[theThinker (OP)]

I have searched the internet all over and cannot get what I want. It just say things like... chemical energy is released as heat like in burning. Everyone knows that! But what is heat?

Take the burning of hydrogen in oxygen to water, in a closed vessel with nothing else:
1) Are there tables of values where we could calculate the amount of energy released?
2) In what exact form is energy released, say kinetic energy, potential energy, photon; don't just mention heat unless you tell what is heat and who gets the heat. If kinetic energy, then it must be the kinetic energy of the molecules of water.   

[alancalverd]

1. Yes, about 286 kJ/mol of hydrogen.

2. In a stoichiometric mixture, mostly kinetic energy of the resulting water molecules, with some photons from the flame area. In an excess of either gas, some to most of the kinetic energy will be dispersed among the molecules of the excess gas.

[theThinker (OP)]

1. Yes, about 286 kJ/mol of hydrogen.

2. In a stoichiometric mixture, mostly kinetic energy of the resulting water molecules, with some photons from the flame area. In an excess of either gas, some to most of the kinetic energy will be dispersed among the molecules of the excess gas.

OK. But I still find it strange that 2 hydrogen molecules would combine with 1 molecule of oxygen to give water molecules which then become "hyperactive". How come? Chemical energy is just electrical energy - chemical bonding in water is just cheaper energy-wise.

[alancalverd]

Science is indeed full of strange happenings. But we invent conserved quantities like energy to give us a rational framework for predicting what might happen next.

[Origin]

OK. But I still find it strange that 2 hydrogen molecules would combine with 1 molecule of oxygen to give water molecules which then become "hyperactive". How come?

You answer that question in your next sentence.

Chemical energy is just electrical energy - chemical bonding in water is just cheaper energy-wise.

That's right the energy of a water molecule is less than the 2 hydrogen atoms and the oxygen atom.  Where did the 'extra' energy go, it didn't just disappear.  The short answer is that it was converted to heat.

[evan_au]

I still find it strange that 2 hydrogen molecules would combine with 1 molecule of oxygen to give water

Part of the answer is that oxygen is more "electronegative" than Hydrogen (O=3.4, H=2.2).
https://en.wikipedia.org/wiki/Electronegativities_of_the_elements_(data_page)

This means that Oxygen tends to grab and hold onto electrons more strongly than Hydrogen does, so the Oxygen tends to steal the electrons from the Hydrogen atom. This is visible in the structure of the water molecule, where the oxygen end is slightly negative, and the hydrogen end is slightly positive; this electrical polarization produces many significant characteristics of water.

The ratio of elements can be explained by looking at electron shells:
- Hydrogen has 1 unpaired electron in its outer shell.
- Oxygen has 2 unpaired electrons in its outer shell.
- These electrons pair up when two Hydrogen atoms bond with 1 oxygen atom, producing the familiar chemical name for water H₂O
- Essentially, an electron paired with another electron of opposite spin has lower energy than two isolated electrons, just like two bar magnets put together with opposite polarity (North poles touching South poles)

Note that you can mix hydrogen and oxygen at room temperature without releasing any significant energy. It actually takes a trigger (eg a tiny spark) to break apart the Hydrogen and Oxygen molecules, so the Hydrogen and Oxygen atoms can find each other. The kinetic energy released by the formation of a small number of water molecules is so great that it breaks apart other Hydrogen and Oxygen molecules to form water, which in turn break apart other Hydrogen and Oxygen molecules in a chain reaction, likely causing an explosion.

This initial input of energy is called the Activation Energy to start the chemical reaction.
https://en.wikipedia.org/wiki/Activation_energy

[theThinker (OP)]

I still find it strange that 2 hydrogen molecules would combine with 1 molecule of oxygen to give water

Part of the answer is that oxygen is more "electronegative" than Hydrogen (O=3.4, H=2.2).
https://en.wikipedia.org/wiki/Electronegativities_of_the_elements_(data_page)

This means that Oxygen tends to grab and hold onto electrons more strongly than Hydrogen does, so the Oxygen tends to steal the electrons from the Hydrogen atom. This is visible in the structure of the water molecule, where the oxygen end is slightly negative, and the hydrogen end is slightly positive; this electrical polarization produces many significant characteristics of water.

The ratio of elements can be explained by looking at electron shells:
- Hydrogen has 1 unpaired electron in its outer shell.
- Oxygen has 2 unpaired electrons in its outer shell.
- These electrons pair up when two Hydrogen atoms bond with 1 oxygen atom, producing the familiar chemical name for water H₂O
- Essentially, an electron paired with another electron of opposite spin has lower energy than two isolated electrons, just like two bar magnets put together with opposite polarity (North poles touching South poles)

Note that you can mix hydrogen and oxygen at room temperature without releasing any significant energy. It actually takes a trigger (eg a tiny spark) to break apart the Hydrogen and Oxygen molecules, so the Hydrogen and Oxygen atoms can find each other. The kinetic energy released by the formation of a small number of water molecules is so great that it breaks apart other Hydrogen and Oxygen molecules to form water, which in turn break apart other Hydrogen and Oxygen molecules in a chain reaction, likely causing an explosion.

This initial input of energy is called the Activation Energy to start the chemical reaction.
https://en.wikipedia.org/wiki/Activation_energy

Thanks. This is the part that I can't find through googling. They just tell me burning wood converts chemical energy to "heat". Even the aboriginal tribes of long ago knew the physics of "heat".

[theThinker (OP)]

NOTE: I think this thread is more suitable for General Science as it involves energy.

I think chemical energy to kinetic energy seems problematic.

Let's assume 2 H + 1 O atoms with negligible KE in the CM frame. Somehow, they combine to give H₂O. There is no way the single H₂O molecule could acquire any kinetic energy relative to the center-of-mass frame. This would require an external force acting on the H₂O, but any forces during the formation of this single molecule may only be internal forces.

How to solve this classical mechanics paradox?     

[Eternal Student]

Hi.

    The energy released during the formation of H₂O can be accommodated and accounted for in various ways.   As you said, it won't really be translational kinetic energy for the molecule because that will violate conservation of momentum.   However, it can be vibrational energy in the molecule,  infra-red photons flying about the place,   electrons in the Oxygen atom in an excited state (temporarily in a higher then usual orbital)  ---  many different ways to account for that energy.
    When there is a collection of molecules, then there are various theories about how energy tends to be equally partitioned between all the various modes of supporting that energy   (equi-partition theory) - that happens fairly quickly and the ensemble of particles reaches an equilibrium such that a well defined temperature for the ensemble of particles emerges.   Temperature is considered to be a measure of the average energy supported by a degree of freedom that a system has (for example, kinetic energy in one direction, say the x-axis, might be a degree of freedom the system has).   When there is a large number, "an ensemble", of particles - then they will have a non-zero average translational k.e. -  that's just a consequence of the equi-partition theory and random interactions that take place between the particles.
    For the single molecule you were describing, there's no such ensemble of particles, there's no well defined temperature and the energy never really does get to be exhibited as translational k.e. for that molecule.   Instead it remains in the various other forms previously described  (vibration in the molecule, excited electrons, infra-red photons etc.)

Best Wishes.

[theThinker (OP)]

Hi.

    The energy released during the formation of H₂O can be accommodated and accounted for in various ways.   As you said, it won't really be translational kinetic energy for the molecule because that will violate conservation of momentum.   However, it can be vibrational energy in the molecule,  infra-red photons flying about the place,   electrons in the Oxygen atom in an excited state (temporarily in a higher then usual orbital)  ---  many different ways to account for that energy.
    When there is a collection of molecules, then there are various theories about how energy tends to be equally partitioned between all the various modes of supporting that energy   (equi-partition theory) - that happens fairly quickly and the ensemble of particles reaches an equilibrium such that a well defined temperature for the ensemble of particles emerges.   Temperature is considered to be a measure of the average energy supported by a degree of freedom that a system has (for example, kinetic energy in one direction, say the x-axis, might be a degree of freedom the system has).   When there is a large number, "an ensemble", of particles - then they will have a non-zero average translational k.e. -  that's just a consequence of the equi-partition theory and random interactions that take place between the particles.
    For the single molecule you were describing, there's no such ensemble of particles, there's no well defined temperature and the energy never really does get to be exhibited as translational k.e. for that molecule.   Instead it remains in the various other forms previously described  (vibration in the molecule, excited electrons, infra-red photons etc.)

Best Wishes.

I am most interested in the conversion of chemical energy (which basically is electrical potential energy) into heat which nearly all in the internet talk about.

But "heat" is kinetic energy. There is only one formula for kinetic energy of a particle and it is 1/2mv² without exception. Even rotational kinetic energy is the sum of all kinetic energy of the particles of the body. In the case of the product of burning producing one single molecule of H₂O, kinetic energy must only be translational 1/2mv² unless all its kinetic energy is now rotational; there cannot be "vibrational" kinetic energy.

I am not familiar with where infra-red photons appear from.   

[theThinker (OP)]

It seems "vibration in the molecule, excited electrons" in combustion product of one water molecule may not be able to transfer such energy to other atoms/molecules.

[alancalverd]

......eppur si muove.

And if I want to be pedantic, 2H + O ↔ H₂O + E doesn't actually happen. The reaction is between molecular H₂ and O₂ so the resultant is at least 2H₂O, allowing the product molecules to fly apart from the CM without violating the conservation laws.

[theThinker (OP)]

......eppur si muove.

And if I want to be pedantic, 2H + O ↔ H₂O + E doesn't actually happen. The reaction is between molecular H₂ and O₂ so the resultant is at least 2H₂O, allowing the product molecules to fly apart from the CM without violating the conservation laws.

May there not be other combustion reactions where only one product molecule is formed?

[Eternal Student]

Hi.

It seems "vibration in the molecule, excited electrons" in combustion product of one water molecule may not be able to transfer such energy to other atoms/molecules.

   You might be right, it doesn't seem like it could - but it does.   Firstly everything discussed is only a very simple and idealistic model of what is happening.   A molecule can have energy stored in it that is more complicated and/or doesn't really fit any explanation assuming rigid little particle balls on springs etc.
    The equi-partition theorem is discussed in various places   (Here's Wikipedias entry:   https://en.wikipedia.org/wiki/Equipartition_theorem).    For a large ensemble of particles,  energy will not stay in the form of vibration in the molecules and excited electrons or just some other small number of forms,  it will be equally distributed.   We reason that this happens because there is at least one mechanism by which energy can be exchanged between the degrees of freedom.   However, you don't need to spend too long trying to find that mechanism,  it just does happen and the equi-partition of energy in a system at equilibrium is what we observe and what allows a temperature to be defined exactly as it is (a measure of the energy in any degree of freedom,  typically the average translational k.e. of the particles).   In those rare situations where energy isn't equally partitioned between the degrees of freedom of a system then it isn't in a suitable equilibirum (yet, or ever) and we don't try to assign it a temperature.
   
Best Wishes.

[theThinker (OP)]

Hi.

It seems "vibration in the molecule, excited electrons" in combustion product of one water molecule may not be able to transfer such energy to other atoms/molecules.

   You might be right, it doesn't seem like it could - but it does.   Firstly everything discussed is only a very simple and idealistic model of what is happening.   A molecule can have energy stored in it that is more complicated and/or doesn't really fit any explanation assuming rigid little particle balls on springs etc.
    The equi-partition theorem is discussed in various places   (Here's Wikipedias entry:   https://en.wikipedia.org/wiki/Equipartition_theorem).    For a large ensemble of particles,  energy will not stay in the form of vibration in the molecules and excited electrons or just some other small number of forms,  it will be equally distributed.   We reason that this happens because there is at least one mechanism by which energy can be exchanged between the degrees of freedom.   However, you don't need to spend too long trying to find that mechanism,  it just does happen and the equi-partition of energy in a system at equilibrium is what we observe and what allows a temperature to be defined exactly as it is (a measure of the energy in any degree of freedom,  typically the average translational k.e. of the particles).   In those rare situations where energy isn't equally partitioned between the degrees of freedom of a system then it isn't in a suitable equilibirum (yet, or ever) and we don't try to assign it a temperature.
   
Best Wishes.

I think I cannot dispute your argument: 1) I have no knowledge of statistical thermodynamics 2) a water molecule has many electrons+(protons+neutrons) though it seems the nucleus should not be involved as we exclude nuclear energy when discussing chemical energy in combustion.

Still, your argument seems the most convincing.   

[Deecart]

I am not sure where the energy go but i am sure where it come from.
Energy come from the deficit of mass.

Like in any thermonuclear raction, when mass disapear it is tranformed into energy (kinetic or and radiation, not sure if we can always easyly distinguish between the two form, wave or particle).
But for some chemistry reaction, involving, not the core of the atoms but the electrons (covalent bounds), the default of mass is very very small.
Therefore the energy released from some chemical reaction (if exothermic) is very very small compared to some nuclear reactions.
But fundamentaly, the origin of the energy is the same : Loss of mass.

[alancalverd]

But fundamentaly, the origin of the energy is the same : Loss of mass.

There is no mass loss in the combustion of hydrogen, nor any mass gain in the electrolysis of water.

[chiralSPO]

NOTE: I think this thread is more suitable for General Science as it involves energy.

I think chemical energy to kinetic energy seems problematic.

Let's assume 2 H + 1 O atoms with negligible KE in the CM frame. Somehow, they combine to give H₂O. There is no way the single H₂O molecule could acquire any kinetic energy relative to the center-of-mass frame. This would require an external force acting on the H₂O, but any forces during the formation of this single molecule may only be internal forces.

How to solve this classical mechanics paradox?     

I don't think that purely internal forces causing changes to vibrational motions would be prohibited by any conservation laws.

By analogy, perhaps could imagine an arrangement of magnets (or springs) with negligible KE in the CM frame, that when allowed to combine and "react" with each each other would result in a system with significant internal kinetic energy. This extra KE could well be in the form of a symmetrical vibrational mode, in which there is no net motion of the center of mass.

The forces involved in chemical reactions are often viewed very abstractly, but I think this is a mistake. On a molecular level, chemical reactions are extremely violent events (series of events, really), involving lots of smashing, and rending, and ricocheting. More like cars crashing into each other (repeatedly) than building with legos...

[Eternal Student]

Hi.

There is no mass loss in the combustion of hydrogen, nor any mass gain in the electrolysis of water.

    That doesn't seem like an entirely fair comment @alancalverd . However, a discussion of mass-energy equivalence probably isn't useful for the OP.

Best Wishes.

[Deecart]

There is no mass loss in the combustion of hydrogen, nor any mass gain in the electrolysis of water.

There is always a loss of mass in this case.
Energy can not come from nowhere.

In nuclear reactions (changes to the nucleus of atoms), there is enough energy released or absorbed that the change in mass is significant and must be accounted for. In contrast, chemical reactions (changes to only the electrons in atoms) release or absorb very little energy compared to nuclear reactions, so the change in mass of the system is often so small that it can be ignored. As a reasonable approximation only, therefore, chemists often speak of the conservation of mass and use it to balance equations.

But strictly speaking, the change in mass of the system during a chemical reaction, though small, is never zero.
If the change in mass were exactly zero, there would be no where for the energy to come from.

Chemists like to speak of "chemical potential energy" and talk as if the energy released in a reaction comes from the potential energy. But "chemical potential energy" is just an old-fashioned term for what we now know is mass.

Fundamentally, when chemists say "potential energy" they mean "mass". There is not some bucket of potential energy in an atom from which a reaction can draw. There is just mass.

https://www.wtamu.edu/~cbaird/sq/2013/10/21/why-is-mass-conserved-in-chemical-reactions/

Therefore, if someone want to know what sort of energy fundamentaly create the heat in an exothermic reaction, he has to investigate how the loss of mass produce some energy or at least in what form.
With nuclear reactions i think that mass is transformed into kinetic energy.
It it probably (but i am not sure) the same with chemical reactions.