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During water electrolysis, the electrochemical reaction on the cathode side can be expressed as protons combined with electrons to form hydrogen. I believe this electrochemical reaction is a heat release process, but I don't know the reason.

Why protons combined with electrons will release heat?

It will be better if you could provide some references.

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closed as off-topic by Buck Thorn, Mithoron, Todd Minehardt, Poutnik, user55119 Jun 26 at 18:18

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    $\begingroup$ Well, there is the binding energy to consider... $\endgroup$ – Jon Custer Jun 25 at 17:06
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If the process of uniting protons and electrons is assumed to occur isothermically in a vessel of constant volume, then the heat released will be equal to the change in internal energy of the system:

$$q_v=(\Delta U)_v$$

Now if the mixture were to behave as an ideal gas, the change in internal energy would be zero and therefore so would the heat. But we can expect a mixture of oppositely charged species such as protons and electrons to behave as an extremely non-ideal mixture, consistently with the large Coulombic potential between the particles. Since $\Delta U$ for the process of uniting protons and electrons (and thereby also minimizing repulsions) is negative, the process is then expected to be exothermic ($q<0$).

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  • $\begingroup$ Thank you for the reply. Do you have references for me to read in order to have a better understand of the mechanism? $\endgroup$ – king Jun 27 at 0:32
  • $\begingroup$ I recommend an introductory physical chemistry or thermodynamics textbook that explains basic electrochemistry. Perhaps if your background in basic electrostatic theory is not so strong, also reading a introductory physics textbook (undergraduate level). $\endgroup$ – Buck Thorn Jun 27 at 18:33
  • $\begingroup$ I checked the Wikipeida, it said the electron affinity is the heat released (negative energy term) when a free electron combines with a neutral atom to make a negatively charged ion, and this process could release heat. In my case, protons (not neutral atom) combined with electrons to form hydrogen (not a negatively charged ion). Can I still use electron affinity to explain my problem? $\endgroup$ – king Jun 27 at 20:36
  • $\begingroup$ The thing about your questions is that you attempt to take leaps between different scenarios. Take my answer for instance: I answered a very narrow (specific) question, what is the heat released when you combine proton and electron in an isothermal chamber. I think however you are more interested in what happens at an electrode when hydrogen is reduced: $\ce{2H+ + 2e- -> H2}$. There is at least one important additional step between this reaction and the case I present and that is the formation of an H-H bond. $\endgroup$ – Buck Thorn Jun 28 at 9:09
  • $\begingroup$ So the affinity of a proton for an electron is definitely an important step of the process, so I'd have to answer that you should use the electron affinity, but that is not enough. $\endgroup$ – Buck Thorn Jun 28 at 9:11
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One way to think of it is the electrical potential energy of the separated charges is released (as heat energy, eventually) when the charges unite. First the energy will be released as photons, and these photons will be absorbed by nearby molecules, causing them to increase in kinetic energy, which on the macro scale, is heat.

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    $\begingroup$ If an isolated proton and electron combine when initially widely separated, energy is conserved and they will just separate again as total energy will be above dissociation energy. No photons emitted as initial state is a continuum. In a medium, say a liquid or gas, energy will be taken away from pair primarily by collisions and ultimately end up as thermal energy, ie translation, vibrational and rotational motion. Some emission is possible from excited levels but is slow process compared to collisions so not a major pathway $\endgroup$ – porphyrin Jun 25 at 16:32
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A proton combining with an electron doesnt necessarly release energy.All the posts forget that there is the weak and strong interaction.For example let's take hydrogen atom.If the electron from the 1s orbital is combined with the proton(core) it will leave a neutrally charged neutron.But neutron is unstable when it isnt in the core of elements , so it will decay into a proton , an electron and an electron anti-neutrino . So you see not all cases are what you describe.

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  • $\begingroup$ This is chem.SE. We're not in the business of synthesising elements. $\endgroup$ – Karl Jun 25 at 22:10
  • $\begingroup$ Hmm?Why synthesise elements?I didnt say anything wrong.It doesnt always release energy. $\endgroup$ – Warrior Jun 25 at 22:17
  • $\begingroup$ Honestly? You cannot produce neutrons via chemistry. $\endgroup$ – Karl Jun 25 at 22:35
  • $\begingroup$ Yes there is always the possibility of something like this happening.In Hydrogen atom the electron is at the 1s subshell so it is possible for it to be fused with the proton of the core. $\endgroup$ – Warrior Jun 29 at 20:59
  • $\begingroup$ Ah, the famous inverse neutron decay. A very powerful and versatile cooling device. Only it doesn't work, because it can only take heat in chunks of 0.8 MeV. $\endgroup$ – Karl Jun 30 at 9:22

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