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### Gibbs free energy of reaction vs. standard Gibbs free energy of reaction

Posted: Tue Jan 30, 2018 11:31 pm
Can someone explain what the textbook means when it says the standard Gibbs free energy of reaction is the difference between pure products and reactants, and therefore doesn't change over the course of a reaction as opposed to Gibbs free energy of reaction which does change over the course of a reaction (because the chemical composition of molecules is changing)?

### Re: Gibbs free energy of reaction vs. standard Gibbs free energy of reaction

Posted: Wed Jan 31, 2018 10:11 am
(ΔG) = (ΔH⁰) - (T)ΔS is the equation you use to calculate Gibbs free energy under standard conditions. You use this equation when the free energy change associated with the formation of the substance from the elements in their most stable forms exist under the "standard conditions" of 1 atm pressure and 298K.

Under non-standard conditions, you would use this equation: ΔG = ΔG⁰ + RT (ln Q), where R is the ideal gas constant 8.314 J/(mol)(K), Q is the reaction quotient, and T is the temperature in Kelvin.

### Re: Gibbs free energy of reaction vs. standard Gibbs free energy of reaction

Posted: Wed Jan 31, 2018 10:11 am
Not entirely sure what you're asking, but I'll give it a shot.

I think ΔG is 0 for pure reactants and products, e.g. graphite or a metal in its standard state.