# How is the change in Gibb's free energy a condition for spontaneity for reactions not happening under constant pressure conditions?

According to what I have studied, the change in Gibb's free energy is only a condition for spontaneity for processes happening at constant pressure and temperature conditions.

Consider a reversible reaction happening in a closed vessel at a constant temperature. We check the feasibility of the reaction at any instant using the following equation:

$$\Delta G = \Delta G^{.} + RT\ln(Q)$$

As the reaction happens, the partial pressures of each gaseous component will keep on changing as well as the total pressure. Then how can we use Gibb's free energy for checking spontaneity?

• Why would it be a problem for partial pressures to change? Apr 14 '20 at 9:00
• What about Helmholtz energy A = U - TS ? Apr 14 '20 at 10:41

Gibbs energy have an another equation which is:- So from this we can see that Enthalpy change is negative and entropy change is positive for a spontaneous reaction to take place and the overall equation of 