Hess's Law問題

Hess's Law 同combustion / formation 冇咩關係

hess's law 係講由反應物變做生成物既enthalpy 與路徑無關
具體d姐係:

圖片參考：http://imgcld.yimg.com/8/n/HA00184332/o/701011130175113873410760.jpg

H1 = H2+H3

而點樣寫番條equation呢....
就要知道個enthalpy個定義先
好似enthalpy of combustion
就係: 一犘爾物質在氧中完全燃燒時的焓變
The enthalpy change when one mole of the substance is completely burnt in oxygen.

用CH3CH3CH2OH 做例子 先寫左條反應式
CH3CH3CH2OH(l) + O2(g) ---> CO2(g) + H2O(l)
之後平衡條equation
2 CH3CH3CH2OH(l) + 19 O2(g) ----> 6CO2(g) + 9H2O(l)

但係個定義係1mole substance
所以要寫成
CH3CH3CH2OH(l) + 19/2 O2(g) ----> 3 CO2(g) + 9/2 H2O(l)

而enthalpy of formation 既定義係
由組成某物質的成分元素(在其標準態下)生成一犘爾該物質時的焓變
The enthalpy change when one mole of the substance is formed from its constituent elements in their standard state. 同enthalpy change of combustion 一樣 寫左條equation 之後平衡
再約成substance 係1mole 3C(s) + 9/2 H2(g) + 1/2 O2(g) ---> CH3CH3CH2OH(l)