How is chemical energy stored in carbon I'm fairly new to chemistry, and I have a hard time understanding how chemical energy is stored in carbon (I'm aware that the question can be generalised to 'how is chemical energy stored', but I'm trying to understand it in the example of carbon). In the example in my book, carbon and oxygen react to form carbon dioxide, releasing energy in the form of warmth in the process.

My book says that the energy released in this process mainly comes from carbon, which is originally the solar energy stored by plants. I have to tried to look for explanations how that energy is stored (not how the process of photosynthesis works, but in what form or what is the meaning of the 'stored energy'), but I still can't really wrap my head around it. The best answer so far was What is the nature of chemical energy?

If I understand the answer there correctly, the bond potential energy of a carbon dioxide molecule is lower than the combined bond potential energy of a carbon atom and an oxygen molecule. Given that, there are still two things I don't understand:


Why is the bond potential energy in the carbon atom and/or oxygen molecule higher than that of a dioxide molecule? How can that explained? Is this directly related to the chemical traits of the elements carbon and oxygen and the compound carbon dioxide?
Why, or better said how is said bond potential energy converted to warmth in the process?


I'm aware that there might be concepts in physics here I don't know or understand to understand a possible answer, and I would be thankful for any pointers to what subjects I should be first reading to understand such an answer.

Edit: I would like to quote here something I came across on wikipedia. I'm not providing it as an answer nor claiming it is correct, just trying to illustrate how confusing it is to find a satisfying answer for this matter. This comes from https://en.wikipedia.org/wiki/Chemical_bond#Overview_of_main_types_of_chemical_bonds:


  In the simplest view of a covalent bond, one or more electrons (often a pair of electrons) are drawn into the space between the two atomic nuclei. Energy is released by bond formation. This is not as a reduction in potential energy (sic), because the attraction of the two electrons to the two protons is offset by the electron-electron and proton-proton repulsions. Instead, the release of energy (and hence stability of the bond) arises from the reduction in kinetic energy due to the electrons being in a more spatially distributed (i.e. longer de Broglie wavelength) orbital compared with each electron being confined closer to its respective nucleus.


Italic and bold added by me. Note that wikipedia itself has a '[clarification needed]' tagging in the middle of that text, right after 'two protons', which I removed.

So if I understand that correctly and assuming that info is correct, any formation of a covalent bond would result in energy being released. But that still doesn't explain why my book is claiming the energy is particularly stored in carbon (and not in oxygen), and why it is released as warmth. In addition, the explanation on wikipedia would contradict what I referred to earlier, namely that the energy released comes from differences in bond potential energy.
One of the most important issues in understanding bond energy in chemistry is the question: energy relative to what?

The formation of bonds from (mostly hypothetical) atomic elements releases energy (or they wouldn't be bonded.) Solid carbon (graphite or diamond) has less energy than a cloud of carbon atoms so it could be said graphite has less energy relative to a cloud of atomic carbon atoms. We would describe this energy as "bond energy". But this is the energy released by the hypothetical process of forming lists of C-C bonds from isolated carbon atoms. 

But there is another subtle complication if we want to understand reactions. sometimes, when chemists write a reaction scheme they don't specify all the detail. So C +O2->CO2 doesn't actually mean atomic carbon atoms plus oxygen molecules give carbon dioxide. Most chemists would assume that the carbon starts in the state it is normally found as in laboratories: graphite (so not isolated atoms but a molecular solid containing lost of C-C bonds). This terminology seems to have confused some people. But chemists don't like to waste space in formulae redundantly specifying things most of their colleagues will already know. The default state of an ingredient in a reaction is assumed unless specified otherwise and this usually means the form the compound or element takes in a bottle on the lab shelf.

So how can we say that carbon stores energy? Well we have to ask the question, relative to what? In the case of the question the answer is relative to carbon dioxide gas. The point here is that the double bonds in CO2 are stronger than the combined bonds in oxygen gas and graphite (so they are lower energy) This means that the reaction between carbon and oxygen releases energy (it takes less energy to break all the O-O and C-C bonds than is released when new C=O bonds are formed. So elemental carbon "stores" energy relative to carbon dioxide. 

The claim that carbon "stores" solar energy absorbed by plants is a little simplistic (mainly because plants don't store carbon but usually store more complicated molecules like sugars or poly-sugars like lignin or cellulose). Dead plants are converted to more carbon like compounds (oil is mostly hydrocarbons, but coal is mostly carbon) under some geological conditions. What plants actually do at the start of this process is to convert carbon dioxide into sugars using energy input from sunlight. They do this to store energy and to grow by converting the sugars into the structures of their leaves, stems and trunks. 

We can reverse that process by burning the plants or their degradation produce like oil and coal in air to reproduce the carbon dioxide and release energy. In this highly simplified sense, carbon is a store of "solar" energy. 

But the key to understanding what is meant is to understand the chemical reaction involved in the creation or burning of the carbon. Then you can understand what energy is stored and how it is stored.