Chem MT3 2008 KEY - Name_GSI_ Chemistry 1A, Fall 2008 KEY...

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Name_______________GSI________ Page 1 of 9 Chemistry 1A, Fall 2008 KEY Midterm Exam III-Version A November 12, 2008 (90 min, closed book) N a m e : T A : S I D : S e c t i o n : Please read this first: Write your name and that of your TA on all 9 pages of the exam Test-taking Strategy In order to maximize your score on the exam: Do the questions you know how to do first. Go back and spend more time on the questions you find more challenging. Budget your time carefully -- don't spend too much time on one problem. Show all work for which you want credit and don't forget to include units. Question Page Points Score A 2 10 Bi-ii 3 10 Biii-vii 4 17 C,D 5 21 E 6 14 F (MC) 7-9 28 Total 100 Useful Equations and constants: G ° = H ° - T S ° H ° = Σ n H ° f (products) - Σ n H ° f (reactants) S ° = Σ nS ° (products) - Σ nS ° (reactants) G ° = Σ n G ° f (products) - Σ n G ° f (reactants) PV=nRT 1L·atm = 0.1kJ w = - P ext V G ° = - RTln K G = G ° + RTln Q G ° = - nF Є º Є = Є º - (RT/nF) lnQ or Є = Є º - (0.05916V/n) logQ oxidation occurs at the anode reduction occurs at the cathode E cell = E cathode -E anode R S T R H K ° + ° = 1 ln N 0 = 6.02214 x 10 23 mol -1 T (K) = T (C) + 273.15 F = 96,485 C / mol 1 V = 1 J / C R = 8.31451 J K -1 mol -1 R = 8.20578 x 10 -2 L atm K -1 mol
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Name_______________GSI________ Page 2 of 9 A.) Bond Energy and Potential Energy The following figure is a plot of the potential energy as a function of separation (r) between atoms. The black line corresponds to a hydrogen atom (H · ) and a chlorine atom (Cl · ) approaching each other. H · (g) + Cl · (g) HCl (g) Potential Energy + 0 - i. Which curve would correspond to hydrogen and fluorine (H · + F · ) atoms? same as H+Cl Curve B ii. Explain your choice in terms of the separation distance. The minimum in the curve corresponds to the bond length of the molecule. HF has a shorter bond length than HCl because fluorine is smaller and more electronegative than chlorine. Therefore the separation distance between atoms should be shorter for HF than for HCl which corresponds to Curve A. iii. Explain your choice in terms of the minimum potential energy. The depth of the well in the curve corresponds to the bond strength of the molecule. HF has a stronger bond than HCl because fluorine is smaller and more electronegative than chlorine.
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Chem MT3 2008 KEY - Name_GSI_ Chemistry 1A, Fall 2008 KEY...

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