(delta)G and Spontaneity

Gibbs free energy formula requires only temperature to be constant. But, if pressure changes, why does a negative (delta)G not ‘necessarily’ indicate a spontaneous reaction?

Its my understanding -G always correlates with a spontaneous rxn. Based on the 2 formulas, pressure does not factor into either equation…and temperature can in fact change spontaneity. I think “pressure” affects gases most, versus other types of molecules. This is my gut response. Lets see what others have to say:)

ok, you made me look this up…at work. Apparently, Gibbs free energy assumes constant temp and pressure.

the calculation of deltaG is done with parameters (generally deltaH and deltaS) that have specific value in particular conditions. It is easy to understand why changing temp can change deltaG (temp is in the equation). Now in the case that a reactant or product is a gas, assuming a temperature, deltaH and deltaG, if the pressure changes then you have to apply le chatelier principle for equilibrium with gases (please refer to Gen Chem 2) material. I will conclude by saying that if you change pressure, you also change the equilibrium constant and for gases it is calle Kp (ratio of pressures of the products/pressures of reactant). There is a relation between deltaG and Kp such as deltaG = -RT ln Kp.

So the bottom line is that if you change pressures, you change deltaG (when you have gases in your reaction). This is information that is meant to get you started on the topic.

I hope this helps.