lecture 2

lecture 2 - Todays Lecture Announcements Prologue: Activity...

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Today’s Lecture Announcements Prologue: Activity Today’s lecture in 3 acts Act 1 – We work together Act 2 – Gibb’s and his energy Act 3 – I’ll have a proton please
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Announcements Office hours Monday’s, 4-5:30 pm, in my office (ENGR 208) Or as announced Google Group – Are you a member? (would blog comments work better? Problem sets PS 1 due Today? PS 2 out Today, Due 9/20 (Tuesday) Reading for Today’s Lecture Benjamin Chapter 2, focus on 2.4, 2.5, and 2.6 Chapter 3, 3.1, 3.2, 3.4, 3.6-3.9 Chapter 4, sections 4.1, 4.2 (covered today – and we will work on this again next time)
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Prologue – Recap of Activity What is the concentration of Pb 2+ in water in equilibrium with Pb(OH) 2 (s) in 0.1 M NaCl at pH 11 and 25 C? Pb(OH) 2 (s) = Pb 2+ + 2 OH - K sp = 10 -10.9 (some solubility products are found in Benjamin text)     i ii   Pb 2+       22 2 { }{ } { }{ }  sp sp Pb OH K Pb OH s K Pb OH
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Activity What is the concentration of Pb 2+ in water in equilibrium with Pb(OH) 2 (s) in 0.1 M NaCl at pH 11 and 25 C? Pb(OH) 2 (s) = Pb 2+ + 2 OH - K sp = 10 -10.9 activity coefficients by Davies equation I = 0.1 M 2 2 2 2 2 2 2 2 22 { }{ } [ ] [ ] [] sp Pb OH sp Pb OH K Pb OH Pb OH K Pb OH    [Pb 2+ ] = 6.0x10 -5 M     i ii  
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Activity Calculating activity coefficients by iteration Dissolution or precipitation of lead hydroxide Pb(OH) 2 (s) = Pb 2+ + 2 OH - K sp = 10 -10.9 What if the ionic strength was not specified as 0.1 M NaCl but you measured [Pb 2+ ] and pH? 2 2 2 2 2 2 2 2 22 { }{ } [ ] [ ] [] sp Pb OH sp Pb OH K Pb OH Pb OH K Pb OH   
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Activity Calculating activity coefficients by iteration To calculate activity coefficient, need ionic strength To calculate ionic strength, need ion concentrations To calculate ion concentrations, need activity coefficients 1/ 2 2 1/ 2 log 0.2 1 i I Az I I      2 1 2 ii i I c z 2 2 22 []  sp Pb OH K Pb OH 
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Activity Calculating activity coefficients by iteration Assume an initial ionic strength a guess based on reactions should not matter if it is high or low Use this I to calculate activity coefficients Use activity coefficients to calculate concentrations Use concentrations to calculate “real” ionic strength Use “real” ionic strength to calculate better activity coefficients Repeat… until I varies from previous iteration by less than some criterion (e.g., less than 1%)
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Act 1 – A large combo #6, please aka working with reactions A generic reaction: a A + b B = c C + d D Example: Pb(OH) 2 (s) = Pb 2+ + 2 OH -                 cd CD eq a b a b ab AB K   2 2 2 2 2 2 2 2 2 { }{ } { }{ } [ ] [ ] { ( ) ( )} 1 sp Pb OH Pb OH Pb OH K Pb OH Pb OH s   solid : already at the reference state, {Pb(OH 2 )} = 1
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Working with Reactions Adding reactions: Pb(OH) 2 (s) = Pb 2+ + 2 OH - K sp = 10 -10.9 2 H + + 2 OH - = 2 H 2 O ( K w ) -2 = (10 -14 ) -2 = 10 28 Pb(OH) 2 (s) + 2 H + = Pb 2+ + 2 H 2 O K = 10 17.1               2 2 22 2 2 2 17.1 () [] 10   Pb H Pb H O Pb K
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lecture 2 - Todays Lecture Announcements Prologue: Activity...

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