Why doesn't CH4+2N2+7O2->4HNO3+CO2+1428 kJ/mol happen?

[Pds314]

I was trying to determine the most efficient way to power an aspirated engine of some sort in titan's 95% N, 5% CH4 lower atmosphere, and I stumbled into what seems to be a way to make methane burn quite violently (more so per mass than anything else I can think of besides H2 reactions). HNO3, Nitric acid, is the primary burn product, as the reaction produces 4 of them for each methane burned.

CH4+2N2+7O2->4HNO3+CO2+1428 kJ/mol seems to be one possible result that would work well in OUR atmosphere (it is awful on titan. You have to bring 7 O2 for every CH4 and two N2 molecules you want to burn, making it about half as efficient as just bringing O2 and burning it with the methane itself.)

My question is: if Methane can output 1428 kJ per mole, or about 90 kJ per gram... Why doesn't igniting it produce a deadly cloud of HNO3?

Can it be forced into happening by varying temperature, pressure and stoichiometry?

Is there an even more energetic reaction that happens instead given the same reactants in the same ratios? E.G. Burning the methane with the oxygen normally still only produces 808 kJ or so and leaves us 2N2+5O2. What can we do with that?

Did I make an error in my calculations?

4*413+941+7*495-4*607-4*463-8*201-2*799 = -1428 (potential energy, more negative is more boom).

Is it just not happening because of some absurd activation energy or requirement for a two-stage burn?

EDIT: I found the error. I forgot to count both N2s.

[nhnifong]

I'm not a chemist, but I would assume that it has a high activation energy because of the relative stability of N2.

[Raven.]

Methane, along with every other Alkane, will always combust into CO2 gas and water. Anything else that results is a byproduct of something else mixed in with the methane, not the methane itself.

Here:

CH4 + 2O2 -> CO2 + 2H2O

The reason is does not produce a deadly cloud of HNO3 is because CO2 and water are being produced instead. In addition, HNO3 is an ionic compound, methane is not. HNO3 needs to be formed from two other ionic compounds in a solution such as an aqueous (in water) solution, so your equation is impossible. The nitrogen in your equation will also never react.

Source: Currently enrolled in engineering chemistry; we're required to know the combustion of the first ten alkanes, which includes methane.

Edited February 12, 2014 by Raven.

[spudman2]

EDIT: I found the error. I forgot to count both N2s.

Umm, Nitrogen gas has an energy of formation of 0 kJ/mol. Crunching your numbers now...

EDIt: Okay, here we go... All of this is in kJ/mol, btw

(HNO₃) 4 x 207 + (CO₂) 393.5 = 1221.5 kJ/mol for the output products

(CH₄) 74.9 + (O₂) 7 x 0 + (N₂) 2 x 0 = 74.9 kJ for the input reactants

1221.5 - 74.9 = 1146.6 kJ/mol

Somebody check this for me, but I'm fairly certain that it's right... And Raven, the methane would be more likely to burn with the oxygen, but I see no reason why there can't be a non-water containing reaction that produces Nitric Acid...

Edited February 12, 2014 by spudman2

[Raven.]

Umm, Nitrogen gas has an energy of formation of 0 kJ/mol. Crunching your numbers now...

EDIt: Okay, here we go... All of this is in kJ/mol, btw

(HNO₃) 4 x 207 + (CO₂) 393.5 = 1221.5 kJ/mol for the output products

(CH₄) 74.9 + (O₂) 7 x 0 + (N₂) 2 x 0 = 74.9 kJ for the input reactants

1221.5 - 74.9 = 1146.6 kJ/mol

Somebody check this for me, but I'm fairly certain that it's right... And Raven, the methane would be more likely to burn with the oxygen, but I see no reason why there can't be a non-water containing reaction that produces Nitric Acid...

If methane does react with nitrogen, then it won't be a combustion reaction.

Now a much heavier Alkane, or the right mixture, might be a different story. And yes, there are ways to produce nitric acid outside of water, one way is from nitric oxide. In his case, if you want to get nitric acid, then the nitrogen has to react with the oxygen first in order to form nitric oxide, which will then react with oxygen to form nitric acid. But that's two reactions, not one. The reason I mentioned water is because water forces the ions to disassociate, which makes forming ionic compounds very easy.

[Kialar]

Splitting N2 will need roughly 1500 kJ/mol to overcome the binding enthalpy. That's why nitrogen is used as inert gas.

Spoken without even trying to be scientifical correct: the activation energy for the nitrogen triple binding is to high. The OP's formular wouldn't work. One of the reasons is this.

Even chemical reactions (combustions, redox reactions, ionic formations and so on) are quite human: the chose the most easy path.

[spudman2]

Even chemical reactions (combustions, redox reactions, ionic formations and so on) are quite human: the chose the most easy path.

Or are humans like chemical reactions? Or is this an overarching rule behind the universe?

[lajoswinkler]

I was trying to determine the most efficient way to power an aspirated engine of some sort in titan's 95% N, 5% CH4 lower atmosphere, and I stumbled into what seems to be a way to make methane burn quite violently (more so per mass than anything else I can think of besides H2 reactions). HNO3, Nitric acid, is the primary burn product, as the reaction produces 4 of them for each methane burned.

CH4+2N2+7O2->4HNO3+CO2+1428 kJ/mol seems to be one possible result that would work well in OUR atmosphere (it is awful on titan. You have to bring 7 O2 for every CH4 and two N2 molecules you want to burn, making it about half as efficient as just bringing O2 and burning it with the methane itself.)

My question is: if Methane can output 1428 kJ per mole, or about 90 kJ per gram... Why doesn't igniting it produce a deadly cloud of HNO3?

Can it be forced into happening by varying temperature, pressure and stoichiometry?

Is there an even more energetic reaction that happens instead given the same reactants in the same ratios? E.G. Burning the methane with the oxygen normally still only produces 808 kJ or so and leaves us 2N2+5O2. What can we do with that?

Did I make an error in my calculations?

4*413+941+7*495-4*607-4*463-8*201-2*799 = -1428 (potential energy, more negative is more boom).

Is it just not happening because of some absurd activation energy or requirement for a two-stage burn?

EDIT: I found the error. I forgot to count both N2s.

Whether a reaction will occur or not, depends on far more factors than just activation energy. It's fairly complicated and I really don't feel like explaining a whole semester of chemical kinetics on a forum.

But think about what should happen. Just think about the actual event. What are the odds of all three particles (O2, N2 and CH4) smashing together at the same time, at the right angles and positions? Very, very small.

These reactions, even if they're energetically favorable, will proceed with at least two steps, where one step is the slowest and determines the overall speed.

What will happen if you put a short spark through a mixture of oxygen, nitrogen and methane? If the ratios are favorable, methane and oxygen will combust in a detonation. Nitrogen will do nothing. You'll be left with CO2, water (steam) and nitrogen. They will not react any futher at room temperature.

In addition, HNO3 is an ionic compound, methane is not.

Nitric acid is not an ionic compound. It's molecular compound which is polar. HNO3 is a neutral molecule with covalent bonds between atoms and dipole-dipole forces between the molecules.

Calcium oxide is an ionic compound because it's made of ions.

Or are humans like chemical reactions? Or is this an overarching rule behind the universe?

Every reaction in our body, from those in our guts to the ones in our brain, determining our sense of "myself", they're all following that rule. Increasing entropy, and following the path of the least resistance. That's how things work.

[Pds314]

Whether a reaction will occur or not, depends on far more factors than just activation energy. It's fairly complicated and I really don't feel like explaining a whole semester of chemical kinetics on a forum.

But think about what should happen. Just think about the actual event. What are the odds of all three particles (O2, N2 and CH4) smashing together at the same time, at the right angles and positions? Very, very small.

These reactions, even if they're energetically favorable, will proceed with at least two steps, where one step is the slowest and determines the overall speed.

What will happen if you put a short spark through a mixture of oxygen, nitrogen and methane? If the ratios are favorable, methane and oxygen will combust in a detonation. Nitrogen will do nothing. You'll be left with CO2, water (steam) and nitrogen. They will not react any futher at room temperature.

Nitric acid is not an ionic compound. It's molecular compound which is polar. HNO3 is a neutral molecule with covalent bonds between atoms and dipole-dipole forces between the molecules.

Calcium oxide is an ionic compound because it's made of ions.

Every reaction in our body, from those in our guts to the ones in our brain, determining our sense of "myself", they're all following that rule. Increasing entropy, and following the path of the least resistance. That's how things work.

Indeed. But even if it isn't explosive, if the reaction were as I originally described it, burn products should overwhelmingly be attracted to do this.

The real problem is that it is about 3 times less efficient than this because I made a symbolic error.