My Physics textbook gives the following definition of energy :

The capacity of a body to do work is defined as the energy possessed by it

This was the definition that was used to derive $U_G = mgh$ and $\text{KE} = \dfrac{mv^2}{2}$

Now, consider the following statement

Exothermic reactions release energy in the form of heat

I am unable to see how the first definition and second statement relate to each other.

As I see it, an object losing energy means that it's capacity to do work decreases. What is the object that's loosing energy here? And, what does this energy get transferred to? The atmosphere in which the reaction is taking place? I seem to have some silly misconception.


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    $\begingroup$ You might be mixing two different examples together.. In your first statement, you are talking about the physical energy of a body whereas in the second case you are taking a reaction which causes a change in chemical potential. $\endgroup$ Commented Jul 19, 2020 at 8:00
  • $\begingroup$ @Safdar I'm afraid I don't know what chemical potential is, can you provide a brief definition of it that can be interpreted by a layman? Thanks. $\endgroup$ Commented Jul 19, 2020 at 8:07
  • $\begingroup$ icsm.fr/Local/icsm/files/286/JFD_Chemical-potential.pdf Does this help? $\endgroup$ Commented Jul 19, 2020 at 8:17
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    $\begingroup$ Then wait until next year.. In simple terms what happens is when you have a chemical reaction, you first supply some energy to break the bonds between atoms in a molecule. This creates a compound with a higher amount of energy. that is unstable. After this the compound reacts with the other reactants to give a molecule that is more stable. This is done by releasing energy. If the energy produced is more than the energy given, it is released into the surroundings that we observe as heat. Such a reaction would be exothermic in nature. $\endgroup$ Commented Jul 19, 2020 at 8:35
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    $\begingroup$ You are perfectly right to be confused. That first definition is a pointless generalisation, bordering on being plain wrong. Usually (=in the terminology that 99% of the physicists and chemists in the world agree on) "work" $W$ excludes thermal energy transfer "Q". $\endgroup$
    – Karl
    Commented Jul 19, 2020 at 10:55

2 Answers 2


Look energy has various forms, the formulas you listed are applicable to macroscopic objects and are due to position and the velocities they posses (for potential energy and the kinetic energy your'e talking about) respectively.

But when we zoom into the molecules and atoms the energies they posses are there due to completely different reasons. If you know molecules tend to vibrate and/or rotate about specific axes that contributes to one kind of energy they posses, then you can have energy due to the bonds through which they are bonded because of how the orbitals overlap (If you don't know what orbitals are, they are just regions around the nucleus where electrons are found), atoms and molecules sometimes form lattice you could say these are just clusters if them arranged in a specific manner, and a lot many things the list goes on.

Now a reaction involves two things reactants and products. So many a times it happens that the product formed is way much stable due to various different reasons than the reactant by stable itself. Now imagine a scenario you have pencil, now if I ask you to make it stand on it's tip you won't be able to but lay it horizontally it will easily rest like that so you may ask why? What happens is that the horizontal position is way stable than than the vertical because in the latter position is unstable than the former why you'd ask, it is because when you try to make it stand it on it's tip you have to correctly align the center of gravity with the tip so it perfectly balances. Same applies to atoms and molecules some configuration is more stable than the other due to various different reasons including some that I talked about above. Now when you try to keep the pencil up on it's tip it takes more effort hence more energy is required to keep it that, in way same when a chemical configuration is unstable than the other it needs more energy, when it becomes more stable it discards the excess energy it required, 'it' here means the molecule so the molecule is what gives off the energy as heat. And if you don't know fire is a result of this kind of reaction so you can probably guess were the heat/energy goes.


I assume that as you are in 10th grade you know that bonding between atoms is due to forces of attraction. To separate the atoms in the bond, you need to exert a force enough to overcome the attractive force between them. Now, as distance is being covered in the direction of the force separating the bonds. Hence, work is done. As work is done by the force, it means energy equal to the work done on breaking the bond was supplied to the molecule. Thus, energy is absorbed (It can be in any form; heat, light etc.).

Now, during bond formation, (taking the formation of a diatomic molecule) two atoms come close to one another due to their attractive force. Distance covered in the direction of the force is decreasing, actually having negative values according to the law of vectors. Then, as the distance covered is opposite to direction of force separating them, work is done on the force by the atoms forming the bond. The attractive force is doing work. Thus, energy is supplied by the bonding atoms to the environment providing separation force, equal to the work done. This is energy released to environment in the form of heat.

In exothermic reactions, energy released during bond formation is greater than energy absorbed during bond breaking. Concludingly, the extra energy is released in the form of thermal energy. I think this is sufficient for you. You will learn in AS and A levels that chemical bonds' thermodynamics is also related to kinetic energy and potential energy.

As distance between atoms increases, potential energy increases between them, and hence, work has to be done so energy is absorbed to separate them and vice versa.

Hope this will be helpful to you at your stage.


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