This preview has intentionally blurred sections. Sign up to view the full version.
View Full DocumentThis preview has intentionally blurred sections. Sign up to view the full version.
View Full DocumentThis preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
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 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. 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?...
View
Full Document
 Spring '08
 BIGHAM
 Thermodynamics, Reaction, Chemical reaction

Click to edit the document details