12 - Chemistry 2000 Lecture 12: Free energy Marc R. Roussel...

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Unformatted text preview: Chemistry 2000 Lecture 12: Free energy Marc R. Roussel Gibbs free energy I Suppose that we have a thermodynamically allowed process at constant T and p . I The entropy change of the universe is positive, so S system + S surroundings I The heat transferred to the system is H system (constant p ), so the heat transferred to the surroundings is- H system . I The surroundings are assumed to be sufficiently large that this heat transfer has a negligible effect on them, i.e. is reversible: S surroundings = q rev T =- H system T S system- H system T I All of the quantities in the inequality now refer to the system, so we drop the subscripts: S- H T or T S- H or H- T S I Define the Gibbs free energy G = H- TS I At constant T , G = H- T S I For a thermodynamically allowed process at constant T and p , we just saw that H- T S G 0 for a thermodynamically allowed process at constant T and p . The standard state I The Gibbs free energy change is very sensitive to the reaction conditions. I We therefore define a standard state we will use as a reference. Standard temperature and pressure (STP): 25 C and 1bar (100000Pa) Gas: at STP and ideally behaving Solid: at STP Liquid: pure, at STP Solute: 1mol/L concentration at STP and ideally behaving Standard molar Gibbs energy of formation I Consider the reaction C 2 H 4(g) + Br 2(g) C 2 H 4 Br 2(l) I We could imagine decomposing this reaction into the following steps: 1. Disassemble the reactants into the most stable forms of the elements at 25 C and 1 bar. 2. Reassemble the elements into the product. I Because G is a state function, r G would be the same for either the direct reaction, or for the process in which the elements act as intermediates. I Define the formation reaction for a compound as elements in most stable forms at STP 1 compound I The Gibbs free energy change in the formation reaction elements in most stable forms at STP compound is called the standard (molar) Gibbs energy of formation, f G or f G m ....
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This note was uploaded on 03/03/2012 for the course CHEM 2000 taught by Professor Roussel during the Fall '06 term at Lethbridge College.

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12 - Chemistry 2000 Lecture 12: Free energy Marc R. Roussel...

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