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Grandpa -

When, for example, carbon reacts with oxygen to produce carbon dioxide and heat, does any of the heat released come from the conversion to heat of some small portion of the masses of the carbon or oxygen atoms involved?

A. D. Reichle


Sent from AllExperts.com at 24 Sep 2000

Note: Grandpa is listed as an expert in physics on this site.  Some of the examples come from there.


Dear A. D. -

No - this is not an atomic reaction, but a simpler chemical reaction.  The heat comes from electrons changing the amount of what is essentially kinetic energy - from changing the energy of their "orbits."  (They're not really orbits - but the motion of electrons is kinetic energy.)  There are other energies in atoms and molecules - bonds, etc., which may change also during the reaction.

But also:

Yes -  If we consider that energy and mass are simply two forms of the same thing, then the energy lost could be viewed as mass lost.  This is not quite like an atomic reaction, wherein the mass itself is altered.  In the chemical reaction, the measured mass remains the same if we make sure we have all the product of the reaction to measure.

This is a good question, and there is much we don't understand.  Since energy is "given off" in this reaction, some of the kinetic energy internal in the molecules (atoms) is converted to kinetic energy of the motion of surrounding molecules (heat).  This is called an exothermic reaction - one in which heat is given off during the reaction.

There are other chemical reactions that "take on" energy rather than giving it off.  We call these endothermic reactions.

An example is the reaction required to separate oxygen and hydrogen in water.  This is endothermic.  We put energy in to cause the reaction.  We can do this by causing an electric current to flow through the water.  Oxygen will collect at one electrode and hydrogen at the other.  Now we may put the oxygen and hydrogen back together to get water - this is exothermic - it gives off energy.  In fact, it's an explosion.  Just mix hydrogen gas and oxygen gas and light it - bang.  The only "waste" product is water.  What a great way to run an engine - if we could obtain hydrogen gas cheaply enough.  If we obtained the hydrogen by separating the oxygen and hydrogen in water, it would take more energy to obtain the hydrogen than the burning of it could give us back.

Keep thinking this way.


- Grandpa

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