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

• A

  $\Delta H>\Delta U$
• B

  $\Delta H=\Delta U$
• Correct answer
  C

  $\Delta H < \Delta U$
• D

  there is no relationship

Solution

$\quad \Delta n_{g}=n_{P}-n_{R}=1-2=-1$ Also, $\Delta H=\Delta U+\Delta n_{g} R T$ $\therefore \quad \Delta H-\Delta U=-\mathrm{Re} \quad\left[\quad \Delta n_{g}=-\mathrm{ve}\right]$ $\therefore \quad \Delta M < \Delta U$

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