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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: mahela007 on May 15, 2010, 11:42:22 PM

Title: Why does Gibbs free energy require constant pressure?
Post by: mahela007 on May 15, 2010, 11:42:22 PM

All text books and other sources say that the equation for Gibbs free  energy requires that the process by carried out under constant pressure. Why is this?

Title: Re: Why does Gibbs free energy require constant pressure?
Post by: Grundalizer on May 16, 2010, 12:15:04 AM

http://en.wikipedia.org/wiki/Gibbs_free_energy

Title: Re: Why does Gibbs free energy require constant pressure?
Post by: Yggdrasil on May 16, 2010, 02:38:13 AM

Gibbs free energy (G) is a function of the temperature and pressure of a system as well as the number of molecules of each chemical species (this has to do with the way G is defined).  Therefore, the G is useful for reactions occurring at constant temperature and pressure, because the change in G will reflect only free energy released/absorbed by the reaction you want to study.

There are other definitions of free energy that are useful in other situations.  For example, the Helmhotz free energy depends on the volume of the system and is therefore useful for reactions that occur under conditions of fixed volume.

Title: Re: Why does Gibbs free energy require constant pressure?
Post by: OrganicSynthesis on May 17, 2010, 02:36:03 PM

Enthalpy and Entropy changes with pressure, so G = H - ST changes with pressure.