4b - Josiah Willard Gibbs (1839-1903) Motivation The second...

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1 Josiah Willard Gibbs (1839-1903) Motivation The second law: 0 univer surroundings system se S S S ∆= + A system state function A surroundings state function Can the second law be expressed solely in terms of system state functions?
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2 The Gibbs free energy ,. Consider a system in thermal and mechanical equilibrium with the surroundings: The change of entropy of the surroundings ( can be ex : p surr surr surr s sys sys sys sys sys sy urr s s urr surr TP qH T q T T S S == = 00 Define the Gibbs energy (a ressed in terms of system state functions!) (we c system an drop the "sys" label fro state f m no u wo ct n) n sys sys sys sys sys sys sys sys s univ ys H ST S H T HT S S S ∆∆ ⇒∆ = ≥ ⇒ ⇒− ⇒≤ ion): For processes that take place under constant pressure and temperature: G0 0 univ H S GT S = ∆⇔ whereas is a state f (1) The main advantag unction of the univer e of using instead of has to do wi se, is a state function of the th the fact tha system. t Thus, the Gibbs energ univ univ G G S S y makes it possible to describe spontaneous processes solely in terms of system state functions. (2) Whereas increases in a spontaneous process, decreases. (3) Equilibrium is defin d y e b univ G S , which is equivalent to 0. (4) is an extensive state function and has units of energy. (5) 0 only 0 , if u s niv uni y surr ss y s surr v dG G dS S = ∆≥ = Properties of the Gibbs energy
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3 Enthalpic and entropic driving forces 0 univ TS G S H T ∆= =− < Favors processes that increase the entropy of the system ( 0). The relative weight of this term increases with temperature sys SS > surr Favors exothermic processes ( H<0) since they increase the entropy of the surroundings ( S H/T). “Enthalpy wins at low T Entropy wins at high T” S and H are also temperature Caution depend t! : en Standard molar Gibbs energy of formation The standard molar Gibbs energy of formation of a substance is equal to the standard (at P=1 bar) Gibbs energy change, involved in obtaining one mole of the substance, at a given temperature, from the elements that constitute it in their most stable allotropic form at the same temperature and at P=1 bar.
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This note was uploaded on 06/13/2009 for the course CHEM 260 taught by Professor Staff during the Spring '08 term at University of Michigan.

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4b - Josiah Willard Gibbs (1839-1903) Motivation The second...

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