2011-07-11, 09:00 AM
As people have said above, there is almost no hydrogen on the air.
Also, sometimes a reaction is very exothermic and has negative gibbs energy (and thus very favorable thermodynamically), but the kinetics of the reaction are so slow that it never actually happens.
For example, diamond at 1 atm and 25°C should spontaneously become coal, since coal is more stable in that condition. However, the kinetics of the reaction are so slow that diamond lasts pretty much forever without changing.
Also, sometimes a reaction is very exothermic and has negative gibbs energy (and thus very favorable thermodynamically), but the kinetics of the reaction are so slow that it never actually happens.
For example, diamond at 1 atm and 25°C should spontaneously become coal, since coal is more stable in that condition. However, the kinetics of the reaction are so slow that diamond lasts pretty much forever without changing.