[kensten32]

Okay so my understanding (could be wrong) is that CH4 when mixed with Oxygen will yield CO2 and H2O. What I would like to know is why? Why wouldnt the yield be CO2 + 2H2? Also why doesnt the H2O just react with another Oxygen atom in the air to form Hydrogen Peroxide? H2O2. Is it because the electron balance of H2O? or a Activation levels?

[curiouscat]

My guess is energetics. The alternatives will be thermochemically too uphill or would have a subsequent reaction possibility that is energetically very exothermic.

[Corribus]

Calculate the ΔG° for each of those reactions and see which one is most negative.

EDIT: But, thermodynamic favorability doesn't necessarily translate into a faster reaction.

[curiouscat]

EDIT: But, thermodynamic favorability doesn't necessarily translate into a faster reaction.

But the converse holds. i.e. thermodynamically unfavorable cannot be  a fast reaction. Right?

[Corribus]

Sure it can - a thermodynamically unfavorable reaction can reach equilibrium either quickly or slowly, just like a thermodynamically favorable reaction can.  The kinetics depend significantly on the reaction barrier, which doesn't necessarily have a relationship to the relative energies of the products and reactants.  Other kinetic factors (collision frequency) have nothing at all to do with the thermodynamics. 

In any case, of the three potential reactions mentioned by the OP, all of them have significantly negative ΔG°, although the typical combusion reaction is by far the most negative.  All of them are primarily enthalpically driven at room temperature - only the conversion to hydrogen gas is entropically favorable.  While the conversion to hydrogen peroxide actually has a pretty negative ΔG° (-584.4 kJ/mol), the stoichiometry requires 3 oxygen molecules and a methane for the mechanism (whatever it is) which probably has a low probability of occuring.  Compare to the water and hydrogen gas conversions which would have a much more straightforward mechanism and hence be more kinetically favored.  The conversion to hydrogen gas would seem to have the simplest mechanism, and may indeed have the fastest rate, but it is the least thermodynamically favored, which means that once it's all said and done, most of the conversion favors formation of water, even if it might be a kinetically slower reaction.  I would advise the OP to remember than these three alternatives are not happening independently.  They may all be happening in the combustion of methane (in fact, probably are, as well as others, such as formation of CO). What the primary product will be, and what the concentrations of each product is as a function of time, depends on both kinetics AND thermodynamics.

[curiouscat]

The kinetics depend significantly on the reaction barrier, which doesn't necessarily have a relationship to the relative energies of the products and reactants. 

It does (I think) : The (forward) barrier has to be at least as large as the energetic uphill jump of a thermodynamically uphill step.

That's why I said if the step is truly thermodynamically unfavorable it'd be hard for the forward reaction to be fast. (Unless the pre-exponential is so large that it compensates but I think that'd be unlikely for a truly high activation barrier, say 3 eV)

Am I making a blunder?

[Corribus]

In a specific case like you mention, yes that's true, but I don't think it's possible to really generalize over all possible scenarios.  Very thermodynamically favorable reactions can have enormous reaction barriers - the combustion reaction mentioned here is a fine example. Mix methane and oxygen together and you get.... nothing.  Equilibrium takes virtually forever to reach without giving the reaction a kick (a spark, say).  On the other hand, thermodynamically unfavorable reactions can have pretty fast rates.  If you wait an infinitely long amount of time, the thermodynamically favorable reaction will dominate the ultimate products if these two processes are in competition with each other.  But this of course assumes a closed, idealized system where nothing can react with the products of the thermodynamically unfavorable but kinetically slow reaction. Which the atmosphere decidedly isn't.

[curiouscat]

In a specific case like you mention, yes that's true, but I don't think it's possible to really generalize over all possible scenarios. 

The generalization I always use is this (and I'd love to see an exception):

A thermodynamically favorable reaction may or may not be feasible due to kinetic barriers. But OTOH if your thermodynamics itself is unfavorable there is very little kinetics can  do to rescue you.

Point being, thermodynamics & kinetics are not somewhat independent constraints like is often taught. Thermodynamics places a lower bound on how low a barrier can be for an uphill step.

[gianegizelle]

The kinetics depend on the reaction barrier in which it doesn't necessarily have a relationship to the relative energies of the products and reactants some other kinetic factors the collision frequency have nothing at all to do with the thermodynamics.

[Corribus]

Point being, thermodynamics & kinetics are not somewhat independent constraints like is often taught. Thermodynamics places a lower bound on how low a barrier can be for an uphill step.

This is true, but again the rate depends on more than just the activation barrier.  Even if you neglect that, just because an endergonic reaction has a lower limit to what its reaction barrier is, this doesn't mean that any given endergonic reaction is going to have a slower rate than any given exergonic reaction. 

I understand your point, but I'm not sure it's a useful generalization.  The number of reactions with trivial energy difference between the transition state and either the reactants/products is fairly small, I would think.

[curiouscat]

I understand your point, but I'm not sure it's a useful generalization.  The number of reactions with trivial energy difference between the transition state and either the reactants/products is fairly small, I would think.

Here's one (perhaps trivial) situation where I've found it useful: In comparing relative contributions of competing pathways on a reaction network, highly endergonic pathways can be rejected at the outset. Without even bothering to calculate the kinetics.