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Unformatted text preview: Some practice while you wait: What is the value for H (from Bs), the sign on S (from n ) and the sign of G for each of the following reactions? H (g) + C H (g) C H (g) n gas = n final n initial n gas = 1  2 n gas = 1 S < 0 H = B HH +B C=C +4B CH B CC 6B C H H = (436)+(612)+4(413)(347)6(413) H = 125 KJ H < 0 n gas = n final n initial n gas = 3  2 n gas = 1 S > 0 H = 4B OH 2B HH B O=O H = 4(463) 2(436) (495) H = 485 KJ H > 0 Section 9.6 Free Energy G = H T S (See book for derivation) Free Energy ( G) a combination of heat and disorder that can be used to predict the spontaneity of a reaction. Think about the conditions that you know are favorable exothermic ( H < 0) and disorder ( S > 0) G = H T S () = () (+)(+) G < 0 is favorable! Think about the conditions that you know are less favorable endothermic ( H > 0) with disorder ( S < 0) G = H T S (+) = (+) (+)() G > 0 is not favorable! Section 9.6 Free Energy G = H T S G < 0 is favorable! G > 0 is not favorable! If n gas = 0, T S 0 and G ~ H (Just consider H term) At T H term is most important At T S term is most important So, if exothermic ( H<0) but disorder ( S<0), favorable at T but unfavorable at T. H < 0 & S > 0 H > 0 & S < 0 So, if endothermic ( H>0) but disorder ( S>0), unfavorable at T but favorable at T. H T S G Spontaneous ? < 0 () exothermic + > 0 (+) more disorder < 0 () YES! < 0 () exothermic + < 0 () less disorder depends on T Low T: YES High T: NO > 0 (+) endothermic + > 0 (+) more disorder depends on T Low T: NO High T: YES > 0 (+) endothermic + < 0 () less disorder > 0 (+) NO! Free Energy Summary Red = favorable Purple = unfavorable Some practice while you wait: What is the value for H (from Bs), the sign on S (from n ) and the sign of G for each of the following reactions?...
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 Spring '08
 BIGHAM

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