Suppose we have the reaction $$\mathrm{A + B \leftrightharpoons C}$$ The system starts with reactants (or products) and reaches equilibrium. Can we measure the change in Gibbs energy $ΔG$ at a moment where the system isn't in equilibrium? I mean the initial state will have some $G$ equals to $G_1$ (Gibbs free energy of the reactants). As the reaction keeps going then at some point we will have both products and reactants (still not in equilibrium). Is now Gibbs free energy still defined? Can we know measure $ΔG$ as:
$$ΔG=G_2 -G_1$$
I am asking because we say that thermodynamic variables can only calculated at equilibrium. So the above states ($1$ and $2$) must be equilibrium states. But they aren't as the reaction still keeps going.
What I want to clarify is why we are free to draw the Gibbs free energy of the system as a function of the reaction coordinate and calculate difference of Gibbs free energy at different point of the extent of reaction. How we can measure a thermodynamic potential at a point (in the extent of reaction coordinate) where the system is not in equilibrium. I looked also in this post $\Delta G$ and reaction coordinate where in an answer there is a diagram of Gibbs free energy of the system as function of the reaction coordinate.