Confused about gibbs free energy

[jd12345]

Wikipedia says :
In thermodynamics, the Gibbs free energy (IUPAC recommended name: Gibbs energy or Gibbs function; also known as free enthalpy[1] to distinguish it from Helmholtz free energy) is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure (isothermal, isobaric).

So is gibbs free energy only valid for constant pressure and temperature?
I tried to find change in gibbs free energy of a chemical reaction under these conditions and it always comes zero.

ΔG = ΔH - TΔS ... temperature is constant
ΔG = ΔH - TΔH/T... at constant pressure q = ΔH
ΔG = 0

there is something wrong in this as reactions do happen at constant pressure and temperature adn gibbs free energy change is not zero

 

[Mike H]

The Gibbs (free) energy is used in cases of constant pressure and temperature, just as the Helmholtz (free) energy is used in cases of constant volume and temperature.

Other than that, I am not entirely sure what you are trying to do with those equations. Certainly, some reactions do occur spontaneously under constant temperature and pressure while others don't, depending on the value of the Gibbs (free) energy for that process.

 

[DrDu]

Various people already tried to convince you that in case of an irreversible reaction it is not correct to equate Delta S as q/T or Delta H/T.
A chemical reaction in equilibrium will in deed not be able to do work.

 

[jd12345]

Got it - thanx