## 5J.15

$\Delta G^{\circ}= \Delta H^{\circ} - T \Delta S^{\circ}$

$\Delta G^{\circ}= -RT\ln K$

$\Delta G^{\circ}= \sum \Delta G_{f}^{\circ}(products) - \sum \Delta G_{f}^{\circ}(reactants)$

Goyama_2A
Posts: 107
Joined: Sat Aug 24, 2019 12:17 am

### 5J.15

The question asks to calculate the equilibrium constant at 23 C and 100 C for two reactions using the data from appendix 2A.
Even though the standard Gibbs of formation for the molecules is given in the appendix, the solutions manual says we have to use the enthalpies of formation and entropies of formation in order to calculate the deltaH and deltaS for the reactions and then use deltaG=deltaH-TdeltaS to find the Gibbs for the reaction. I was wondering why we had to do this instead of just using the Gibbs of formations for the molecules to find the deltaG of the reaction? I understand that the equation for deltaG uses temperature, but please conceptually explain it.

Connie Chen 1E
Posts: 51
Joined: Mon Jun 17, 2019 7:24 am

### Re: 5J.15

At different temperatures, delta G changes, which means that you cannot use the standard Gibbs free energy values at 100 degrees C, as those are only the values at 25 degrees C. This means in order to find the delta G at a different temperature, you would need to use the equation that relates delta G and delta G standard to get the correct value at a different temperature. Additionally, K also changes with temperature so the K at delta G standard is not the same as at a different temperature.

Nathan Nakaguchi 1G
Posts: 102
Joined: Wed Sep 18, 2019 12:22 am
Been upvoted: 1 time

### Re: 5J.15

You have to use the H's and S's instead of G's because you have to calculate G for temperatures different than 25C. So you have to use $\Delta G=\Delta H-T\Delta S$ where T= the different temperature.

Goyama_2A
Posts: 107
Joined: Sat Aug 24, 2019 12:17 am

### Re: 5J.15

Connie Chen 1E wrote:At different temperatures, delta G changes, which means that you cannot use the standard Gibbs free energy values at 100 degrees C, as those are only the values at 25 degrees C. This means in order to find the delta G at a different temperature, you would need to use the equation that relates delta G and delta G standard to get the correct value at a different temperature. Additionally, K also changes with temperature so the K at delta G standard is not the same as at a different temperature.

Why does delta G standard change with temperature when standard conditions are 25C, meaning one temperature? Wouldn’t delta G at any other temperature just be delta G rather than delta G standard? So why is delta G standard impacted by temperature?