Setting Up the Equations for Equilibrium

When the equations for the equilibrium are set up, usually this procedure runs or 

A thermodynamic model suitable to establish the thermodynamic relations is set up. For instance, in the case of an osmotic cell, the osmotic pressure in reality is established due to the action of gravity of a liquid column. However, the thermodynamic model reduced the liquid column in that simply a pressure is exerted to the membrane. 

From the thermodynamic model, the relevant thermodynamic variables and the constraint equations are fixed. 

The equations of energy, enthalpy, free energy, free enthalpy, are set up, and the suitable thermodynamic function is minimized under the equations of constraint. 

From the minimum of the thermodynamic function, the equilibrium conditions in terms of the intensive variables are obtained, e.g  

---