[luysion]

Hi, ive recently started studying chemistry and have being going over gibbs free energy.. I understand it is a driving force causing chemical and physical changes but during my course of reading I did have some queries stem to mind.

1) In relation to equilibrium why is it that a reaction occurs such that G is minimum ? I.e. i read that the reason neither the forward nor reverse reaction proceeds to completion during equilibrium is because Gibbs Free Energy is at a minimum. Is this due to no driving force being provided?

2) When gibbs free energy is positive it indicates the forward reaction (e.g. A --> B) is non-spontaneous why does it still occur? I read something about the reaction still proceeds but the pressure of the product does not read 10^5 P i.e. standard pressure?
im really confused on this bit can somone please simplify this

cheers for any help

[renge ishyo]

When  :delta: G = 0 the system is at equilibrium. What this means more or less is that the rate of the forward reaction and reverse reaction are the same at this point so that, even though both reactions still take place back and forth, you don't see any net change taking place in the amounts of chemicals present. If you have a negative  :delta: G the system is out of balance, and so reactants are spontaneously converted into products until the  :delta: G reaches zero at which point you again see no net change. Conversely, if the  :delta: G is positive then the products are spontaneously converted back into reactants until  :delta: G reaches zero where you again see no change.

[roya_kindflower]

2) When gibbs free energy is positive it indicates the forward reaction (e.g. A --> B) is non-spontaneous why does it still occur? I read something about the reaction still proceeds but the pressure of the product does not read 10^5 P i.e. standard pressure?
im really confused on this bit can somone please simplify this

cheers for any help

I don't know if your country have this syllabus.But for me in Hong Kong, in this topic, there is still a thing called "entropy"(the degree of randomness of the molecules,delta S). Even though  :delta: G is positive, provided that  :delta: S is positive,ie,entropy increases, the reaction still occurs owing to the nature prefers messy disorder to orderness. Eg. dissolution of ammonium nitrate in water.

P.S.My exam is called "Hong Kong Advanced level Examination(HKALE),have you ever heard of it?

[renge ishyo]

Roya, the Gibbs free energy depends on the entropy change as well as the enthalpy change.

:delta: G =  :delta: H - T :delta: S

[Borek]

Even though  :delta: G is positive, provided that  :delta: S is positive,ie,entropy increases, the reaction still occurs owing to the nature prefers messy disorder to orderness.

As Renge wrote - entropy change is part of the gibbs free energy change. It is Gibbs free energy that defines what will happen. Entropy change can be used to explain why some endothermic reactions occur (here your example with ammonium nitrate works), but just because entropy grows doesn't mean reaction proceeds; that's not enough, you need negative Gibbs free energy change for that.

[roya_kindflower]

As Renge wrote - entropy change is part of the gibbs free energy change.

My godness, thanks for reminding me!
So, can the poster(Is that the Eng for 樓主?) provide me some reactions that still proceed with a +ve  :delta:G?