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

 

[LudusRex]

I can clarify that Gibbs free energy is a thermodynamic property that measures the amount of energy available to do useful work in a system. It takes into account both the enthalpy (heat content) and entropy (degree of disorder) of a system. The equation you have mentioned, ΔG = ΔH - TΔS, is the standard formula for calculating the change in Gibbs free energy.

To answer your question, yes, Gibbs free energy is only valid for systems at constant temperature and pressure, as stated in the definition. This is because the equation is derived from the first and second laws of thermodynamics, which assume these conditions. Therefore, if the temperature and pressure of a system are not constant, the equation will not accurately calculate the change in Gibbs free energy.

Additionally, it is not always the case that ΔG = 0 for a chemical reaction under constant temperature and pressure. This only happens when the reaction is at equilibrium, meaning that the forward and reverse reactions are occurring at equal rates and there is no net change in the system. In most cases, chemical reactions are not at equilibrium, and therefore ΔG will not be zero.

I recommend further research and understanding of thermodynamics and the concept of Gibbs free energy to fully grasp its application and limitations.