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Author Topic: How do chemical bonds store energy?  (Read 11770 times)

Offline LukyTom

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How do chemical bonds store energy?
« on: 03/03/2009 05:39:42 »
Hey there.

We always learn in school that bonding between certain particles (atoms/molecules) contain energy, and that when this bond is broken, energy is released. What does this means in terms of electron interaction?
« Last Edit: 03/03/2009 09:12:37 by chris »


 

Offline Chemistry4me

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Re: How do chemical bonds store energy?
« Reply #1 on: 03/03/2009 05:54:29 »
What does this means in terms of electron interaction?
Can you expand on that one?
 

Offline Chemistry4me

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Re: How do chemical bonds store energy?
« Reply #2 on: 03/03/2009 06:14:22 »
The bond energy is the energy required to break one mole of isolated bonds to give isolated atoms or other uncharged fragments.
 

Offline LukyTom

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Re: How do chemical bonds store energy?
« Reply #3 on: 03/03/2009 06:14:22 »
For say combustion of methane, how does this release energy? How is heat energy (flame) produced here?
 

Offline LukyTom

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Re: How do chemical bonds store energy?
« Reply #4 on: 03/03/2009 06:32:11 »
Perhaps my question is a bit hazy. I meant in terms of whats is precisely happening in the electron level in its relation to the releease of energy. I.e. persay with Ionic Bonded compound, when broken up, electrostatic? forces are converted into some other form of energy?
 

Offline lancenti

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How do chemical bonds store energy?
« Reply #5 on: 03/03/2009 11:56:41 »
The "English" answer is at the bottom, but I always like to begin with a Mathematical Demonstration. It's a kind of OCD I have.

Taking your example for the Combustion of Methane,

CH4(g) + O2(g) → CO2(g) + 2 H2O(l)

This reaction could also be seen as going through an extra step, namely:

CH4(g) + O2(g) → C(s) + 4 H(g) + 2 O(g)

If I remember right, that step is called atomization. The energy required to atomize methane is four times the bond energy of the C-H bond. According to the data booklet I ripped from here, this value is 413 kJmol-1 per bond giving us a total of 1652 kJmol-1. The O=O on the other hand, takes 498 kJmol-1. This gives us a grand total of 2150 kJmol-1 for the atomization reaction.

Now, the second step of the reaction would then be

C(s) + 4 H(g) + 2 O(g) → CO2(g) + 2 H2O(l)

We would be forming 2 C=O and 4 O-H bonds which would take -740 kJmol-1 and -366 kJmol-1 per bond respectively. This gives us a grand total of -2944 kJmol-1.

Now, if we were to apply Hess' Law, we would find that the Overall Reaction's Enthalpy is equal to the sum of the two. This gives us a net energy change of -794 kJmol-1 of methane combusted. Thus energy is released.

But you're probably thinking that doesn't answer your question.

All things want to move down to a lower energy state as this is the same as stability. So the more stable something is, the more energy it takes to raise it back to its unstable state. Now, in Methane, the bonds are fairly strong and this is why Methane and most hydrocarbons are inert. However, when you combust Methane, the electrons find that they can enter a lower energy state by forming C=O bonds and O-H bonds rather than staying in their state of C-H and O=O bonds. So they 'rearrange' themselves to leave CO2 and H2O. But there is an amount of energy that is not accounted for in the bonds, and that would be the 794 kJmol-1 floating about. This is released as heat which we can then use to cook our food among other things.

... in short, the electrons go into a lower energy state and the excess energy they release as heat.
« Last Edit: 03/03/2009 11:59:08 by lancenti »
 

Offline lightarrow

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How do chemical bonds store energy?
« Reply #6 on: 03/03/2009 12:03:49 »
We always learn in school that bonding between certain particles (atoms/molecules) contain energy, and that when this bond is broken, energy is released. What does this means in terms of electron interaction?
There aren't electrons only, there are nuclei too...
Simplistically, it works this way:
chemical energy stored in chemicals is electromagnetic potential energy: when chemicals reacts, charged particles rearrange themselfe so that the total electromagnetic potential energy decreases (in which case energy is released) or increases (in which case energy is absorbed from the environment).
 

Offline LukyTom

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How do chemical bonds store energy?
« Reply #7 on: 10/03/2009 10:01:57 »
Wow, thank you guys so much for the really detailed answers.
 

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How do chemical bonds store energy?
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