Lecture 42

# Lecture 42 - Topics for the day Administrative stuff Gibbs...

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Topics for the day Administrative stuff Gibbs free energy Calculating Gibbs free energy Compound stability Spontaneity from Δ G Temperature for switching spontaneity Homework Quest HW6, due Wednesday 10 am ALEKS Objective 7, due Friday 11:59 pm Practice exam posted. Answers online at noon today. Last week of lectures Today on thermodynamics, Wed exam review Fri next week will be course wrapup, your assessment of the class, ±nal exam discussion, and maybe some surprises. Administrative stuff Grades posted in Blackboard gradebook You should have known all of them already Clicker grade isn’t posted yet. Will come today. My current grade for you for HW (dropping worst 2, and counting ±rst two Quest HW x 2) will be posted soon also. ALEKS prep+clicker+HW+ (Sum of four exams ÷ 8) = 70 Making 88/90/92 % of 70 = 61.60/63.00/64.4 points will get you a B+/A–/A, respectively, without the ±nal. Administrative stuff 1 2 3

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Exam Four is Wed May 5th, 7 – 9 pm Susan’s Exam Review: Monday 4 - 5 pm, WEL 3.502 2009 Formula sheet on BB. 2010 will be posted today. UPDATE!!! The new exam room isn’t big enough for the whole class. We are splitting into two rooms. 105 If your surname starts with A – L go to GEA 105 If your surname starts with M – Z go to GAR 0.102 Administrative stuff Surnames M–Z GAR 0.102 Surnames A–L GEA 105 Where were we? We talked through the different possible signs of Δ S sys and Δ H sys (and hence Δ S surr ) to see the different possibilities for spontaneity. We’ll see an easier way to do this today. Note that Δ S sys and Δ H sys are independent of each other, all possible combinations of +/+, +/–, –/+, –/– are possible. Δ S surr and Δ H sys ARE completely dependent on each other. They must be opposite sign to each other. Another source of confusion: Δ S surr = – Δ H sys T At constant T and P Δ S sys = q T For an isothermal process (constant T) Δ S sys = Δ H T At a phase change (constant T and P) Δ S reaction is dominated by the Δ n for gaseous reactants. If Δ n gas = 0, then the more complex gas has the higher entropy (more bonds = more ways for energy to be dispersed). Δ rxn = S˚(products) – S˚(reactants) 4 5 6
A. Yes, always B. No, never C. Sometimes iClicker Time Is an exothermic reaction spontaneous? The sign of Δ S surr is opposite to the sign of Δ H, so for exothermic reactions, Δ S surr is positive. When both

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## This note was uploaded on 08/27/2011 for the course CHEM 301 taught by Professor Wandelt during the Spring '08 term at University of Texas.

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Lecture 42 - Topics for the day Administrative stuff Gibbs...

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