首先, 這涉及到 Hess's Law:
The enthalpy change of a chemical reaction is not affected by the path it takes. (By the conservation of energy)
然後再將已知的三個 reaction 記錄如下:
4NH3(g) + 3 O2(g) → 2N2(g) + 6 H2O(l) △H1
N2O(g) + H2(g) → N2(g) + H2O(l) △H2
H2(g) + (1/2)O2(g) → H2O(l) △H3
最後劃出以下的energy cycle (Born-Haber cycle):
圖片參考:
http://i117.photobucket.com/albums/o61/billy_hywung/Chem/Hesslaw.jpg
【此圖乃本人自製圖片,未經本人同意勿擅自連結或使用】
在上圖中藍色的為本題所需求出的能量轉變. 根據 Hess's law, 順時針方向 (clockwise direction) 的 enthalpy change 應和逆時針方向 (anti-clockwise) 的 enthalpy change 相同, 所以:
△H = (1/2) × △H1 + 3 × △H2 + 3× △H3
= (1/2) × (-1536) + 3 × (-367.4) + 3 × (-285.9)
= -2727.9 kJ