# 1106 - Some practice while you wait What is the value for...

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Unformatted text preview: Some practice while you wait: What is the value for ∆ H (from B’s), 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 H-H +B C=C +4B C-H –B C-C –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 O-H – 2B H-H – 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. ΔH T ΔS ΔG 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 B’s), the sign on ∆ S (from ∆ n ) and the sign of ∆ G for each of the following reactions?...
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1106 - Some practice while you wait What is the value for...

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