Lecture12 - Chemistry 132 Lecture 12 Thermodynamic...

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Chemistry 132 – Lecture 12 – Thermodynamic Description of the Equilibrium State Thermodynamics views a chemical reaction as the spontaneous “flow” of atoms from reactants to products carried out at constant T and P and requires that G<0 to be spontaneous. When the reaction is at equilibrium, G = 0 . Think of Free energy as available energy . The T S term is the unavailable energy discarded in the form of heat, no matter what. Only G is available (free) to do useful work . Equilibrium Expression for Reactions in the Gas Phase Recall that if the pressure of an ideal gas is changed from P 1 to P 2 at constant T, G = (H – TS) = H – T S (1) Because the reaction is isothermal, H = 0. Therefore, G = -T S (2) 1
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Chemistry 132 – Lecture 12 – Thermodynamic Description of the Equilibrium State But S = nR ln(P 1 /P 2 ) = - nR ln(P 2 /P 1 ) (3) Substitute equation (3) into (2). G = nRT ln(P 2 /P 1 ) (4) If P 1 is the reference state of 1 atm, G = nRT ln(P 2 /P ref ) = RT ln(P 2 /P ref ) n which is the change in Gibb’s free energy in
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