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Unformatted text preview: L ECTURE 20. T HERMODYNAMIC OVERVIEW A Q UALITATIVE A PPROACH Todays lecture is a general overview of thermodynamics from a qualitative perspective. To really be able to understand thermo, you need to look at a chemical reaction and talk about it in the context of relative changes in state functions. The lecture actually parallels a hand you can download called, Thoughts on Thermodynamics. If you can actually learn what is on this handout, and be able to explain it unassisted to a classmate, you will do well. How to Know if a Reaction Happens First, one of the important consequences of thermodynamics is the ability to explain whether a reaction occurs or not. Note that for every spontaneous reaction, there is a reverse non-spontaneous reaction and it would be nice to look at a reaction and tell whether it is going to happen as written or as the reverse. For example: 2H 2 + O 2 2H 2 O exploding balloon is spontaneous 2H 2 O 2H 2 + O 2 water becomes H 2 , O 2 is not spontaneous The state function that determines spontaneity is free energy, G: So when you look at a reaction, if you know it occurs from experience, you know it has G = . Example. Consider AgNO 3 + NaCl AgCl + Na + + NO 3 You know from solubility rules that AgCl is insoluble and the reaction shifts to the right, so you know that for this reaction, G is ___ Rxn is not spontaneous _ + Free energy G ___ Rxn is spontaneous Signs and thermoBe the System So what is the source of this idea about thermodynamic signs? Why was G = for a spontaneous reaction defined as spontaneous. Is it arbitrary? NO....
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- Spring '07