Looking for intuition on exothermic reactions

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All,

I am looking for a logical step that I must be missing in order to understand how heat energy is produced in an exothermic reaction. All of the standard explanations--more heat is given off than is taken in--don't seem satisfying. If heat is the result of the kinetic energy of molecules, how does the creation of new, more stable bonds produce this kinetic energy in the surroundings? I hope this makes sense. I cannot understand how the molecules forming new bonds are imparting this kinetic energy to their neighboring molecules.
 

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  • #2
DrClaude
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If heat is the result of the kinetic energy of molecules,
Be careful here. This is a common, but often misleading picture. At a given temperature, energy is distributed among all degrees of freedom, not just kinetic energy (look up the equipartition theorem).

how does the creation of new, more stable bonds produce this kinetic energy in the surroundings?
Following from what I wrote above, the extra energy does not necessarily go immediately into kinetic energy. When a bond breaks, you can get bits flying off with more kinetic energy, but you can also get an excited molecule where the conversion to kinetic energy will take, e.g., collisions with other molecules.
 
  • #3
hilbert2
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It's about electromagnetic potential energy being converted to kinetic energy in the form of random motion of the molecules. Just like the potential and kinetic energy of a falling meteorite is converted to heat when it comes down.

EDIT: yes, initially some of the released energy can be in the form of sound waves but even them quickly turn into heat by viscous dissipation.
 
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Thanks for the replies. One source seems to suggest that as more stable bonds are forming, the electrostatic forces between these atoms are accelerating them together and thus increasing the kinetic energy of their surroundings, too. Would this be an accurate description?
 
  • #5
DrClaude
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Thanks for the replies. One source seems to suggest that as more stable bonds are forming, the electrostatic forces between these atoms are accelerating them together and thus increasing the kinetic energy of their surroundings, too. Would this be an accurate description?
Somewhat. The bonds will not be formed in their ground state, and that vibrational excitation will eventually be transferred to the surroundings. (But again, I strongly advise against thinking of "heat" as "kinetic energy." They are not the same thing.)
 
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Hmm, thanks. I definitely need more reading on this. What do you recommend I look up to investigate heat in forms that is not kinetic energy?
 
  • #8
Ygggdrasil
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Here's my attempt at a simplified explanation in a previous thread (https://www.physicsforums.com/threads/why-does-the-potential-energy-get-lower-as-atoms-get-closer.897854/):
Think of the potential energy curve as an actual landscape, and imagine rolling a ball down the PE curve. The ball will roll down to the minimum of the curve, but it's kinetic energy will carry it past the equilibrium and up the steep repulsive slope of the curve. The ball will eventually fall back down, and if no energy was lost, the ball will have enough momentum to carry it back out to where PE = 0. This happens a lot in intermolecular interactions—two molecules colliding in an unproductive reaction.

How a chemical bond forms is when your molecules have collided together and collide with a third molecule, transferring some of their kinetic energy to that molecule. Now, the total kinetic + potential energy of the system will not allow the two molecules to fly apart. If you think back to the rolling ball analogy, consider the effect of friction. Friction will cause the ball to lose some of its initial energy, so that when the ball rolls back up the initial slope, it cannot get to the original PE = 0 position, so it rolls back down and is trapped in the "potential energy well" formed by the intermolecular attraction. Just like the ball will roll back and forth in the PE well, the molecule will vibrate as the atoms move toward and away from each other as if connected by a spring. Further collisions with the surroundings will gradually remove energy from the system until the total energy of the system is close to the minimum PE energy on the potential energy curve. (For reasons related to quantum mechanics, the molecules never stop vibrating, leading to a "zero point energy").
The transfer of vibrational energy from the bond to the surrounding molecules is how an exothermic reaction can transfer heat to the surrounding.
 
  • #9
In an exothermic reaction heat is evolved as the molecules gain stability to form compound.
 

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