### Calculating G if H and S are gven

Posted:

**Sun Feb 17, 2019 1:09 pm**Can someone please explain why we assume G is 0 when H and S are given?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=135&t=42313

Page **1** of **1**

Posted: **Sun Feb 17, 2019 1:09 pm**

Can someone please explain why we assume G is 0 when H and S are given?

Posted: **Sun Feb 17, 2019 1:17 pm**

We aren't assuming that the Gibbs Free Energy is equal to 0, rather we are putting in 0 for its value to see when and are equal to one another in order to determine at what temperatures the Gibbs Free Energy would be negative (spontaneous reaction) or positive (non spontaneous reaction).

Posted: **Sun Feb 17, 2019 2:51 pm**

If we have the value when the system is in equilibrium and delta G =0, then we can find the behavior (in terms of spontaneity) for any temperatures above or below that value.

Posted: **Sun Feb 17, 2019 8:19 pm**

Why can we use the same deltaH and deltaS to calculate deltaG at any temperature? (like in this problem: 9.53 Calculate the change in molar Gibbs free energy for the process NH3(l) --> NH3 (g) at 1atm and (a) 15.0 C; (b) 45. C (see Tables 8.3 and 9.1 for standard enthalpy and entropy of vaporization))

I thought that deltaH and deltaS would be different for different temperatures? Are the changes in enthalpy and entropy for a phase change the same at any temperature?

I thought that deltaH and deltaS would be different for different temperatures? Are the changes in enthalpy and entropy for a phase change the same at any temperature?