Gibbs Free Energy Change of a Reaction

In summary, the conversation discusses the concept of Gibbs free energy change in a reaction and the confusion surrounding the statement of "constant chemical potentials". It is noted that in a continuous stirred tank reactor, the composition and chemical potentials remain constant, but the reaction is still occurring. However, in this case, the reaction never reaches equilibrium. It is also mentioned that in equilibrium, the change in free energy is zero, but a reaction can still occur near equilibrium with constant chemical potentials. The conversation concludes by discussing the difficulty of maintaining constant chemical potentials and how it affects the estimation of useful work in a chemical reaction.
  • #1
Amok
256
2
So, in my thermo book, it says that the gibbs free energy change of a reaction is the free energy received by the system at constant T,p and constant chemical potential when the extent of the reaction varies by one mol. The part that is confusing me is the "constant chemical potentials"statement. If the extent of the reaction is changing, aren't the chemical potentials changing as well? Can the change in free energy of a reaction be used to predict how much work one can get out of a chemical reaction?
 
Chemistry news on Phys.org
  • #2
Think of a reaction taking place in a continuous stirred tank reactor. The composition of the mixture in the tank remains constant and so do all the chemical potentials. Nevertheless, as both educts are continuously added and products removed, a chemical reaction is constantly taking place.
 
  • #3
But that means you never reach an equilibrium state (unless you started at one). It's funny because people always seem to forget this when they talk about G free energy, they usually only mention constant T and P.
 
  • #4
Yes, in equilibrium Delta G is zero. However, a chemical reaction can also occur (arbitrarily near) equilibrium with constant chemical potentials of the reactants.
Consider a reaction chamber which where H2 O2 and H2O are in equilibrium and H2 and O2 can diffuse into the chamber via semi-permeable membranes from their respective reservoirs and H2O can be removed by diffusion through a similar membrane semipermeable for water only.
By increasing infinitesimally the pressure (i.e. the chemical potentials) of H2 and O2 and reducing the pressure of H2O in the respective reservoirs you can synthesise an arbitrary amount of water reversibly with Delta G =0 and the chemical potentials of the components hold constant.
 
  • #5
Yes, that's ture. I'm just saying that those conditions are not always present, and people still use delta-g to estimate how much useful work one can get out of chemical reaction. Having constant p and T is easy, having constant chemical potential is harder.
 
  • #6
Yes, you are right. When you are burning a mixture of hydrogen and oxygen the chemical potentials change during that process and you have to integrate delta G over the reaction coordinate to get the maximal amount of work you can extract.
 

What is Gibbs Free Energy Change of a Reaction?

Gibbs Free Energy Change of a Reaction, also known as ΔG, is a measure of the amount of energy available to do work in a chemical reaction. It takes into account both the change in enthalpy (ΔH) and the change in entropy (ΔS) of a reaction.

How is Gibbs Free Energy Change of a Reaction related to spontaneity?

The sign of ΔG indicates the spontaneity of a reaction. A negative ΔG indicates a spontaneous reaction, meaning the reaction will occur without any external input of energy. A positive ΔG indicates a non-spontaneous reaction, meaning the reaction will not occur without an input of energy.

What is the equation for calculating Gibbs Free Energy Change of a Reaction?

The equation is: ΔG = ΔH - TΔS, where ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy. This equation is also known as the Gibbs-Helmholtz equation.

What is the significance of the magnitude of ΔG?

The magnitude of ΔG determines the direction and extent of a reaction. A larger negative value of ΔG indicates a more spontaneous and energetically favorable reaction, while a smaller negative value indicates a less spontaneous and less energetically favorable reaction.

How does temperature affect ΔG?

Temperature has a significant effect on ΔG. As temperature increases, the value of ΔG becomes less negative, making the reaction less spontaneous. This is because the increase in temperature leads to an increase in ΔS, which counteracts the negative contribution of ΔH in the equation.

Similar threads

Replies
4
Views
1K
Replies
10
Views
1K
Replies
8
Views
2K
Replies
11
Views
2K
Replies
13
Views
3K
Replies
3
Views
2K
Replies
1
Views
2K
Replies
6
Views
2K
Back
Top