Question

Nitric acid can be formed in two steps from the atmospheric gases nitrogen and oxygen, plus hydrogen prepared by reforming natural gas. In the first step, nitrogen and hydrogen react to form ammonia: N2 (g) + 3H2 (g) â 2NH3 (g) =ÎHâ92.kJ In the second step, ammonia and oxygen react to form nitric acid and water:

NH3 (g) + 2O2 (g) â HNO3 (g) + H2O (g) =ÎHâ330.kJ

Required:
Calculate the net change in enthalpy for the formation of one mole of nitric acid from nitrogen, hydrogen and oxygen from these reactions.

Answer 1

Answer:

-376 kJ

Explanation:

The first step equation:

$\mathsf{N_{2(g)} + 3H_2{(g)} \to 2NH_3{(g)} \ \ \ \Delta H = -92\ kJ}$    ---- (1)

The second step equation:

$\mathsf{NH_{3(g)} + 2O_2{(g)} \to HNO_3{(g)} +H_2O_{(g)} \ \ \ \Delta H = -330\ kJ}$      ---- (2)

To determine the enthalpy of formation for 1 mole of HNO₃ (nitric acid), we have the following.

From the above equations; let multiply equation (1) by 1 and equation (2) by 2.

$\mathsf{N_{2(g)} + 3H_2{(g)} \to 2NH_3{(g)} \ \ \ \Delta H = -92\ kJ}$     ---- (3)

$\mathsf{2NH_{3(g)} + 4O_2{(g)} \to 2HNO_3{(g)} +2H_2O_{(g)} \ \ \ \Delta H = 2(-330)\ kJ}$      ----- (4)

adding the above two equations, we have:

$\mathsf{N_{2(g)} + 3H_2{(g)}+ 2NH_{3(g)} + 4O_{2(g)} \to 2HNO_{3(g)} + 2NH_3{(g)} +2H_2O_{(g)} \ \ \ \Delta H = (-660 \ kJ -92\ kJ)}$$\mathsf{N_{2(g)} + 3H_2{(g)} + 4O_{2(g)} \to 2HNO_{3(g)} +2H_2O_{(g)} \ \ \ \Delta H = (-752 \ kJ)}$

Now, from the recent equation, we have:

2 moles of nitric acid = -752 kJ

∴

1 mole of nitric acid will be: = (1 mole × (-752 kJ)) ÷ 2 moles

1 mole of nitric acid will be: = -376 kJ