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I'm not a chemist, so this is a very low knowledge post, but the carbon/hydrogen link in carbohydrates seems to be a rather weak one, we reconfigure it all the

by usrusr 3y ago

I'm not a chemist, so this is a very low knowledge post, but the carbon/hydrogen link in carbohydrates seems to be a rather weak one, we reconfigure it all the time on the long and winding process paths from crude to the many, many things we create from it (these days, pretty much everything that isn't metal or concrete). The majority of the energy we use from burning carbohydrates comes not from oxidating carbon but from oxidating hydrogen: oxidating carbon does release slightly more energy per atom than oxidating hydrogen, but the hydrogen atoms outnumber the carbon ones by far. On the atom level, our society already is mostly hydrogen powered, but unfortunately limited to hydrogen packaged in carbon and we can't burn them separately. With this technology, if it scales (big if, no question about that!), we can burn the oxygen for energy while retaining the carbon as stable, even somewhat useful (perhaps not in the amounts post-scaling...) solids that are relatively easy to keep out of the atmospheric cycle. If that really works, it will be a great improvement over what we do and certainly worth spending some of the net energy we'd get from the hydrogen on. And even if the net energy would be zero it could still be worth it, run the plant only at times of intermittent sources surplus. At zero energy balance it would be a 100$ efficiency storage solution (that would solve some of our plastic waste problems on the side)

adrian_b 3y ago
It cannot be said that the energy obtained from burning carbohydrates, or any other hydrocarbon derivatives comes from oxidizing hydrogen. There is a simplified view and a more accurate view, and neither involves oxidizing mostly hydrogen. The simplified view, which is good enough for a qualitative explanation and for approximate quantitative computations, is that, because carbon is more electronegative than hydrogen, all the hydrogen in carbohydrates or hydrocarbons is already completely oxidized. When those burn, only the carbon is oxidized and the hydrogen ions just move from carbon to oxygen, making water. In this simplified view, a carbon atom from carbohydrates gives on average 4 electrons to oxygen, while a carbon atom from fatty substances or saturated hydrocarbons gives around 6 electrons to oxygen. With the energy gained by moving an electron from carbon to oxygen of slightly more than 1 electron-volt, you can compute approximate values for the combustion heats, based on the chemical formulas. The more accurate view is that the difference in electronegativity between hydrogen and carbon is small, so the hydrogen in carbohydrates is only partially oxidized, i.e. in each hydrogen-carbon bond the electron does not completely belong to carbon, but it belongs e.g. 60% to carbon and 40% to hydrogen. While taking into account that the hydrogen-carbon bonds are not ionic, but they are mostly covalent, with only a small ionic component, is necessary in many contexts, for understanding burning and any other redox reactions this is not necessary and the simpler view with the completely oxidized hydrogen is more useful and it is good enough. In any case, it is not acceptable to say that hydrogen is the one that is mostly oxidized when burning carbohydrates, because if the electrons grabbed by oxygen were attributed to hydrogen that would correspond to an energy per electron of more than 1.25 electron-volt, resulting in much higher combustion heats than obtained experimentally. Also, the energy per atom produced by burning carbon is not slightly higher, but it is much higher than when burning hydrogen. Because hydrogen is more electropositive, the energy per electron is a little higher, but a carbon atom gives many times more electrons when oxidized (4 electrons when pure, up to 8 in compounds). Where hydrogen is better, is at energy per mass, because its atoms are much lighter. Because hydrogen is more electropositive than carbon, non-oxidized hydrogen, i.e. pure hydrogen, produced either directly or indirectly by the internal heat of the Earth or produced by the photolysis of water done by the phototrophic living beings, e.g. plants, can spontaneously (i.e. without additional energy) reduce the carbon dioxide into organic substances, so in this sense it can be said that all life is hydrogen-powered, because it is the universal intermediate between the primary energy sources and the organic matter. However, once the organic substances have been synthesized, the energy surplus contained in pure hydrogen has been consumed and the remaining energy, which can be obtained by burning the organic substances, to recover the water and the carbon dioxide from which they have been made, is the energy corresponding to the oxidation of the included carbon, not of the included hydrogen.
- usrusr 3y ago
  Thanks. Can I read this as there's no way this process could run any other way than as a massive energy sink? Or are the graphene bindings sufficiently low energy that the hydrogens will happily hop away to their "higher" bindings in h2? (that's my mental model of chemistry: stuff "wants" to go down but there are plenty of local minima where certain combinations can rest until "shaken out", never made it to the point where I could reason about why certain combinations are local minima)
  - adrian_b 3y ago
    Any hydrocarbon derivative can be converted by pyrolysis into hydrogen and carbon (and small amounts of other substances, if it is not a pure hydrocarbon), where the carbon may be graphite or other forms, e.g. graphene. This requires a great amount of energy. By burning the obtained hydrogen, depending on the amount of hydrogen in the original hydrocarbon, there are good chances to obtain more energy than used for pyrolysis, if all the processes are done carefully, to avoid losses (the reason why energy can be gained is that every new hydrogen-oxygen bond corresponds to an old hydrogen-carbon bond replaced by a half of a hydrogen-hydrogen bond and a half of a carbon-carbon bond, and even when ignoring the other bonds and ignoring the weak old oxygen-oxygen bond, a hydrogen-oxygen bond is strong enough in comparison to a hydrogen-carbon bond to ensure a positive energy balance). Nevertheless, the amount of energy obtained in this way would be much less than the energy obtained by burning the hydrocarbon derivative at the beginning, so this would be a waste that could be justified only if the graphene could be sold at a high price. If it is desired to not release carbon dioxide, then the original hydrocarbon derivative should be left alone and hydrogen should be obtained from water. If the hydrocarbon derivative needs to be destroyed, it is easier to ensure a clean process for burning followed by carbon dioxide capture, than for pyrolysis, which can produce many dangerous substances.

by usrusr 3y ago

adrian_b 3y ago

It cannot be said that the energy obtained from burning carbohydrates, or any other hydrocarbon derivatives comes from oxidizing hydrogen. There is a simplified view and a more accurate view, and neither involves oxidizing mostly hydrogen. The simplified view, which is good enough for a qualitative explanation and for approximate quantitative computations, is that, because carbon is more electronegative than hydrogen, all the hydrogen in carbohydrates or hydrocarbons is already completely oxidized. When those burn, only the carbon is oxidized and the hydrogen ions just move from carbon to oxygen, making water. In this simplified view, a carbon atom from carbohydrates gives on average 4 electrons to oxygen, while a carbon atom from fatty substances or saturated hydrocarbons gives around 6 electrons to oxygen. With the energy gained by moving an electron from carbon to oxygen of slightly more than 1 electron-volt, you can compute approximate values for the combustion heats, based on the chemical formulas. The more accurate view is that the difference in electronegativity between hydrogen and carbon is small, so the hydrogen in carbohydrates is only partially oxidized, i.e. in each hydrogen-carbon bond the electron does not completely belong to carbon, but it belongs e.g. 60% to carbon and 40% to hydrogen. While taking into account that the hydrogen-carbon bonds are not ionic, but they are mostly covalent, with only a small ionic component, is necessary in many contexts, for understanding burning and any other redox reactions this is not necessary and the simpler view with the completely oxidized hydrogen is more useful and it is good enough. In any case, it is not acceptable to say that hydrogen is the one that is mostly oxidized when burning carbohydrates, because if the electrons grabbed by oxygen were attributed to hydrogen that would correspond to an energy per electron of more than 1.25 electron-volt, resulting in much higher combustion heats than obtained experimentally. Also, the energy per atom produced by burning carbon is not slightly higher, but it is much higher than when burning hydrogen. Because hydrogen is more electropositive, the energy per electron is a little higher, but a carbon atom gives many times more electrons when oxidized (4 electrons when pure, up to 8 in compounds). Where hydrogen is better, is at energy per mass, because its atoms are much lighter. Because hydrogen is more electropositive than carbon, non-oxidized hydrogen, i.e. pure hydrogen, produced either directly or indirectly by the internal heat of the Earth or produced by the photolysis of water done by the phototrophic living beings, e.g. plants, can spontaneously (i.e. without additional energy) reduce the carbon dioxide into organic substances, so in this sense it can be said that all life is hydrogen-powered, because it is the universal intermediate between the primary energy sources and the organic matter. However, once the organic substances have been synthesized, the energy surplus contained in pure hydrogen has been consumed and the remaining energy, which can be obtained by burning the organic substances, to recover the water and the carbon dioxide from which they have been made, is the energy corresponding to the oxidation of the included carbon, not of the included hydrogen.

usrusr 3y ago
Thanks. Can I read this as there's no way this process could run any other way than as a massive energy sink? Or are the graphene bindings sufficiently low energy that the hydrogens will happily hop away to their "higher" bindings in h2? (that's my mental model of chemistry: stuff "wants" to go down but there are plenty of local minima where certain combinations can rest until "shaken out", never made it to the point where I could reason about why certain combinations are local minima)
- adrian_b 3y ago
  Any hydrocarbon derivative can be converted by pyrolysis into hydrogen and carbon (and small amounts of other substances, if it is not a pure hydrocarbon), where the carbon may be graphite or other forms, e.g. graphene. This requires a great amount of energy. By burning the obtained hydrogen, depending on the amount of hydrogen in the original hydrocarbon, there are good chances to obtain more energy than used for pyrolysis, if all the processes are done carefully, to avoid losses (the reason why energy can be gained is that every new hydrogen-oxygen bond corresponds to an old hydrogen-carbon bond replaced by a half of a hydrogen-hydrogen bond and a half of a carbon-carbon bond, and even when ignoring the other bonds and ignoring the weak old oxygen-oxygen bond, a hydrogen-oxygen bond is strong enough in comparison to a hydrogen-carbon bond to ensure a positive energy balance). Nevertheless, the amount of energy obtained in this way would be much less than the energy obtained by burning the hydrocarbon derivative at the beginning, so this would be a waste that could be justified only if the graphene could be sold at a high price. If it is desired to not release carbon dioxide, then the original hydrocarbon derivative should be left alone and hydrogen should be obtained from water. If the hydrocarbon derivative needs to be destroyed, it is easier to ensure a clean process for burning followed by carbon dioxide capture, than for pyrolysis, which can produce many dangerous substances.