KE and PE for Endothermic and Exothermic Reactions

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magdi1989

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Hi all I've been searching for this answer but i couldn't find it, so please help me with it. Thanks!

So an.....
Endothermic Reaction has Higher PE and Lower KE?
&
Exothermic Reaction has Higher KE and Lower PE?

IM CONFUSED AS HELL ON THIS!!!! LOL

Thanks again everyone for all your help! 🙂

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Okay, I hate you bringing this up, because it's physics related, and physics is the spawn of satan.

All the concepts like this are interrelated, so I'll use a concept to see if it helps you understand/remember how this works better.

I'll just focus on kinetic energy, since potential and kinetic energies are opposites, you can deduce that when kinetic energy increases, potential energy decreases, etc (you got that?). This is how to relate kinetic energy to everything in chemistry: as temperature increases so does your average kinetic energy. There, that's it, simple.

So lets relate temperature to exothermic/endothermic and bonds. When you boil water, or melt ice, what two things are generally occuring? You're breaking bonds and you're increasing the temperature; just from my mention of temperature, you should automatically go AHAH, an increase in temperature means an increase in kinetic energy! When you break bonds, you put energy in to force those bonds to break, energy in is endothermic, right? Putting heat in (to raise the temperature) is endothermic, right? So endothermic processes increase kinetic energy (while PE is obviously decreasing).

Going the opposite way, as you freeze something, bonds are being formed and temperature is decreasing (exothermic, losing heat), so considering temp decreases, so is our kinetic energy decreasing (not much movement going on in that block of ice).

Well, hope that deconfuses you.

Cheers

P.S. No more physics questions, NO MORE. *flails madly*
 
Okay, big update here on this topic, this was a reply from the famed Chad, and I think it clears things up, but god, it also made things a tad more confusing to me, but I think it is a very very good clarification of all this craziness.
The main reason for confusion here is the terminology. First, in physics we often talk about situations where mechanical energy is conserved so if an object is losing potential energy, it must be gaining kinetic energy as when an object is dropped off a building. But in chemistry we typically view what your bio book refers to as potential energy as internal energy ('stored' energy) and so it isn't defined in the same way as we would in physics.

In chemistry we often talk about a situation in which we heat a substance which results in greater molecular motion and therefore greater kinetic energy of the atoms/molecules in that substance. A gas tends to 'store' energy as kinetic energy and so in this way the internal energy of the gas is being 'stored' as kinetic energy. Your bio review book refers to what is properly called 'internal energy' as 'potential energy' and might then call this kinetic energy stored by a gas potential energy which would make no sense whatsoever from a physics perspective. So much confusion!!!

In applying this to chemical rxns, heat isn't simply increasing molecular motion but is often converted to what I'll call chemical energy (or chemical potential energy) which involves making and breaking bonds. Chemical potential energy can be released in the context of a chemical rxn and is released as heat in an exothermic rxn. It is in this context that your bio review book stated that an exothermic rxn decreases potential energy and that an endothermic rxn increases potential energy. Not the normal terms we might use in chemistry but it is conceptually correct.

I'll agree with you that this is confusing but I hope this helps.

So I guess in this whole exothermic/endothermic KE/PE for chemistry context, we can think of heat as potential energy (internal energy), so with that in mind, just think of heat lost as potential energy lost (exothermic), and heat gained as potential energy gained. Crazy, but apparently it works in this situation (although it seems based on Chad's explanation, that we should be calling it "internal energy" not potential energy, as mis-labeling of sorts, probably the root of all the confusion actually since it clashes with physics).
 
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Kahr
Temp Increase = KE Increase = Endothermic = PE Decrease

Chad
Temp Decrease "Heat Lost"= Exothermic = PE Decrease "PE Lost"

Am I reading that correctly? If so then 😕
 
Kahr
Temp Increase = KE Increase = Endothermic = PE Decrease

Chad
Temp Decrease "Heat Lost"= Exothermic = PE Decrease "PE Lost"

Am I reading that correctly? If so then 😕

You are reading that correctly. The reason it is confusing and conflicts is because my explanation was based on a purely physics point of view, which is correct. Chad was explaining why the OP of this thread and my AP Biology review book was saying the opposite. It was focusing on heat or internal energy as potential energy. Which is why the OP's resource and my review book were saying that exothermic rxn = PE decrease, because heat is given off/lost. Chad was also saying that they are incorrectly calling it potential energy, when they should only be calling it internal energy, which is really the real cause for confusion here. So the key is to think only about PE/internal energy here, not relate it to KE at all, keep the two as separate concepts for now(although if they asked a question regarding KE and related it to temperature, I would still go with the increase in temperature is an increase in the avg KE, etc); and just forget physics. Sorry if this doesn't clear anything up for you.
 
You are reading that correctly. The reason it is confusing and conflicts is because my explanation was based on a purely physics point of view, which is correct. Chad was explaining why the OP of this thread and my AP Biology review book was saying the opposite. It was focusing on heat or internal energy as potential energy. Which is why the OP's resource and my review book were saying that exothermic rxn = PE decrease, because heat is given off/lost. Chad was also saying that they are incorrectly calling it potential energy, when they should only be calling it internal energy, which is really the real cause for confusion here. So the key is to think only about PE/internal energy here, not relate it to KE at all, keep the two as separate concepts for now(although if they asked a question regarding KE and related it to temperature, I would still go with the increase in temperature is an increase in the avg KE, etc); and just forget physics. Sorry if this doesn't clear anything up for you.
👍
 
ok guys, Chad said, temp decreases= exothermic since heat is lost but on destroyer its saying temp decrease=endothermic since the soln got cooler and heat was absorbed. Its number 57 on the 2011 destroyer.
This is so Frustrating!!
 
ok nvm. 93 on destroyer is saying decreasing temp means its exothermic. So I guess, we should just stick with Chad.
Let me know if I'm reading it incorrectly. This stuff is really confusing!
 
Sorry to bring this back up but can someone clarify this?

If a reaction is EXOTHERMIC, it decreases Potential Energy AND Kinetic Energy simultaneously?
 
Sorry to bring this back up but can someone clarify this?

If a reaction is EXOTHERMIC, it decreases Potential Energy AND Kinetic Energy simultaneously?

Its all relative to the system or surroundings. Endothermic reaction, system increases in heat, temperature increases, surroundings temp decrease.
 
I've always related it to energy diagrams. Is that incorrect?

An exothermic reaction ends up lower than it began, so it "fell", meaning energy was released and it has less potential energy to give now.

For the endothermic reaction, energy was gained at the end of the reaction, so since it's "higher" than the reactants it means it gained potential energy.

Is this flawed?
 
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