## When to use the different equations

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

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

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

Omar Selim 1D
Posts: 108
Joined: Sat Jul 20, 2019 12:16 am

### When to use the different equations

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

Shivam Rana 1D
Posts: 106
Joined: Fri Aug 09, 2019 12:16 am

### Re: When to use the different equations

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

GFolk_1D
Posts: 101
Joined: Fri Aug 09, 2019 12:15 am

### Re: When to use the different equations

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)

Christineg1G
Posts: 115
Joined: Fri Aug 09, 2019 12:15 am
Been upvoted: 1 time

### Re: When to use the different equations

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."

Altamash Mahsud 1I
Posts: 50
Joined: Fri Aug 09, 2019 12:16 am

### Re: When to use the different equations

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.

Caroline Zepecki
Posts: 101
Joined: Fri Aug 09, 2019 12:16 am

### Re: When to use the different equations

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