Gibbs free energy






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Shanzey
Posts: 120
Joined: Wed Sep 18, 2019 12:20 am

Gibbs free energy

Postby Shanzey » Sat Feb 08, 2020 10:31 pm

I don't really understand what Gibbs free energy is conceptually, and how it is related to enthalpy and entropy. I understand that, by looking at the equation, an increase in enthalpy increases Gibbs free energy and an increase in entropy decreases Gibbs free energy, but why is that?

Tiffany_Chen 2K
Posts: 106
Joined: Fri Aug 30, 2019 12:15 am

Re: Gibbs free energy

Postby Tiffany_Chen 2K » Sat Feb 08, 2020 10:34 pm

Really depends on specific values (using the equation). We should cover the +/- conditions post midterms. For entropy, you can think of the second law, where spontaneity and increase in entropy relates (-deltaG corresponds with spontaneous reactions).

Andres Merlos 2L
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Joined: Wed Sep 18, 2019 12:17 am

Re: Gibbs free energy

Postby Andres Merlos 2L » Sun Feb 09, 2020 12:36 pm

Gibbs free energy is the energy in a chemical reaction that can be used to do work. Since enthalpy is the total heat content of a system, you would subtract entropy, which is the unavailable energy that is used for work, with temperature. This gives us the energy that can be used for work.

DLee_1L
Posts: 103
Joined: Sat Aug 17, 2019 12:17 am

Re: Gibbs free energy

Postby DLee_1L » Sun Feb 09, 2020 2:46 pm

Andres Merlos 2L wrote:Gibbs free energy is the energy in a chemical reaction that can be used to do work. Since enthalpy is the total heat content of a system, you would subtract entropy, which is the unavailable energy that is used for work, with temperature. This gives us the energy that can be used for work.


So to clarify, a negative Gibbs free energy means that the reaction is spontaneous and the energy is actually "free"? And if the Gibbs free energy is positive, that means that to get an energy release, you need to first put some energy into the system?

Lauren Stack 1C
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Joined: Sat Aug 17, 2019 12:18 am

Re: Gibbs free energy

Postby Lauren Stack 1C » Sun Feb 09, 2020 6:31 pm

DLee_1L wrote:
Andres Merlos 2L wrote:Gibbs free energy is the energy in a chemical reaction that can be used to do work. Since enthalpy is the total heat content of a system, you would subtract entropy, which is the unavailable energy that is used for work, with temperature. This gives us the energy that can be used for work.


So to clarify, a negative Gibbs free energy means that the reaction is spontaneous and the energy is actually "free"? And if the Gibbs free energy is positive, that means that to get an energy release, you need to first put some energy into the system?


A negative Gibbs free energy value means the the reaction is spontaneous and will proceed without an input of energy. If it is a positive value, you must input energy to cause it.
Here are some diagrams that helped me understand the basis of delta G in the context of a reaction
Image
Here you see the energy at the beginning of the exergonic reaction is higher than the end, showing a negative delta G value and denoting that it will proceed without an energy input. Conversely, if you look at the endergonic graph, the energy in the products is higher than the reactants, showing an input of energy would be required to get there.


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