H2o and o2 do not react at room temperature
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The activation energy barrier to the reaction is indeed very large. The Arrhenius equation allows the rate constant, k, for a reaction to be determined:
k = Ae^(-Ea/RT)
where A = Arrhenius pre-exponential factor (depends on the system in question)
Ea = activation energy barrier in J/mol
R = 8.314 J/(K·mol)
T = absolute temperature in K
If Ea is sufficient large, k can be so close to zero that the reaction essentially will not occur at the temperature in question.
If you look at the situation in terms of collision theory, the collision of H2 and O2 needs to have enough energy to overcome the Ea barrier, and if the probability of this is sufficiently small then the reaction will essentially not occur. It's the same idea with spontaneous human combustion... the thermodynamics (energies) favour it occurring, but the kinetics make it essentially impossible to occur at room temperature. So, it is not so much that the reaction CANNOT occur, it is that the chances of it occurring are so small that, for all practical purposes, it DOES NOT occur.
Now, the question becomes why can you set of the explosion with only a spark... well, the spark gives enough energy to a small group of molecules for them to react, and the reaction releases alot of energy, which in turn allows other nearby collisions to have enough energy to overcome the Ea barrier, and so on.
Best Answer
The activation energy barrier to the reaction is indeed very large. The Arrhenius equation allows the rate constant, k, for a reaction to be determined:
k = Ae^(-Ea/RT)
where A = Arrhenius pre-exponential factor (depends on the system in question)
Ea = activation energy barrier in J/mol
R = 8.314 J/(K·mol)
T = absolute temperature in K
If Ea is sufficient large, k can be so close to zero that the reaction essentially will not occur at the temperature in question.
If you look at the situation in terms of collision theory, the collision of H2 and O2 needs to have enough energy to overcome the Ea barrier, and if the probability of this is sufficiently small then the reaction will essentially not occur. It's the same idea with spontaneous human combustion... the thermodynamics (energies) favour it occurring, but the kinetics make it essentially impossible to occur at room temperature. So, it is not so much that the reaction CANNOT occur, it is that the chances of it occurring are so small that, for all practical purposes, it DOES NOT occur.
Now, the question becomes why can you set of the explosion with only a spark... well, the spark gives enough energy to a small group of molecules for them to react, and the reaction releases alot of energy, which in turn allows other nearby collisions to have enough energy to overcome the Ea barrier, and so on.
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