Gibbs energy.pdf - Gibbs Free Energy Suniv > 0 Suniv = 0 Suniv < 0 Irreversible spontaneous Reversible spontaneous Impossible We want to have a SYSTEM

# Gibbs energy.pdf - Gibbs Free Energy Suniv > 0 Suniv = 0...

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74 Gibbs Free Energy 0 = Δ univ S 0 > Δ univ S Reversible, spontaneous Irreversible, spontaneous 0 < Δ univ S Impossible We want to have a SYSTEM STATE function that tells us the spontaneity of the reaction. Lets suppose a process occurring under constant T and P: T H S sys surr Δ = Δ T S T H T H S S S S sys sys sys sys surr sys univ ) ( - Δ Δ = Δ Δ = Δ + Δ = Δ
75 Gibbs Free Energy 0 = Δ univ S 0 > Δ univ S Reversible, spontaneous Irreversible, spontaneous 0 < Δ univ S Impossible We want to have a SYSTEM STATE function that tells us the spontaneity of the reaction. Lets suppose a process occurring under constant T and P: At constant pressure condition, the system is in mechanical equilibrium (p = p ext ). This does not mean that the change is reversible! T H S sys surr Δ = Δ T S T H T H S S S S sys sys sys sys surr sys univ ) ( - Δ Δ = Δ Δ = Δ + Δ = Δ sys sys P surr P H q q Δ = = , , for reversible and irreversible change For the system entropy change, the reversible change can occur only when T = T trs trs trs rev sys trs surr T H T q T T S Δ = = = Δ , ) ( ( ) T H dT T T C T C T H T S trs T T B p A p trs surr Δ + Δ = Δ ) ( ) ( ) ( , ,
76 GIBBS FREE ENERGY (defined only for system) sys sys sys TS H G G = = T G S sys univ Δ = Δ For constant T and P Δ G < 0 ! Spontaneous Process Δ G = 0 ! Reversible, or equilibrium Δ G > 0 ! Does not happen
77 Gibbs Free Energy:H 2 O(s) ! H 2 O(liq) Δ H o =+6.03 kJ/mol, Δ S o =22.1 JK -1 mol o o o S T H G Δ Δ = Δ Spontaneous Reaction!!! The direction of chemical reaction is determined by the H and S.
78 K 333 J/K.mol 0 . 93 kJ/mol 31.0 , 0 = = Δ Δ = = Δ Δ = Δ o o o o o S H T S T H G Ex) Boiling Point Br 2 (liq) ! Br 2 (g) Δ H o =31.0 kJ/mol, Δ S o =93.0 J/K.mol What is the normal boiling point? Soln) At phase transition temperature, the process is reversible and Δ G=0. 0 Δ G 333 K Boiling point Prefers gas form Prefers liquid form •T > 333 K : entropy dominates G. •T < 333 K : enthalpy dominates G.
79 Free Energy and Chemical Reactions Standard molar Free Energy Change: the change in free energy that will occur if the reactants in their standard states are converted to the products in their standard states.

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