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Originally Posted by MRiGnS  Well, I did not mention it on purpose in my example, because you could argue that it contains an O[2] molecule whereas methane CH[4] and nitrogen N[2] doesn't. |
You're forgetting one of the basic foundation principles of chemistry: elements and compounds have different chemical properties. Oxygen and carbon dioxide are simply not the same.
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Originally Posted by MRiGnS It might be possible that in some kind other evolved plants and animals could use methane and chlorine instead of carbon dioxide and oxygen for exothermic chemical reactions. |
I would be highly surprised to see a methane-chlorine reaction. 1) There are very few situations in which both reactants could be properly synthesised and isolated to react, and 2) the reaction in a lab setting is remarkably violent.