How to calculate Gibbs energy of formation at a temp T using standard state data?

In summary, the individual was looking for information on calculating the energy of formation for a reaction, and found a helpful article on the Gibbs-Helmholtz equation. He was also wondering if it would be worthwhile to calculate the ΔH value for a reaction at a far different temperature than the standard temperature. The individual concluded that it would be worthwhile to do so.
  • #1
pa5tabear
175
0
I hope this isn't deemed school related. It is something I learned in school, but I'm now trying to figure out a reaction model for my work.

In the process of calculating the reaction constant, usually you first determine the Gibbs energy of formation of each species, find the difference between the reactants and products to find the energy of reaction, then use relationship:

ln K = (-grxn)/(R*T)

where K is your equilibrium constant, T is your temperature, and R is the gas constant with appropriate units.

If I have data for the standard state, and want to calculate the energy of formation for a different temperature, how do I do this?

I know I can adjust the equilibrium constant for temperature, but I'm wondering if it is possible to also calculate the energy of formation at a non standard temperature.
 
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  • #2
pa5tabear said:
I hope this isn't deemed school related. It is something I learned in school, but I'm now trying to figure out a reaction model for my work.

In the process of calculating the reaction constant, usually you first determine the Gibbs energy of formation of each species, find the difference between the reactants and products to find the energy of reaction, then use relationship:

ln K = (-grxn)/(R*T)

where K is your equilibrium constant, T is your temperature, and R is the gas constant with appropriate units.

If I have data for the standard state, and want to calculate the energy of formation for a different temperature, how do I do this?

I know I can adjust the equilibrium constant for temperature, but I'm wondering if it is possible to also calculate the energy of formation at a non standard temperature.

I was looking at this one day when I was preparing to TA my thermodynamics section. What I think you want to use is the Gibbs-Helmholtz equation.

http://en.wikipedia.org/wiki/Gibbs–Helmholtz_equation

This should allow you to calculate Gibbs free energies of reactions at other temperatures than STP.

-James
 
  • #3
jfizzix said:
I was looking at this one day when I was preparing to TA my thermodynamics section. What I think you want to use is the Gibbs-Helmholtz equation.


This should allow you to calculate Gibbs free energies of reactions at other temperatures than STP.

-James

Thanks!

I think this is exactly what I needed. I just didn't know such a derivation existed. I was trying to think of ways to do it using the definition ΔG=ΔH-T*ΔS but I wasn't sure how to calculate the T*ΔS term.

I do have one question. In the wikipedia article, it says to treat ΔH as independent of temperature. I'm guessing they say this for ease of calculation. The reaction I'm looking at (silicon epitaxy) occurs at around 1150 °C... very far from 298 K. Do you think it would be worthwhile to calculate a ΔH value at 1150 °C using heat capacity data, and switch that in? I think doing so would only give a more accurate value, and I see no issue.
 

1. What is Gibbs energy of formation?

Gibbs energy of formation, also known as Gibbs free energy of formation, is the amount of energy released or absorbed when one mole of a substance is formed from its constituent elements in their standard states at a given temperature and pressure.

2. How is Gibbs energy of formation calculated?

Gibbs energy of formation can be calculated using the formula ΔG°f = ΣΔG°f(products) - ΣΔG°f(reactants), where ΔG°f is the standard Gibbs energy of formation for each component in the reaction.

3. What is the significance of standard state data in calculating Gibbs energy of formation?

Standard state data refers to the thermodynamic properties of a substance at a specific temperature and pressure. It is important in calculating Gibbs energy of formation because it provides a baseline for comparison and allows for consistent and accurate measurements.

4. Can Gibbs energy of formation be negative?

Yes, Gibbs energy of formation can be negative. A negative value indicates that the reaction is exothermic, meaning that energy is released during the formation of the compound. This is a favorable condition for the reaction to occur.

5. How does temperature affect the calculation of Gibbs energy of formation?

The value of Gibbs energy of formation is dependent on temperature. As temperature increases, the value of ΔG°f also increases. This is because the formation of a compound at higher temperatures requires more energy, making it less favorable.

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