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Topic: Free Energy vs. Enthlapy  (Read 4629 times)

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Offline gatohedonista

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Free Energy vs. Enthlapy
« on: November 12, 2007, 03:42:33 PM »
Hey,

      Can someone please explain the difference between Free Energy and Enthalpy?

My idea of the concept is the following,

Enthalpy is the amount of heat given off in a chemical reaction, whereas Free Energy is the work that you can do with the heat from that chemical reaction.

Any suggestions?

Thanks,
GH

Offline TheWall04

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Re: Free Energy vs. Enthlapy
« Reply #1 on: November 12, 2007, 08:57:23 PM »
Hey!
There is actually quite a difference between Free energy and Enthalpy. I'll try and explain it the best i can to you.

Free Energy (Also known as Gibbs free energy) and represented by the symbol "G" is a thermodynamic function designed to produce a kind of "rule" or "principal" for spontaneous change. It is defined by the equation;
G = H - TS
Basically in thermochemistry, free energy is a measure of the amount of mechanical (or any other) work that can be extracted from a system. It is a subtraction of the entropy of a system ("useless energy") from the total energy, yielding a thermodynamic state function which represents the "useful energy".

On the other hand theres enthalpy (or heat content), represented by the symbol "H," which is a thermodynamic funtion used to describe constant-pressure processes using the equation;
H = U + PV . and at a constant pressure, DeltaH = DeltaU + P DeltaV
In short, enthalpy is a quotient or description of thermodynamic potential of a system, which can be used to calculate the "useful" work that can be obtained from a closed thermodynamic system under constant pressure.

I hope this helps you understand a little better. If not let me know and i can try and approach it differently for you.



Offline gatohedonista

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Re: Free Energy vs. Enthlapy
« Reply #2 on: November 13, 2007, 12:36:53 AM »
Hi TheWall04,

     First of all, thanks for replying.  :) Well, all the information that you posted looks very similar to what I got from wikipedia. That's the reason why I posted this question here, because in wikipedia they don't really compare and contrast both of them. I wish you could explain it to me excluding the jargon, I think that's what throws me off. This is what I got so far,

Enthalpy, heat content, refers to the amount of heat a certain reaction can release or absorb. Let's stick with an exothermic reaction for now. As far as I know, the enthalpy in an exothermic reaction would be the heat given off when the reaction takes place. Now, this heat is also energy, and energy can be used to do "work" on a different system.

Gibbs free energy, using the last example, refers to the work that can be done with that heat or that energy from the exothermic reaction. However, in Gibbs free energy you take the enthalpy from that exothermic reaction, but you subtract the entropy. That's why, when entropy is neglected because it is too small, Enthalpy nearly equals Gibbs free energy.

I wish you could explain it to me using your words and using examples if it's not too much trouble.

Thanks,
GH

Offline TheWall04

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Re: Free Energy vs. Enthlapy
« Reply #3 on: November 13, 2007, 01:27:45 AM »
Here is a comparison i found online:
A system’s enthalpy is only entropy change (after DH is divided by T) if it is transferred to the surroundings and no work of any sort is done there in the surroundings. A surroundings’ enthalpy is only entropy change (after DH being divided by T) when it is transferred to the system and no work is then performed in the system. Gibbs free energy change, DG, is only considered entropy change(after being divided by T) when no useful work of any kind is done by the heat transfer in the system or in the surroundings.

But if that doesn't make sense to you, then in short, the main difference between them is that the free energies of formation of most substances do change significantly as the temperature changes. And the enthalpies of formation of most substances do not change significantly as temperature changes. For the enthalpies of formation, it is often quite reasonable to estimate values on the basis that the change with temperature is insignificant.. that being said, enthalpy reactions change much less with temperature change, than that of free energy.

As for examples, just type into google "free energy" and "enthalpy" and you will get a whole bunch of worked and unworked examples. Just compare them is my advice and you may better understand it. Theres only so much you can learn from words. Attempt to do some problems for each and then you may better grasp the exact difference between them in your own words. And have a visual understanding to go along with it.

I hope this helped a little more?

Offline CT101

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Re: Free Energy vs. Enthlapy
« Reply #4 on: November 13, 2007, 02:50:40 PM »
When the students I tutor ask me this question this is how I answer it: Enthalpy is the amount of heat absorbed or released by a chemical reaction. Free energy is a measurement of whether or not said reaction will occur by itself or not, ie spontaneity. Does it have enough juice to occur on its own? The next question is to what extent will this reaction occur (will I get a little product or a lot?), which is a equilibrium topic. Free energy is always used to calculate the equilibrium constant of a reaction.

It is misleading, however, to say delta G = delta H in an exothermic reaction.

Use creating electricity as an example. A generator is powered by the combustion of coal. Everyone knows combustion reactions are exothermic....when the coal is burned it gives off a lot of heat. This is the enthalpy. This very enthalpy, like you mentioned earlier, is what is used to do work on another system, powering a generator.

Now, is that coal burning spontaneous? No. It doesn't have enough free energy to do that. Why not? Because of coal's low entropy. Those carbon carbon bonds are rigid and very stable, so coal probably has a low (negative) entropy value. Multiply that by the high temperature necessary to break a bond that stable, and you get a negative TdeltaS. The absolute value of that TdeltaS is higher than the enthalpy of burning the coal, so when you crank out the math, you end up with a positive delta G which = a non-spontaneous reaction.

I think if you were to think of it like that it would get a lot less confusing.

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