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  Gibbs free energy

Gibbs free energy ‘G’:

According to II law of thermodynamics, in order to expect the spontaneity of process entropy of universe is considered. ΔS universe is the sum of ΔS system and ΔS surroundings. It is hard to find out ΔS surroundings in most of the physical and chemical procedures. Thus a thermodynamic function that reformulates the spontaneity criterion considering only the system under study is needed.

For this reason, “a free energy function” has been introduced by the II law of thermodynamics. The free energy function, termed as the Gibbs free energy function, symbolized by the symbol ‘G’ is mathematically stated as,

G = H - TS

Here,
H = enthalpy or heat content of the system,
T = Temperature in Kelvin and
S = entropy

This expression is applicable for constant temperature and pressure processes. In an isothermal process, when ΔH and ΔS are the modifications in enthalpy and entropy of the system, then free energy change ΔG is specified by,

ΔG = ΔH – TΔS

When 1 and 2 refer to the initial and final states of the system throughout the isothermal process, then

ΔG = (G2 - G1) = (H2 - H1) - T(S2 - S1)
from I law of thermodynamics,
ΔH = ΔE + PΔV
Hence, ΔG = ΔE + PΔV - TΔS.

For spontaneous process, the enthalpy change at constant pressure will be negative. This is since in an exothermic process, the enthalpy of the final state (i.e., H2) is lower than the enthalpy of the initial state (i.e., H1) and hence (H2 - H1) is negative and the process takes place spontaneously to achieve the lower enthalpy state. Likewise, the entropy change (ΔS) raises in a spontaneous process as entropy of the final state S2 will be more than the initial state S1 and hence (S2 - S1) = ΔS, is positive. Combining negative ΔH and positive ΔS, in the expression for free energy change ΔG, at constant temperature, the overall magnitude of ΔG becomes negative for a spontaneous process. Here, ΔH and ΔS words refer only to the system.

ΔG = ΔH – TΔS

Therefore, criterion for the prediction of feasibility of a reaction or the prediction of thermodynamic spontaneity of a procedure based on the free energy change (i.e., ΔG) of the process is specified as:

Whenever at constant temperature and pressure of the system, if,

ΔG < 0, ΔG is –ve, the process is spontaneous and feasible

that is, ΔG = 0, the process is in equilibrium

i.e. ΔG > 0, ΔG is +ve,  the process is non-spontaneous and non-feasible.

In chemical thermodynamics, spontaneous processes are also termed as irreversible or feasible processes whereas non spontaneous processes are termed as non feasible processes as time factor of the procedure is not considered here.

All reversible processes are considered as equilibrium processes.

1571_gibbs energy.jpg

 

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