Below is a text over which I have my questions which I have written below this text:

Chemical energy stored by molecules can be released as heat during chemical reactions when a fuel like methane, cooking gas or coal burns in air.

The chemical energy may also be used to do mechanical work when a fuel burns in an engine or to provide electrical energy through a galvanic cell like dry cell.

Thus, various forms of energy are interrelated and under certain conditions, these may be transformed from one form into another.

The study of these energy transformations forms the subject matter of thermodynamics. The laws of thermodynamics deal with energy changes of macroscopic systems involving a large number of molecules rather than microscopic systems containing a few molecules. Thermodynamics is not concerned about how and at what rate these energy transformations are carried out, but is based on initial and final states of a system undergoing the change. Laws of thermodynamics apply only when a system is in equilibrium or moves from one equilibrium state to another equilibrium state. Macroscopic properties like pressure and temperature do not change with time for a system in equilibrium state.

  1. What is chemical energy ? 1st paragraph

What I read online: In conclusion to all :

Chemical energy is energy stored in the bonds of chemical compounds, like atoms and molecules. This energy is released when a chemical reaction takes place. Usually, once chemical energy has been released from a substance, that substance is transformed into a completely new substance.

Now , we know energy is Force * displacement and chemical means a phenomenon relating to chemistry. Therefore , does chemical energy mean that when separation between atoms in a molecule increase or decrease, Chemical energy increases or decreases.

  1. Chemical energy released as heat ? 1st paragraph

What does this mean

  1. What does it mean by

thermodynamics is not concerned by how and at what rate these transformation are carried out ?

At what rate change in temperature happened. I.e did temp change in even proportion every second or uneven ? Doesn’t matter in this chapter. Is this what this statement mean to say.

  • $\begingroup$ @AJN Ok. Thank you. So , my definition or thinking of chemical energy is correct. $\endgroup$
    – S.M.T
    Jun 6, 2021 at 13:34
  • $\begingroup$ @AJN It is in my physics textbook that energy and work done mean the same thing or at least have the same formula. $\endgroup$
    – S.M.T
    Jun 6, 2021 at 13:35
  • $\begingroup$ @AJN It states “ Energy possessed by a body is the measure of amount of work done that the body can perform “. $\endgroup$
    – S.M.T
    Jun 6, 2021 at 13:36
  • $\begingroup$ @AJN It has to be always true right ? $\endgroup$
    – S.M.T
    Jun 6, 2021 at 13:36
  • $\begingroup$ @AJN It states at heading: (Energy as work done). Moreover in Q , formula of is used in place of energy. But I think your argument has to be correct since there is no usage of having energy = work done. $\endgroup$
    – S.M.T
    Jun 6, 2021 at 13:38

1 Answer 1


Our categorization systems and atomic models are all an approximation, but they help relate the things we can't see to things we can visualize and understand.

We define a chemical reaction as one where chemical "bonds" are altered. This is where electron configurations between atoms are altered. While electrons are technically indistinguishable and it makes more sense to think of them in an overall configuration probabilistic cloud, I still think of it like nuclei taking over/giving up electrons from other atoms, as opposed to a physical reaction where this exchange does not happen but the electron configuration is still altered.

Example of a physical reaction is heating something up causing the electrons (and nuclei) to move further apart. Example of a chemical reaction is one nucleus taking an electron from another thereby forming a bond.

For the ideas of energy, protons and electrons attract (electrons and electrons repel, protons and protons repel), so it takes energy to pull opposite charges apart or push same charges together, like a spring getting compressed. Similarly this stored energy is released when going in the opposite direction - moving same charges apart or opposing charges together. Released energy has to go somewhere else. This is often going to heat in the form of "physical energy" moving the electrons apart and making them faster (these are related). It can also be in another form such as an electromagnetic wave or light, but even with then, heat is usually generated somewhere to increase entropy.

Personally, (although I think it is important to distinguish between physical and chemical reactions) I think that making a distinction between "physical" and "chemical" energy is not as important as tracking the overall change in energy from a configuration change. Chemical energy is stored or released by a chemical reaction- where two atoms form or break a bond (give/receive electron(s) from the other). Physical is rearranging electrons without exchange. (Nuclear is a change in nucleus composition).

As for the rate of change, it's not important in scenarios where you consider only the points where the configurations are stable. Some intermediary unstable atomic configurations may exist, but considering them won't produce any different results when you only care about the stable fuel being fed in and stable products leaving. The nuances of those unstable configurations are best left to people trying to figure out the minutia of the sub-atomic level in hopes of controlling it for the next equivalent of the nuclear revolution, than people trying to figure out how much fuel they need to burn.


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