Understanding Thermochemical Reactions: Breaking and Forming Bonds

[cam875]

Im not sure if I am understanding this correctly, when atoms in a molecule break apart is it the energy that was previously contained within the bond holding the atoms together that is released causing heat? and when creating a molecule out of atoms, does it require energy in order to use to form the bonds? or am I confusing forces and energy?

 

[Ygggdrasil]

In general, breaking bonds costs energy while forming bonds releases energy.

 

[cam875]

so how is it that when ATP is broken by losing a phosphate into ADP it releases energy for a cell?

 

[Borek]

Some bonds are broken, but then some new are formed. Overall effect is energy production, but initially you need to put some energy into the process.

 

[Ygggdrasil]

The phosphodiester bond between the beta and gamma phosphates (i.e. the last two phosphates) in ATP is a very weak bond thermodynamically. Therefore, breaking this bond and forming a bond with -OH from water is a very exothermic reaction because forming a bond with -OH releases a good amount of energy. The reaction is further made favorable by the increase in entropy. Finally, because the cell maintains a high concentration of ATP and a low concentration of ADP and phoshphate, the reaction has an even more favoarable free energy change.

 

[cam875]

ok I am starting to understand, I am gunna give an example of how I think it works and someone just correct me where i go wrong.

Two atoms contain energy and they decide to bond but when they bond the energy of the atoms leaves them and becomes other energy, and therefore energy is produced?

and when two atoms break apart from a bond they both gain there rightful energy back or something again? and therefore energy is lost? This all seems a little weird.

And in cell respiration where does the initial energy come from to break apart the first phosphate before that phosphate can even think about bonding with OH

 

[Borek]

and when two atoms break apart from a bond they both gain there rightful energy back or something again? and therefore energy is lost? This all seems a little weird.

Nothing is lost - to break them apart you have to give them back their "rightful" energy. Exactly the same amount they gave away when bonding.

Pease note that nomenclature you have used (and I have repeated) has nothing to do with the nomenclature we should be using

 

[cam875]

but why exactly does an atom have to give away energy in order to form a bond with another atom?

 

[Borek]

Lower energy states are more stable. So atoms give away energy, but gain stability.

 

[Fearless]

I don't know how much physics you have been toting yer head against. But here goes an explanition, if something was unclear, please ask away!

1. We could look at the bonds as normal modes of wave-functions. we could for example say that they are strings which resonate with a certain frequency. The higher the frequency the higher the energy.

2. But to actually really know what a bond is (the wave model is somewhat accurate but still bad due to some unforseen quantum effects). A bond is almost like a quantum well. In this quantum well you have a potential. When you break the bond, you must get the "particles in the well" to come up from it and break free (often electrons are used for the simple case). You can only excite the particle to a certain level and breaking the barrier (like E > 0) if you somehow induce the system with energy. When bonds form, the particle(s) gets down this quantum potential and the molecule as a whole gains energy but, the particle gives it away (a very crude description... maybe).

Feel free to comment, I was having dinner and didn't feel like taking books down the shelf. ;)

 

[cam875]

wow, that's actually a pretty cool explanation but I don't understand what you mean by a quantum well? could u try to explain what that actually is, and hopefully ill be able to understand it.

 

[Fearless]

well well, what is the quantum well ;)

it's a way of confining particles that is free in three dimensions to a 2D-surface. Like an electron in a pi-bond.

Now look at this picture. http://upload.wikimedia.org/wikipedia/en/thumb/e/e6/Square_potential.png/350px-Square_potential.png

What you see there is the potential to the left, and a barrier. The finite barrier is of width a. A bond is usually between 1 to 5 Ångström long. A molecule is much longer by comparison.

The bond is essentially this width and the electron usually is in the potential-well because of being there lowers the energy for the molecule at this instant in time and space.

 

[Borek]

it's a way of confining particles that is free in three dimensions to a 2D-surface. Like an electron in a pi-bond.

Do you mean that quantum well has to be 2D? What about 1D? 3D?

 

[cam875]

why would it be 2d if everything is 3d or even 10d according to super string?

 

[Fearless]

Borek: of course it can have infinite dimensions, but for a first touch of QM, it's more pedagogic to just state something. Then, when they get more interested you can add more dimensions.

 

[Borek]

Borek: of course it can have infinite dimensions, but for a first touch of QM, it's more pedagogic to just state something. Then, when they get more interested you can add more dimensions.

Making up justification to your mistake will not work. It is very non-pedagogic to "just state something" that is wrong

 

[chemisttree]

ok I am starting to understand, I am gunna give an example of how I think it works and someone just correct me where i go wrong.

Two atoms contain energy and they decide to bond but when they bond the energy of the atoms leaves them and becomes other energy, and therefore energy is produced?

and when two atoms break apart from a bond they both gain there rightful energy back or something again? and therefore energy is lost? This all seems a little weird.

And in cell respiration where does the initial energy come from to break apart the first phosphate before that phosphate can even think about bonding with OH

Are you still having trouble understanding this?

 

[Fearless]

Making up justification to your mistake will not work. It is very non-pedagogic to "just state something" that is wrong

Ok, so what is a quantum well then? I am eager to be proved wrong.

 

[Borek]

Ok, so what is a quantum well then? I am eager to be proved wrong.

First of all - it is not

a way of confining particles that is free in three dimensions to a 2D-surface.

Quantum well is the space (section, area, volume) to which the particle is confined by barriers on sides. Number of dimensions - be it 1, 2 or 3 - doesn't matter. What matters is that particle is confined.

 

[Fearless]

Borek: Thanks, You have given me a better understanding of the phenomena. :)