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L28-post

# L28-post - Lecture 28 Which Way Enthalpy vs Entropy The...

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1 Lecture 28 Which Way? Enthalpy vs. Entropy Key Idea: Reactions that are exothermic and increase disorder favor products. The “arrow” of time, The direction of “unassisted” chemical reactions 1 st Law Energy can neither be lost nor created; it is always conserved. 3 rd Law Entropy of ordered systems at 0 K is 0; Thus, entropy values are absolute quantities. The Grand Summary The Laws of Thermodynamics Processes occur in the direction of greatest energy dispersion (entropy); the total entropy of the system and surroundings always increases. 2 nd Law S universe = S sys + S surr > 0 Why do these processes all happen in one particular direction? * Br 2 expansion into vacuum * CaCl 2 dissolution: CaCl 2 (s) + H 2 O (l) Ca 2+ (aq) + 2 Cl (aq) * NH 4 Cl dissolution NH 4 Cl (s) + H 2 O (l) NH 4 + (aq) + Cl (aq) These processes are all “product-favored” as written! H rxn = 81 kJ/mol o H rxn = 0 o H rxn = + 15 kJ/mol o From Monday’s lecture... Product-favored: S universe = S sys + S surr > 0 CO (g) + 2 H 2 (g) Æ CH 3 OH (l) Is S o + or ? Is H o + or ? H o rxn = H o f (CH 3 OH (l) ) − ∆ H o f (CO (g) ) 2 H o f (H 2 (g) ) = 238.66 kJ – ( 110.54) – 0 = 128 kJ per mole of CH 3 OH S o rxn = S o (CH 3 OH (l) ) S o (CO (g) ) 2 S o (H 2 (g) ) = 126.8 J/K – (197.7 J/K) – 2(130.7 J/K) =

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