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Describe Gibbs energy and spontaneity. |
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Answer» Solution :In Gibbs FUNCTION, G is an extensive property . `G=H-TS` The change in Gibbs ENERGY for the system, `DeltaG_("sys")` can be written as `DeltaG_("sys") = DeltaH_("sys") - TDeltaS_("sys") - S_("sys") Delta T` (constant temperature, `DeltaT=0` ) `therefore DeltaG_("sys") = Delta H_("sys") - T Delta S_("sys")` `Delta G= DeltaH- T DeltaS` It is referred to as the gibbs equation. Here, `DeltaH and T Delta S` are energy TERMS. Therefore `Delta G` is unit of energy. If the system is in thermal equilibrium with the surrounding, then the temperature of the surrounding is same as that of the system. Also, increase in enthalpy of the surrounding is equal to decrease in the enthalpy of the system. Therefore, entropy change of surroundings, `DeltaS_("surr")= (Delta H_("surr") )/( T) = (- DeltaH_("sys") )/( T)` `DeltaS_("total") = DeltaS_("sys") = ((-DeltaH)/( T) "sys")` Rearranging the above equation : `T Delta S_("total") = T DeltaS_("sys") - Delta H_(sys)` For spontaneous PROCESS, `DeltaS_("total") GT 0` So, `T Delta S_("sys") - Delta H_("sys") gt 0` `- ( Delta H_("sys") - T DeltaS_("sys") ) gt 0` `therefore - Delta G gt 0` `therefore Delta G = Delta H- T Delta S lt 0` `DeltaH_("sys")` is the enthalpy change of a reaction, `T Delta S_("sys")` is the energy which is not available to do useful work. So `DeltaG` is the net energy available to do useful work and is thus a measure of the .free energy.. For this reason, it is also known as the free energy of the reaction. (i) If `Delta G` is negative `( lt G)`, the process is spontaneous. (ii) If `DeltaG` is positive `( gt 0)`, the process is non spontaneous. |
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