如何分辨endothermic同exothermic reaction

Actually , i think the best method to distinguish them is by remembering!!
There are 6 cases that the reactions must be exothermic ( gives out heat):
(1) All compustion reaction
((including your example C + O2 ---> CO2 ))
(2) All precipitation reaction
(3) All displacement reaction
(4) All acid-alkali neutralization
(5) Addition of a little water to an ahydrous salt
(6) Reaction between calcium oxide and water
There are 2 cases that the reactions must be endothermic (takes in heat) :
(1) Cracking of oil fractions
(2) Thermal decomposition of calcium carbonate

Besides, usually near the reaction, there will be a symbol Delta H which represents heat change.
If the Delta H is positive, it is an endothermic reaction.
If the Delta H is negative, it is an exothermic reaction.

Hope this can help u!!

Combustion must be exothermic.

i think it is endothermic.
Combustion is a endothermic reaction.
Usually, there are several example for endo/exothermic reaction.
e.g Neutralization, Dilution ---> exo
Combustion --> endo
Usually, it is difficult to determine whether it is exo or endo reaction from the equation.
You should determine from experiment result.
[delta]H <0 exothermic.
[delta]H>0 endothermic.
Break Bond is a endothermic reaction, while form bond is a exothermic reaction. The H value depends on the overall process involve break bond and form bond.

2008-04-04 15:19:57 補充：
[delta]H大過0 is endothermic
[delta]H細過0 is exothermic