Gibbs free energy during a phase change

[mmorris86]

Hey guys I was trying to figure this out and the best answer that a web search revealed was this: http://pruffle.mit.edu/3.00/Lecture_16_web/node1.html

I dont really understand what those graphs are trying to say... maybe that delta G of the liquid = delta G of the solid at their equilibrium??

Also what is deltaG(naught), I found this equation delta G(naught)= -RTln(K), but i dont understand it. Whats it used for and how does it relate to delta G???

🙁 I hate gen chem....

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[nze82]

Hey guys I was trying to figure this out and the best answer that a web search revealed was this: http://pruffle.mit.edu/3.00/Lecture_16_web/node1.html

I dont really understand what those graphs are trying to say... maybe that delta G of the liquid = delta G of the solid at their equilibrium??

Also what is deltaG(naught), I found this equation delta G(naught)= -RTln(K), but i dont understand it. Whats it used for and how does it relate to delta G???

🙁 I hate gen chem....

Why are you even trying to make sense of these complicated graphs! Stick to the material mentioned in your review books.
DeltaGo represents Gibb's Free Energy, under standard conditions (P = 1atm, T = 273K). In other words, DeltaGo is the STANDARD Gibb's Free Energy. If you try to calculate DeltaG under a different set of conditions, then we use a different formula that relates DeltaG to DeltaGo.

 

[192LT192]

the equation deltaG=-RTlnK just relates standard deltaG to the equilibrium constant, using R constant and Temp.

to get from standard deltaG to just any delta G (meaning delta G at a different set of conditons other than 25C and 1atm and where all concentrations are 1M), you need to use the equation:

deltaG=deltaG(naught) + RT lnQ ---> this is relationship between both types of delta G's

 

[mmorris86]

the equation deltaG=-RTlnK just relates standard deltaG to the equilibrium constant, using R constant and Temp.

to get from standard deltaG to just any delta G (meaning delta G at a different set of conditons other than 25C and 1atm and where all concentrations are 1M), you need to use the equation:

deltaG=deltaG(naught) + RT lnQ ---> this is relationship between both types of delta G's

Thanks so much for both of your replies, but i still need a lil help. So basically at equilibrium, where Keq=1, standard delta G = 0 right? because ln(1)=0 So is that when delta G = delta H - T(delta S) is used?? I am still pretty confused...

 

[americanpierg]

Thanks so much for both of your replies, but i still need a lil help. So basically at equilibrium, where Keq=1, standard delta G = 0 right? because ln(1)=0 So is that when delta G = delta H - T(delta S) is used?? I am still pretty confused...

physical chem was the most useful undergrad class.

anyways, you're mixing up different equations that are used in different situations. the equation with deltaG and delta G0 as others has stated is if they give you a list of G values for reactions at standard conditions and you want to find the G values of one of those reactions at a certain pressure or temperature. Or they give you K values and ask you to find delta G0, etc.

The equation with enthalpy and entropy calculates the change in gibbs free energy if you're given any change in enthalpy and entropy and want to determine if delta G is negative or not, ie spontaneous or not


IF the reaction is at equilibrium, then Q of course equals K. The equation is delga G = delta G0+ RTln(Q). Your equation, "delta G(naught)= -RTln(K)" is only used if its already in equilibrium. Delta G=0, not "standard" delta G, so

delga G= delta G0+RTln(Q)
delta G= 0
0=delta G0 + RTln(K)
delta G0= -RTln(K)