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sample3_sp04_key - Chemistry 105 B Midterm 3 Professor...

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Unformatted text preview: Chemistry 105 B Midterm 3 Professor Bradforth Spring 2004 0 YOU MUST SHOW YOUR COMPLETE WORKING ON EACH OF PROBLEMS 3-6 TO RECEIVE FULL CREDIT. I STATE AND JUSTIFY ANY APPROXIMATIONS YOU MAKE. Useful Physical Constants and Equations TC =TK—273.15 K 9/ R= 8.314JK—1mol'1 Kw = 1.0x10'14 at 25°C pH =pKa + 10g10( [A—l / [HA] ) Thermodynamics ASuniV = AssyS + Assurr Assurr = - AH /T AG = AH - T AS AGoreaction = 2 Up AGfo (products) - 2 Hr AGfo (reactants) Asoreaction = Z np SO (products) - Z r1r SO (reactants) AG = AGO + RT 1n (Q) At equilibrium, AG = 0 therefore, AGO = — RT In (K) and MK =—AH°/RT +AS°/R To solve some questions, you will need data from the following pages. Question 1. (Multiple Choice, 2 points each) /r\ (1) Two buffers are made: <5 A 0.1 M NH3 with 0.05 MNH4C1 (Ka(NH4+ = 5.55 x 101°) and B: 0.03 M HF with 0.03 M F‘ (Kb(F‘) = 1.38 x 10'11 ) Which of the following statements is false: (a) A has a higher buffer capacity (b) A is a basic buffer @ is a basic buffer (d) pH of buffer B is approximately 3.1 (11) Consider which of the following statements is correct: @ I @0th reactions, SF4 (g) + F2 (g) -> SF6 (g) and H28 (g) + NH3 (g) 9 NI-I4S (s), lead to a decrease in system entropy (b) Both transformations, CH30H(I) 9 CH30H(g) and precipitation from solution, lead to an increase in system entropy (c) For a transformation to be spontaneous, the system enthalpy must decrease (d) All of the above are true IE / Question 2. Short answers. (4 points each) (I) Deuterium is a naturally occurring isotope of hydrogen. When light water, H20, and heavy water, D20, are mixed, the following reaction occurs spontaneously: H20(l) + D20(l) 9 2HOD(l) There is little difference in AHf° for H20, HOD and D20. What is the main driving force for the reaction? Explain your answer. Wow DAL he. 9&3 , lDwvencLQW3 MW 3 ‘l‘ mag-3 (11) Solid tin exists in two forms: white and gray. For the transformation Sn (s, white) 9 Sn (s, gray) The enthalpy change is -2.1 kJ mol'1 and the entropy change is -7.4 J K'1 mol'l. I. (a) Calculate the free energy change for conversion of 1.0 mol of white to gray tin at -30°C. l : —1\oo—— (243361?) 2:. -- 307.. 3 OK (b) At what temperature are White and gray tin at equilibrium at 1 atm pressure? AGZO / ANSWER Acme M iatwdclorium -T: 2%): «- fl :. 13‘1- K 4-.- AS ~—TH+ Vt) or iiQC l Miwefl Mow? OK! ® (111) A buffer solution was prepared by adding 8.00 g of sodium acetate NaCH3COO to 250 mL of 0.160 M acetic acid, CH3COOH. Acetic acid has a pKa= 4.74. fimfi Wagon -; 8.06 :O.\mo\—( «Amati L“$W“;’2§O*0JB~=0.0L§MQL go 300:: (a) What major species are present in solution? List them in order of decreasing concentration . 1 _. ~ .f H71) CH C 00 a New _ CH3 on H30 @ '7 3 s 7 /_____ 7 mow \chJJJDE AND oaogg onomuNC Memes (b) What is the pH of the buffer? ‘ t i) P a 0.6% I “'-'-""~—-—-. 12.00 10.00 8.00 4.00 2.00 0.00 0 50 100 150 200 250 300 350 mL NaoH added What is the pKa of the acid? 1 5S What would be a good choice of indicator for this titration? 0 , QJ‘fi/Sd‘i) WM‘W‘ Z O R ”Pia/wk P “Ami 24A 3 LONG ANSWERS (Show all your working to receive partial/full credit) (20 points) Question 3 (a) What is the solubility of Ba(OH)2 in 1 x 10‘3 M BaClz. X For partial credit and full credit, you must write the equation and a table below it. Bold/Q7, U) "ft-:4 36:“ + LCM- ‘Iminck (b) 5.0 mL of 0.10 M K1 (aq) is mixed with 1.00 L of 0.01 M AgN03(aq). Will precipitation % occur? Explain your answer. -10 0.5mmoL OSmM J. : L003 L gnarl} 1r 0 .0\ m . .. , ~S - .0 -:: 5 7'8 V0 9: [I 19C 943in =1 (000053” ‘3 ”a 5' KSAA‘afls "“0 8° Q > V‘ pm‘Pti‘nfl _l_§ W , (c) Agl precipitate can be redissolved in concentrated ammonia solution. Write chemical equations for what is taking place in solution. . + .— A I ‘ @‘ZNH ___,_. (wt. + I 5 U) T 3cm» A3 1m ”0 OR A6511?" -+ + I 2‘ .— M (5) [3"?ng W2) \ _ . ~ + AQ‘WD + ZN Hy M] :_ FECN“3\‘L mm Question 4 (20 points) (a) The rusting of iron is a complex reaction that can be approximated as 4Fe (s) + 3 02 (g) -) 2 Fe203 (s) Over what range of temperatures is rusting spontaneous when each reactant and product is present in its standard state? Show all your working. AW? g 2. A“; (mos) .— um; (Fe) -3 “flog g 20816-0 43 - -1657. k3/M9L Agar = IS.(F‘LOZD “ "'80 (fl) ‘33.,(013 : 1(30) ‘3(1°S) “ 9'03) .. 5:43, 3 Kathe". AGE -, AH: - ms: ._. -wsDx's/ML a: 158K Readwk Shwrmmx wan: 159° n macro“. AG° 5mm less Mfiafiwc. NEH» T CAKES can, 39°th U994: \‘unpergxu-q. is 1", $03: 30‘1“ thT<MlK (b) Will iron parts of a spacecraft rust spontaneously in the oxygen-sparse atmosphere of a planet that has 10'3 atm Oz and an ambient temperature of 1000K? Explain why. AGO 4W wfi‘k T: - - —— 0 k3 AG°= AH°—TAS° = 461,00» (unfit 5n) - \\ 3 4‘ AG WM watwiz— Ly) A9 = AG°+ RT M9 = -\\osx\o‘ -\ RT (‘14 to? = -\\o°;’ooq + (- 3m- Luau} : -\\os\ooo + 341' \A (W!) (6 pts) -\\03 000 + \‘l 2133 = 43m k'slmL. 5 4‘ SW m meme-r A-rmosonme-ar AG<o-X _——————— (C) The preparation of 803 from $02, a step in the manufacture of sulfuric acid, is 302(g) + 1/2 02(3) 9 303(g) What is the equilibrium constant for this reaction at room temperature? Show afl your working. ucjtgcg —_ AGE (5on viAGjCOz) -311 ~— (”$ch 2-. .—:u tar/mat. RT MK ~-.— -— AGO :> K: mpg... AGO/RT) _ ,_—- 11x9 (—‘?\ooo/(&.Z\W)(158)3 .— —- QKP (23‘. £53 Question 5 (25 points) Nitrate salts are usually rather soluble in water. The following graph shows the experimentally determined equilibrium constant for the solubility of barium nitrate, one of the less soluble nitrates, as a function of temperature: Ln (Kw) fi._____'_ 0.003 0.0035 0.004 0.001 0.0015 0.002 0.00 -0.0005 0.0005 l/T ( 1/ Kelvin) (a) The KSp values for Ba(N03)2 in water at 25°C and 509C are 0.015 and 0.13. What is the molar solubility of barium nitrate at the two temperatures? By what factor does the solubility increase on raising the temperature? Ksp : [Bog-*1 CNOf-XL =' (9A {2331 37“ *3 @ scuauttb : x = ji Lt 3 ’2, O - _ : O.l§'3 o @ SC . SOWW- “l . e~ ‘ - .3 0B 0 "3‘5 31‘ \ SOLUGUAT)’ Gram-rm a7 WoROF 1.1,” ‘ -——-,____ (b) From the graph, what is the AH°5°1ution and AS°501ufion for the dissolution reaction of barium nitrate Ba(NO3)2 (s) = Ba2+ (aq) + 2 N03'(aq) S\m = -AH" ~ r H z-Lg—f583 $103 Z %M i “‘9'“: sq tn - - mind tempt: MGHz .= I 1 => A305 “9 =+‘3\ JK 'Wn‘ xplain, based on physical reasoning, the signs of AH°solumn and AS°Solumm Y: '9 3 On dt3§0\\1\3 a Sate, wgbgsnslzm wuimm cor toms ave GEL—um V» 30% A36 WW ® EWZ) Me— Mr CM lather; Wok? WWMJ “$3M ng wk Wt‘r we’re i539“? 30”“ (d) Is the observed tren of increasin solubility w1t temperature, consistent with Le Chatelier’ s principle? If so, why? \a’ 1; PWS IS QAOiD “Wig; Q—‘Vkadanm Sk‘dh bikini)”. ma L2, who» -g:&« M sowing at ML.» kmpf, (e) If AG°f for Ba2+(aq) and N03'(aq) are -56‘1‘kJ/md and -11:J/mol respectively, usmg your earlier answers, find AG°f for Ba(N03)2(s). @ AGE _. nefcef") 4:21:66 (my) —— def (BaCNo-AQ AHor—TAS? -: ,_. S'g\ .\- 'L <.'..\\\\ "— AGE (BO‘QUOZELX k? w 3% " Q3§(9“) = —— Stu + ’L ( -\\\\ -—_. AGE C BQC““331\ i000 Question 5 (15 points) A 100.0 mL buffer solution is 0.15 M Nazl-H’O4 (aq) and 0.10 M KH2P04 (aq). The K32 for phosphoric acid is 6.2 X 108. ? Kat: '31-. ’2.\ (a) What is the pH of the buffer solution? @ W" PKQ1+ Mm ((111: = 1% (b) What is the solution pH after the addition of 8.0 mmol NaOH to the buffer? m #mm\ m saws ismmox HM" lO RANK HLPOH- @ Hflof‘ + OH“ '> HPOLE' + H19 SW 10 Es lS End 7. o 2.3- LHMI‘]= 7.3/‘0010Mh 2 0.23 M w meLWc Ore, CHLPCN‘12 l/looflmk = 001M . 0 .13 . W FK 1: VKOVL“. ‘01:)10 (101).: 12-\+ \'Oé" 2 8 2% @ (c) What is the solution pH after addition of 10.0 mL of 1.0MHN03 to the buffer? =7 lo mmoL 1430‘ _ 1— 9 oddzel H, lDOq ~l— 14 30+ 5 141) UK? + “to Sex '5 ‘0 ' 0 W mew-1; a 5w» s ”00 i)“:—+Z\ +‘mfiko< 7.0) C MLPU‘l‘l 2:. 10 110, O M ; é é) \ ...
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