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Why are reactions exo/endothermic?

What is the difference between these two reactions (and please don't just say 'one gives out energy' :colonhash:) and what causes different reactions to behave in this way?

Edit - I know about bond enthalpies! That isn't my question!
(edited 9 years ago)

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Reply 1
Avatar for Tillsy
Tillsy
7
It basically describes heat change of a reaction under constant pressure.
So if you did an endo, you would feel the test tube getting colder
If you did an Exo, you would feel the test tube getting warmer


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Original post by Tillsy
It basically describes heat change of a reaction under constant pressure.
So if you did an endo, you would feel the test tube getting colder
If you did an Exo, you would feel the test tube getting warmer


Posted from TSR Mobile


Yeah, I know, but what causes certain reactions to take in energy? How are these reactions different from ones that give out energy?
Reply 3
Avatar for Petulia
Petulia
19
Bond breaking is endothermic which absorbs heat.
Bond formation is exothermic which releases heat.
If more bonds are made than broken, then the reaction is exothermic.
If more bonds are broken than made, then the reaction is endothermic.
To break a bond energy has to be taken in so the energy of the reaction goes up and the energy (heat) of the surroundings goes down.

To make a bond energy is released making the energy of the reaction go up and the energy (heat) of the surroundings go up.

If more energy is taken in than given out (each bond requires a certain amount of energy to break/make) then the reaction is endothermic and vice versa.
It's to do with the bonds being formed and broken in a reaction.

Bond breaking is endothermic
Bond forming is exothermic.
So if more energy is needed to break the bonds than is given out when the bonds are formed, the reaction is endothermic.

If more energy is given out when the new bonds are formed than is taken in to break the bonds, the reaction is exothermic.
Original post by Petulia
Bond breaking is endothermic which absorbs heat.
Bond formation is exothermic which releases heat.


Why does bond formation release energy though?

Intuitively I've understood that bond breaking requires energy (because you need to split up the bonds), but never actually understood why bond forming releases energy, apart from it being the reverse of bond breaking.
(edited 9 years ago)
Original post by Arithmeticae
Why does bond formation release energy though?

Intuitively I've understood that bond breaking requires energy (because you need to split up the bonds), but never actually understood why bond forming requires energy, apart from it being the reverse of bond breaking.


It's pretty logical if you think about it. You have to put in work or energy to make anything
Reply 8
Avatar for Petulia
Petulia
19
Original post by Arithmeticae
Why does bond formation release energy though?

Intuitively I've understood that bond breaking requires energy (because you need to split up the bonds), but never actually understood why bond forming requires energy, apart from it being the reverse of bond breaking.


Bond breaking takes in energy because energy is required to break the bonds. Bond making releases energy because the atoms are stable when in a bond.
I don't think you need to know this in detail at AS though.
Original post by Arithmeticae
What is the difference between these two reactions (and please don't just say 'one gives out energy' :colonhash:) and what causes different reactions to behave in this way?


The chemical explanation at igcse level is that making bonds is an ectothermic process, releasing heat energy and breaking bonds is an endothermic process, which requires energy. The amount of energy used to break bonds(-) and the amount of energy released to make them(+) is added(remembering that one is negative the other is positive). This give enthalpy change, if this value is negative then it's exothermic, if it's positive then it's endothermic.
Reply 10
Avatar for saad97
saad97
2
Original post by Arithmeticae
Why does bond formation release energy though?

Intuitively I've understood that bond breaking requires energy (because you need to split up the bonds), but never actually understood why bond forming requires energy, apart from it being the reverse of bond breaking.


ENERGY IS NEVER CREATED NOR DESTROYED!

So everything and has energy within it and when the bonds are MADE*, obviously it will be released and converted into other types of energy. It's pretty straightforward to be honest... I think you're looking into it too much and confusing yourself. :smile:


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(edited 9 years ago)
It's just common sense, some chemical reactions need more energy to break the bonds of the reactants, than the energy that is released when the bonds of the products are formed.

On the other hand, some chemical reactions don't require AS MUCH energy to break the bonds of the reactants, and more energy is released when the bonds creating the products are formed.

For instance if the bonds in the reactants of a chemical reaction are particularly strong, you'll need a lot of heat energy to break them, and when the bonds of the products are formed heat energy will be released but it won't be as much as the energy you had to provide for the breaking to happen.

And vice versa
Original post by easyastau
It's pretty logical if you think about it. You have to put in work or energy to make anything


But bond forming (basically making new molecules) gives out energy O_o

Original post by Petulia
Bond breaking takes in energy because energy is required to break the bonds. Bond making releases energy because the atoms are stable when in a bond.
I don't think you need to know this in detail at AS though.


So the atoms change from a less stable state to a more stable state, because the electrons from the outer orbitals of the products have a lower energy sum than the electrons of the atoms that form the bond?

Original post by GCSEsitter
The chemical explanation at igcse level is that making bonds is an ectothermic process, releasing heat energy and breaking bonds is an endothermic process, which requires energy. The amount of energy used to break bonds(-) and the amount of energy released to make them(+) is added(remembering that one is negative the other is positive). This give enthalpy change, if this value is negative then it's exothermic, if it's positive then it's endothermic.


This isn't what I'm asking, I know this already.
(edited 9 years ago)
Original post by saad97
ENERGY IS NEVER CREATED NOR DESTROYED!

So everything and has energy within it and when the bonds are broken, obviously it will be released and converted into other types of energy.
It's pretty straightforward to be honest... I think you're looking into it too much and confusing yourself. :smile:


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I said released and required, nothing in that post suggests that energy is created or destroyed.

Bond breaking takes energy, it doesn't release it... :/
(edited 9 years ago)
Original post by saad97
ENERGY IS NEVER CREATED NOR DESTROYED!

So everything and has energy within it and when the bonds are broken, obviously it will be released and converted into other types of energy. It's pretty straightforward to be honest... I think you're looking into it too much and confusing yourself. :smile:


Posted from TSR Mobile


This is dangerously wrong ...
Reply 15
Avatar for saad97
saad97
2
Original post by Arithmeticae
I said released and required, nothing in that post suggests that energy is created or destroyed.

Bond breaking takes energy, it doesn't release it... :/


Crap sorry, my bad, I meant bond making*!!!


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Reply 16
Avatar for Borek
Borek
4
Imagine having two balls with opposite charges, kept at some distance. They attract each other, which means they have some potential energy. You allow them to get closer to each other - their potential energy gets lower. What happened to the rest of the energy? It was released - and one of the easiest ways of releasing energy is giving heat.

It is very similar for atoms, even if they are not necessarily initially charged. That's because the charges (electrons) are shifted to different positions and the net effect is very similar.
Original post by Arithmeticae


So the atoms change from a less stable state to a more stable state, because the electrons from the outer orbitals have a lower energy sum than the electrons of the atoms that form the bond?



Dude you're overthinking this waaaaaay too much.

Exothermic = MORE energy given OUT forming bonds, than energy given IN while breaking bonds. Temperature increases.

Endothermic = MORE energy given IN to break bonds, than energy given OUT while forming bonds. Temperature decreases.

That's it. That's all you have to know. Now MOVE ON! This is only 0.00000001% of Unit 2
(edited 9 years ago)
Original post by Borek
Imagine having two balls with opposite charges, kept at some distance. They attract each other, which means they have some potential energy. You allow them to get closer to each other - their potential energy gets lower. What happened to the rest of the energy? It was released - and one of the easiest ways of releasing energy is giving heat.

It is very similar for atoms, even if they are not necessarily initially charged. That's because the charges (electrons) are shifted to different positions and the net effect is very similar.


Thanks, this clears it up :smile:

So it's due to the Coulomb force between charged particles (and the polarization of the charges which still results in a force being created between the two)?
Original post by BarBaBaBaBarBaBaBa


That's it. That's all you have to know. Now MOVE ON! This is only 0.00000001% of Unit 2


Lol, I'm just curious about why it happens XD

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