1-2=-1\\ &\Delta \mathrm{H}=\Delta \mathrm{U}-\mathrm{RT}\\ &\therefore \quad \Delta \mathrm{H} < \Delta \mathrm{U} \end{aligned} $$

Hence, the correct answer is option: 'C'.