Gibbs free energy

Gibbs free energy 

First and second laws of thermodynamics. Together indicate that energy of universe is constant but the entropy continuous to increase to maximum. The combined concept of two laws put forward by the American chemist J. William Gibbs (1878) as follows :

ΔG= ΔH - TΔS

Where- 

• ΔG change in free energy of system (i.e. change during a process in the energy available to do work).
• ΔH change in enthalpy of system or change in heat content at constant pressure Or total energy content of the system (Equivalent to ΔE if change in volume is zero).
• ΔT Absolute temperature at which the process is taking place (K = °C + 273)
• ΔS Change in entropy of system
  
  

The change in free energy of a system that occurs during a reaction can be measured under any number of conditions. If data are collected under standard-state conditions, the result is standard-state free of reaction (ΔG°).

ΔG° = ΔH° - TΔS°

Favorable ΔH°<0  ΔS°>0                   Unfavorable ΔH° >0    ΔS°>0

The entropy term is therefore subtracted from the enthalpy term when calculating ΔG° for a reaction. Because of the way the free energy of the system is defined, ΔG° is negative for any reaction for which ΔH° is negative and ΔS° is positive. 

ΔG° is therefore negative for any reaction that is supported by both the enthalpy and entropy terms. We can therefore conclude that any reaction for which ΔG° is negative should be favorable, or spontaneous.


    Favorable or spontaneous reactions: ΔG°>0

Conversely. ΔG° is positive for any reaction for which ΔG° is positive and ΔS° is negative. Any reaction for which ΔG° is positive is therefore unfavorable.

Unfavorable, or non-spontaneous reaction: ΔG° > 0

Depending on whether they produce or absorb heat, reactions are classified as exothermic (ΔH< 0) or endothermic (ΔH > 0). Reactions can also be classified as exergonic (ΔG < 0) or endergonic (ΔG > 0) on the basis of whether the free energy of the system decreases or increases during the reaction.

When a reaction is favored by both enthalpy (ΔH° < 0) and entropy (ΔS° > 0), there is no need to calculate the value of ΔG° to decide whether the reaction should favored by neither enthalpy ΔH° > 0) nor entropy (ΔS < 0). Free energy calculations become important for reactions favored by only one of these factors.