Chemical energy limit?

[Fish 40]

I know there's a limit to energy production full stop in matter-antimatter pair annihilation, but what is the limit for just chemical energy?

 

If I have a box of one meter cubed, what's the maximum amount of energy extractable chemically by the laws of physics?

Edited November 1, 2011 by Fish 40

[mississippichem]

There is no theoretical limit on how exothermic a reaction can be. As far as percent energy that can be extracted to do work...you are limited by the Carnot efficiency.

 

The second law in a way limits the amount of energy you can harvest as work from a system. For a reversible process:

 

[math]dS=\frac{dq}{T}[/math]

 

[math] \oint \frac{1}{T}dq \leq 0 [/math]

 

If we play around with entropy and the other state-functions we can get:

 

[math]dG=Vdp-TdS [/math]

 

and therefore,

 

[math] dG=\frac{\partial G}{\partial p}dp + \frac{\partial G}{\partial S}dS [/math]

 

Gibbs energy (G) is a measure of spontaneity, and therefore a measure of the amount of energy available to do work. Here I've expressed the Gibbs energy as a function of pressure and entropy only, so one can see that the amount of energy we can extract is really only limited by how low we can get the pressure and how large we can make the change in entropy.

 

Of course practically we are limited by how unstable of a reagent we can have. The decomposition of [ce]CH_{5}^+[/ce] would in theory be hugely exothermic as well as hugely entropically favored but doesn't happen [measurably] because one can never isolate such exotic compounds in the first place. All this rant to say that there is no theoretical limit to the amount of chemical energy we can extract from a 1 meter-cubed box, but practical limitations are very real.

 

There are some hugely energetic materials out there. But the scale of chemical reaction energies is smaller than that achievable by nuclear or annihilation events by orders of magnitude.

[Fish 40]

So there's no way to determine how much more powerful fuels can get in the future?

[mississippichem]

So there's no way to determine how much more powerful fuels can get in the future?

 

Right. There is no theoretical limit on how much chemical energy can be put off by a reaction. Though I think we are already near the practical limit (just my opinion). Alkanes (major component of gasoline, diesel, etc.) are already quite energy dense with respect to combustion. Octane (an alkane with an eight carbon chain) has an enthalpy of combustion of about 5000 kJ/mol which is huge. Luckily alkanes are stable (kinetically anyway) at room temperature and atmospheric pressure. Fuels that have higher enthalpies of combustion such as boranes or hydrogen/oxygen mix are practically an explosion waiting to happen. H₂/O₂ on ploymer support with AlClO₄ has already been used as a rocket fuel and is extremely energy dense, however fuels like this will simply never be safe enough for you to fill up the family sedan with.

 

I think the fuels of the future will not be advantageous based on their energy density. Instead they will be advantageous based on their sustainability (think solar, and hydro-electric; the collection methods are all "passive"). To really make order of magnitude differences to how much energy we can get out of a one meter cubed box, we'll need to look to nuclear processes or other exotic things.

 

I find this to be a strangely provocative (in the good way) question by the way. A very good first first thread for you. Welcome to SFN.

[Moontanman]

Isn't hydrogen and fluorine, when reacted together, the most power reaction for chemical rocket engines?

[imatfaal]

. To really make order of magnitude differences to how much energy we can get out of a one meter cubed box, we'll need to look to nuclear processes or other exotic things.

 

I find this to be a strangely provocative (in the good way) question by the way. A very good first first thread for you. Welcome to SFN.

 

Agree - nice thread.

 

And I think it will be many years before we can rival the energy density of petroleum - a cubic metre of petrol would allow you drive across continents; and just as importantly the apparatus for changing that stored chemical energy into oomph is fairly small, self contained and highly developed. The fact that a car engine and serious size tank could fit in well under 2 cubic metres and produce 100KW over sustained periods is pretty good going. I am not a fan of the internal combustion engine in that I think we use it too much and are both screwing our environment and squandering resources - but there is no doubt it is a remarkably dense combined form of energy storage and conversion

[questionposter]

Shouldn't the limit have something to do with a plasma? I mean, a plasma doesn't have chemical bonds, because chemical bonds can't survive that high of an energy, so doesn't the maximum energy a chemical can store or give off have to be exactly 1 energy level before a that chemical would become a plasma?

[CaptainPanic]

There is no theoretical limit on how exothermic a reaction can be. As far as percent energy that can be extracted to do work...you are limited by the Carnot efficiency.

Why? Redox reactions (for example fuel cells) are not limited by Carnot. I admit that putting the universe's highest energy chemical into a redox is a step we haven't made yet, but we're talking 'theoretically' here anyway.

 

I think we might be able to find some maximum if we approach this from a molecular level: What's the highest energy bond between particles? And what's the lowest? The hydrogen-fluorine combination might be one of the most exothermic reactions, but it's possible to include some ring strain (i.e. get a ring structure with just 3-4 atoms). There might be some additional strain from steric hindrance, which can release even more energy when a structure is broken down. And I probably forget about a bunch of other options to pimp a molecule.

 

Regarding petrol, I think it's not a fair comparison to look at such a fuel in terms of energy per volume, because the majority of the reaction product is in fact oxygen (which isn't in the fuel tank, but in the air). The OP does not seem to give us the option to import some extra reactants into our theoretical cubic meter.

 

[edit] Shouldn't we approach this in terms of energy per mass, rather than energy per volume?

Edited November 4, 2011 by CaptainPanic

[Fish 40]

Either is interesting. I chose volume, because I think it makes a better comparison between vehicles of the same size fitting different fuels in the same tank, assuming hypothetically that the system can run all these fuels (A real car/truck wouldn't). Obviously mass is important, but it seems like the chart I found has most fuels clustered near each other in terms of energy to mass. Hydrogen being a BIG exception.

 

Looking on Wikipedia (how do you link?), it seems that Aluminum has the greatest energy to volume ratio (Hydrogen has the greatest energy to mass ratio), so with the assumption that some day you could extract that energy as quickly, you get more than twice the power you get from Diesel.

 

Course, just because that may be true it doesn't mean that it's the highest in theory, just as diamond is not the hardest -possible- material.

 

I was guessing it was possible to determine this limit based on the bonds, but I'm not much good at math.

Edited November 4, 2011 by Fish 40

[CaptainPanic]

Aluminium is used in places where power is very important, and convenience not so much. Wikipedia (of course) has an article on aluminium batteries.

There are multiple reasons why diesel/gasoline is used more than aluminium... but I guess that the economics are the most important: liquid fuels give a good profit for companies, and they out-compete all other fuels on price per energy. It's easy to compete on a price per energy level if you only have to pump up the fuel from the ground. This may change in the future though (see also the quote under 'commercialization' inth3e wiki article).

 

I am not sure how we can find out if anything real has a higher energy per volume (or weight).

 

p.s. you link by clicking on the small 'insert link' button above the window where you write your text, and then copy-pasting the link into that.

And if you select a piece of text before clicking onto that link, it becomes a hyperlink.

 

[edit: forgot to add the wikipedia link]

Edited November 7, 2011 by CaptainPanic

[YaDinghus]

By Volume, you definitely want to go up the periodic table for reducing agents. Period 7 Metals readily give up lots of electrons and they're extremely compact. But, well, they're heavy, and unlike Hydrogen (which will shed ALL of it's electrons), you need to put lots of Energy into getting all Electrons off any Lanthanide, and you can get much more energy out of them much easier with non-chemical methods (fission), and fissionable material is much more dangerous than most of their fission products - the problem with removing fission products i.e. nuclear waste is mainly a problem because the people profiting from it are not charged with responsibly removing it in the USA, THE BIGGEST USER AND PROVIDER OF FISSION ENERGY. But now we're getting political. As for oxidising power by volume, you're probaly looking at some kind of C-N polymer with hyposulfonic or phosphoric acid groups attached to it. It shuld be reasonable safe for use, too, until it gets close to an electron donator