Question

Give the characteristics of free energy.

Answer 1

Characteristics of Free energy:
i) G is defined as (H-Ts) where H and S are the enthalpy and entropy of the system respectively. T$=$ temperature. Since H and S are state functions, G is the state function.
ii) G is an extensive property while $\Delta G=(G_2-G_1)$ which is the free energy change between the initial $\left(1\right)$ and final $\left(2\right)$ states of the system becomes the intensive property when mass remains constant between initial and final states (or) when the system is a closed system.
iii) G has a single value for the thermodynamic state of the system.
iv) G and $\Delta$G values correspond to the system only. There are three cases of $\Delta$G in predicting the nature of the process. When, $\Delta G<0$ (negative), the process is spontaneous and feasible; $\Delta G=0$. The process is in equilibrium and $\Delta G>0$(positive), the process is non-spontaneous and not feasible.
v) $\Delta G=\Delta H-T\Delta S$. But according to $Ist$ law of thermodynamics, $\Delta H=\Delta E+P\Delta V$ and $\Delta E=q-w$.
But $\Delta S=\frac{q}{T}$ and $T\Delta S=q=$ heat involved in the process.
$\therefore \Delta G=q-w+P\Delta V-q=-w+P\Delta V$
(or)$-\Delta G=w-P\Delta V=$ network.
The decrease in free energy $-\Delta G$, accompanying a process taking place at constant temperature and pressure is equal to the maximum obtainable work from the system other than work of expansion. This quantity is called as the "net work" of the system and it is equal to $(w-P\Delta V)$
$\therefore$ Network $=-\Delta G=w-P\Delta V$.
$-\Delta G$ represents all other forms of work obtainable from the system such as electrical, chemical or surface work etc other than P-V work.
$\therefore \Delta G=q-w+P\Delta V-T\Delta S$.