∆G is net energy available to do useful work and is thus a measure of “free energy”. Show mathematically that ∆G is a measure of free energy. Find the unit of ∆G. If a reaction has positive enthalpy change and positive entropy change, under what condition will the reaction be spontaneous?
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∆STotal = ∆Ssys + ∆Ssurr -∆H sys ∆STotal = ∆Ssys + T T ∆STotal = T ∆Ssys – ∆Hsys For spontaneous change, ∆Stotal > 0 ∴ T ∆ Ssys – ∆Hsys > 0 ⇒ – (∆Hsys – T ∆ Ssys ) > 0 But, ∆Hsys – T ∆ Ssys = ∆Gsys ∴ – ∆Gsys > 0 ⇒ ∆Gsys = ∆Hsys – T ∆Ssys < 0 ∆Hsys= Enthalpy change of a reaction. T ∆Ssys = Energy which is not available to do useful work. ∆Gsys = Energy available for doing useful work. • Unit of ∆G is Joule • The reaction will be spontaneous at high temperature.