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### When to use the different equations

Posted: **Sun Feb 16, 2020 9:02 pm**

by **Omar Selim 1D**

Hi all,

So I'm having trouble figuring out when to use delta G = -RTlnk vs. the other equations for delta G. Is there a reason you would pick one equation over another? and what do you do differently when it is determined at constant pressure vs. volume

### Re: When to use the different equations

Posted: **Sun Feb 16, 2020 9:03 pm**

by **Shivam Rana 1D**

You want to use the equation with ln in it when you are trying to figure out the delta G at non-equilibrium concentrations.

### Re: When to use the different equations

Posted: **Mon Feb 17, 2020 9:16 am**

by **GFolk_1D**

I'm a bit confused about this as well because that delta G equation can also be applied when K is used (the equation is at equilibrium)

### Re: When to use the different equations

Posted: **Mon Feb 17, 2020 10:40 am**

by **Christineg1G**

I found this comment in a previous Chemistry Community post and thought it might be helpful to answer your question: "The standard enthalpy of formation is at 25C, if the temperature of the reaction is different, then you can't use the listed delta G values to calculate G. Then you would use the G=-RTlnK equation. When you are trying to calculate an equilibrium constant, you can mix concentration and gas pressure."

### Re: When to use the different equations

Posted: **Mon Feb 17, 2020 11:47 am**

by **Altamash Mahsud 1I**

When choosing which Gibbs free energy equation to use, you must know whether or not the system you are looking at is in equilibrium. If the system is in equilibrium, then you use the ∆G°=-RTlnK equation. If not in equilibrium, then you are to use one of the other equations.

### Re: When to use the different equations

Posted: **Mon Feb 17, 2020 7:07 pm**

by **Caroline Zepecki**

You can only use the ∆G°=-RTlnK equation when the system in question is at equilibrium, otherwise you have to use the other equations.