# Explain free energy **URL:** \ **Category:** Factual Questions\ **Created:** [May 2, 2007, 3:53pm UTC](https://boards.straightdope.com/t/explain-free-energy/402476 "2007-05-02T15:53:34Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Jelymag](https://avatars.discourse-cdn.com/v4/letter/j/d2c977/32.png) [@Jelymag](https://boards.straightdope.com/u/Jelymag)\ **Post date:** [May 2, 2007, 3:53pm UTC](https://boards.straightdope.com/t/explain-free-energy/402476/1 "2007-05-02T15:53:34Z")
My knowledge of thermodynamics is _lacking_. I find that I can not adequately define free energy. It can’t be measured by any kind of instrument. It has units of kJ. You can calculate it provided you are given the right data, but it’s not possible to directly observe. You can determine the spontaineity of a reaction based on its quantity and sign… So clearly it is very useful, but what IS it??? I’m tempted to say it is some non-existent property that Josiah Gibbs just made up, but I know that isn’t true. I told my students it didn’t matter that I don’t know how to define it, they still have to be able to calculate it 😃 ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [May 2, 2007, 4:09pm UTC](https://boards.straightdope.com/t/explain-free-energy/402476/2 "2007-05-02T16:09:40Z")
Here’s a good place to start > **[Thermodynamic free energy](https://en.wikipedia.org/wiki/Thermodynamic_free_energy)** > > In thermodynamics, the thermodynamic free energy is one of the state functions of a thermodynamic system (the others being internal energy, enthalpy, entropy, etc.). The change in the free energy is the maximum amount of work that the system can perform in a process at constant temperature, and its sign indicates whether the process is thermodynamically favorable or forbidden. Since free energy usually contains potential energy, it is not absolute but depends on the choice of a zero point. Ther... ---
**Author:** ![Jelymag](https://avatars.discourse-cdn.com/v4/letter/j/d2c977/32.png) [@Jelymag](https://boards.straightdope.com/u/Jelymag)\ **Post date:** [May 2, 2007, 9:11pm UTC](https://boards.straightdope.com/t/explain-free-energy/402476/3 "2007-05-02T21:11:01Z")
Thank you **CookingWithGas**. That is interesting. I did not know that Gibbs free energy was not the only kind. I also didn’t know that Josiah Gibbs has his own U.S. postage stamp! ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 2, 2007, 10:40pm UTC](https://boards.straightdope.com/t/explain-free-energy/402476/4 "2007-05-02T22:40:59Z")
> [@](#): > > I told my students it didn’t matter that I don’t know how to define it, they still have to be able to calculate it I hope that _you_ can calculate it, if you’re teaching it? Well, then, there you go. That calculation is the definition. Really, this is the case for almost all quantities in physics: You can’t directly measure total energy, either. All you can do is calculate 1/2 mv[sup]2[/sup] for all objects, and qV, and mgh, and all the rest, and add all of those together. Then, define the result of that calculation to be “energy”. The only difference is that you’ve known about total energy for longer, so you’re more comfortable with it. ---
**Author:** ![scm1001](https://avatars.discourse-cdn.com/v4/letter/s/c4cdca/32.png) [@scm1001](https://boards.straightdope.com/u/scm1001)\ **Post date:** [May 2, 2007, 11:11pm UTC](https://boards.straightdope.com/t/explain-free-energy/402476/5 "2007-05-02T23:11:58Z")
My take (feel free to disagree or to quibble) What determines whether something happens is that the entropy (disorder) of the universe has to increase. That is all. However entropy is very difficult to measure (obviously a stacked deck of playing cards has less entropy than one scattered on the table but there is no meter that will measure it. One can calculate it though) However rather than try and define or measure the entropy of the universe, one breaks down the reaction into two main parts mainly for convenience (i) entropy of reaction (in principle calculable, and is indirectly measurable) (ii) entropy change in the rest of the universe (this usually is measured mainly as a heat gain, and what is known as the enthalpy. However, depending on the conditions (e.g. constant Pressure or Volume) the calculation of this term are slightly different. Hence the different types of free energy. So Gibbs free energy is a theoretical concept, and involves hard to measure items such as entropy of reaction. It works and is used because many of the terms can in principle (though often not in practice) be calculated from first principles. The measured or calculated terms then become extremely useful as they do not change much when changing conditions, so know the entropy and enthalpy at one PTV or set of conditions allows one a good stab at calculating whether the reaction will go at another PTV. By using correction factors such as activities the accuracy can be improved even more. ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [May 3, 2007, 1:11am UTC](https://boards.straightdope.com/t/explain-free-energy/402476/6 "2007-05-03T01:11:19Z")
[QUOTE=Chronos] You can’t directly measure total energy, either. [/QUOTE] Of course you can’t measure total energy. Energy takes its values in an R-torsor 😃 ---
**Author:** ![WarmNPrickly](https://avatars.discourse-cdn.com/v4/letter/w/d78d45/32.png) [@WarmNPrickly](https://boards.straightdope.com/u/WarmNPrickly)\ **Post date:** [May 3, 2007, 1:59am UTC](https://boards.straightdope.com/t/explain-free-energy/402476/7 "2007-05-03T01:59:50Z")
> [@](#): > > What determines whether something happens is that the entropy (disorder) of the universe has to increase. Not quite. (Delta)G = (Delta)H – T\*(Delta)S So a reaction with a decrease in entropy can still be spontaneous if (Delta)H is more negative than -T\*(Delta)S. Now I am not a physical chemist, and I am having trouble locating a reference that proves I am right, but I beleive that this is one reason many reactions go at high pressure go, that would never go at atmospheric pressure. I am specifically thinking of things like the Haber-Bosch proces where several small molecules (H[sub]2[/sub] and N[sub]2[/sub]) are combined into one molecule (NH[sub]3[/sub]), indicating that there is a negative overall entropy. (How do I make a (Delta) sign?) ---
**Author:** ![scm1001](https://avatars.discourse-cdn.com/v4/letter/s/c4cdca/32.png) [@scm1001](https://boards.straightdope.com/u/scm1001)\ **Post date:** [May 3, 2007, 4:12am UTC](https://boards.straightdope.com/t/explain-free-energy/402476/8 "2007-05-03T04:12:03Z")
[QUOTE=Christopher] Not quite. (Delta)G = (Delta)H – T\*(Delta)S So a reaction with a decrease in entropy can still be spontaneous if (Delta)H is more negative than -T\*(Delta)S. [/QUOTE] Not quite I was talking about the entropy of the universe. When you are talking about entropy you are only referring about the entropy of reaction. Huge difference. Reread my post ---
**Author:** ![gladtobeblazed](https://avatars.discourse-cdn.com/v4/letter/g/85f322/32.png) [@gladtobeblazed](https://boards.straightdope.com/u/gladtobeblazed)\ **Post date:** [May 3, 2007, 5:54am UTC](https://boards.straightdope.com/t/explain-free-energy/402476/9 "2007-05-03T05:54:03Z")
This should explain everything 😛 > **[Free Energy](http://free-energy.ws/index.php)** > > Advanced Alternative Energy Solutions by Peter Lindemann, D.Sc. ---
**Author:** ![WarmNPrickly](https://avatars.discourse-cdn.com/v4/letter/w/d78d45/32.png) [@WarmNPrickly](https://boards.straightdope.com/u/WarmNPrickly)\ **Post date:** [May 3, 2007, 5:59am UTC](https://boards.straightdope.com/t/explain-free-energy/402476/10 "2007-05-03T05:59:50Z")
I apologize then. I was taking “The Universe” to be any closed system. I don’t know anything about the thermodynamics of the universe. ---
**Author:** ![scm1001](https://avatars.discourse-cdn.com/v4/letter/s/c4cdca/32.png) [@scm1001](https://boards.straightdope.com/u/scm1001)\ **Post date:** [May 4, 2007, 3:31am UTC](https://boards.straightdope.com/t/explain-free-energy/402476/11 "2007-05-04T03:31:00Z")
[QUOTE=Christopher] I apologize then. I was taking “The Universe” to be any closed system. I don’t know anything about the thermodynamics of the universe. [/QUOTE] actually my post was a little unclear and its not suprising someone took its meaning somewhat different than I intended. Actually its not even clear that the entropy of the entire universe has to always increase (e.g if the universe contracts again). But if one takes any large closed system then the entropy must increase within, even though locally it may not appear to as you say.