CHEM31EX1key_2011

CHEM31EX1key_2011 - N 1:ng l A B C Which of the responses...

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Unformatted text preview: N... 1:ng ' l. A. B. C. Which of the responses below is NOT a statement of the first law of thermodynamics? The energy of the universe is constant. AEuniv = AEsys + AEsurr‘: O AE=q+w ® q=CAT ‘ I 2. 1C. D. 3., B. .0 In lecture, Dr. Lyle performed the “freeze-board” demonstration: An open beaker was placed on a wet piece of wood. Solid barium hydroxide Ba(OH)2 (s) and solid ammonium thiocyanate NH4$CN (s) were added to the beaker and stirred together. After the two solids reacted, a slush was formed in the - beaker, and the beaker became sufficiently cold that it froze to the board, enabling Dr. Lyle to “levitate” the bOard by lifting the beaker. In addition, a distinctive odor indicated the production of gaseous ammonia NH3 (g). 2 Which of the responses below applies to this reaction? AHrxn < O; Wrxn < 0 AHrxn > 0; Wrxn < 0 AHm1 < 0; wt,m > O AHnm > 0; wrxn > 0 Given that the heat of formation of liquid water H200) is -286 kJ/mol, what is the enthalpy change for the reaction ' . 221-120(2) —>2H2 (g) + 02(g) Afon = ? +572 kJ +286 k] —143 kJ —286 kJ ' —572 kJ Name kEY 4. Of the four processes listed below, which two are endothermic? AH 13 [HF( g)] < 0 H20(s)—>H20(Z) 1 01 H200) —-> H20(s) HF(g) ——>1/2H2(g)+l/2F2(g) l/2H2(g)+l/2F2(g) —> HF(g) land3 ewwe B. l and4 C. 2 and 3 D. 2 and 4 5. A CHEM 31 stundent sets up an experiment Where she can illuminate a metal with light beams of various wavelengths, all of which produce electrons through the photoelectric effect. If she increases the wavelength of the light beam while keeping the light intensity constant, she would find A. the kinetic energy of the electrons would increase. 1" V = h Va + E B. the kinetic energy of the electrons would stay the same. . v DC i, ((9 the kinetic energy of the electrons would decrease. A D. the number of electrons produced would increase. 1* f [mm 5 » 06 7 ‘" W5 ’ I d6: E. the number of electrons produced would decrease. 0g ,3 6. Because electrons in atoms behave quantum mechanically, they are not attracted to the nucleous. they can never have precise energies. they do not have a definite mass. DOW? they travel in fixed orbits around the nucleous. (19 there may be certain positions around the nucleous where they will never be found. “NM Name Y 7. Which of the electron configurations below would correspond to a possible excited state of a neutral neon atom? A. 1322s22p5 B. 1 $22s22p6 @ 1322522p53 S1 V D. 1s225221c>63s1 E. 1522522p53sz 8. Which of the following sets of quantuminumbers is NOT possible? n I m, m S 4 1' - —2 +1/2 beoe’e‘ or am V 3 0 1 —1/2 C. 3 0 0 +1/2 D. 2 1 1 —1/2 E 1 0 0 +1/2 As discussed in lecture and in the text, the electron configuration of chromium (Cr) is not What might be expected on the basis of the Aubau principle. Chromium has how many unpaired electrons? Cy'. 4.5, >0 Name K EY 10. Which of the responses below gives the correct ordering for the atomic radii of As, O, P, and S? A. As<P<S<O B. As<S<P<O c. S<P<O<As .O<S<P<As E. O<P<S<As Ni KEY FREE-RESPONSE PROBLEMS: (SHOW YOUR WORK TO RECEIVE PARTIAL OR FULL CREDIT) N I. (10 points) Calcium carbonate deposits form in caves via the reaction: Ca 2+ (aq) + 2HC03'" (aq) —-> CaCO3 (s) + C02 (g) + H 20(1) AH; = +3 91d a. Assuming the inside of the cave is at T = 15 °C and P = 1 atm, determine AE for this reaction. = sat-r -— <1 m:7/3.;MTAWDOE‘31‘19‘JK @ =aak7— 23443 -= arm-24410 3 3'7- k 3" b. Given the heats of formation listed below, determine for HCO; (aq) . Substance: Ca2+(aq)' CaC03(&) , C02(g) H200) AH;(kJ/mol) —543 ~1207.6 —393.5 —285.8 ‘ m.) fin = an: = 0H; {c.4351 may (“1133 tbaffcogj —- AH; Ca“? — ,9 Aflf EH;ng @ abxigifimgle-no‘llo eras-283.73 +543 as] kT/mt T . Osman = ~ea: £52 c. Supposing the concentration of calcium ions Ca2+ (aq) in cave groundwater is 5.8 X 10‘3 M, determine the change in temperature of a liter of groundwater if all of the calcium ions precipitate out via the above reaction. You may assume that a liter of groundwater weighs exactly 1000 g, and that its specific heat capacity is that of pure water, e.g. 4.184 J/g °C. (Be sure to specify the sign of the temperature change; positive for heating, negative for cooling.) For 'Wot 5‘5 $163M flaw/n.9on , w-Q Low-e W mad; Cauéarv') ', we bawe 4 Q j @ AH”), ’3 Eq/kT/ndckn 13.8 >¢Io nun-t =‘ 23 abSO'UBJ} :73 Th read-Car... H fibfiw = CHWAT '3 " 112m ” "224’ J 426’: _ LE“? :aoosa"; Cato 143%“! 32% x "M? AT =‘ Name KEY 11. (10 points) The sun’s atmosphere contains vast quantities of He+ ions. These ions absorb some of the sun’s thermal energy, promoting electrons to excited states of He+. a. For a He+ ion initially in the fifth energy level (ni = 5), calculate the longest wavelength and shortest wavelength of photons that could be emitted when the ion drops to a lower energy level, and identify the Kalue of nf for the final energy level for each transition. Put your answers in the table below. H???“ i y it, was! Tranition x (11m) Final n_ 5 it» Hog-P / =$ ) 53:46“ :2 - . =9 ’h =1 shortest wavelength LN ? ,A @ simmfp); :- lAEl longest wavelength 10 l . Z ‘ =3 )7; : L] I ~ ~ 4, _J_ __ 4? “7;: MEI ' “Kazzlnf m2) - QJHMD 3x11 x fifxizl -7 _ = $.41; x20 ‘ 3 *l a‘g—I} = $.3é4/Mo "7 nizs-amf: A6 <5 -5 3‘ ‘ . ’A: ha. 30.4.24»on 97-53(2‘4‘gx’“ 2‘): 2.7mm“: h~ AF: aawwm‘” 9' _ '8 __ 2333:4131 4&1997’53“ AEQKfiHzxm. 3‘ w lfiffil; = ['TéoMo‘HT , ‘6 ’29 $3135 z-(é.é%‘&w"‘ras)(2fi"i‘6WD EL 1.01%)«0 m OE -) ’ I X“) «3 :,D\3nm_ b. List below all possible values of the quantum number E for n=5. . H Z 4/ @ X ' o 2 l J J 3 D L7, c. It turns out that for a photon to be emitted, the value of the quantum number E must change by i1, e. g. .6 f — fl,- = i1. Given the value of nf that you identified for the shortest wavelength emission in part a, what must have been the value of Z in the initial state with ni=5, e. g. .for this transition was the initial +‘ state55,5p,5d,etc?, Fa Ha. SKOYKSJ' ’X WMSEM 1 41;: I J SD 7* 0 must 1y. is wi 25:0. m New: ,9,- =+l)¢n-q 5“: Ski}. d. The average thermal energy of the sun can be approximated as RT, where R is the gas constant and T the absolute temperature. In order for there to be a substantial number of He+ ions produced, the average thermal energy must exceed the ionization energy for He, e.g. RT 2 11. Given 11 = 2377 kJ/mol for He, estimate the minimum temperature required in degrees Kelvin to produce substantial numbers of He+ ions. ‘(F or reference, the temperature at’the surface of the sun is approximately 6000 K, whereas in the core it is as high as 14 x 106 K.) @ RT7/ I! Name E g Y , III. (10 points) Although hydrogen atoms in most compounds are assigned an oxidation state of +1, in salt-like hydrides such as LiH(s) the hydrogen ions have a charge and oxidation number of -1. This fact can be understood in part by comparing the two electron transfer reactions below: Li<g>+H<g> —>Lz'+(g)+rr<g) AHA” Li<g)+H<g> —>Lz"(g)+H+(g> AHfam a. Given the first ionization energies (ll) and electron affinities (EA) listed below, determine values of AH ,1”, and M3,,” for the above two reactions in kJ/mol. Atom I; (kl/mol) EA ( gl/molz H 1312 -72.8 Li 520. -59.6 k)” k3" AHJM = Lat) + EACH‘X = 520.744 "mime @ ' = 444 mm? . - E1“ _ ‘52“ our?“ < 1:104) wane) v m we 9‘1"? m ~= r252. k‘T/mf‘f b. Based on your answers to part a., explain briefly why reaction 1 would be favored over reaction 2, e. g. why the H in LiH(s might have a charge of—l rather than +1. b 0 DR Y‘Q aod-Caao aw: mated—aura ‘tc ) rectal-Om (L is less so ,mwsng M ‘ ~ ‘0‘. I b i ‘ a H’ \S less (OSHQ +0 m0 [‘2 LL 1" H c. A measure of the stability of a salt like LiH(s) is the “lattice energy,” defined as the energy for the formation of the salt from its gaseous ions. For Lil-1(5) this would correspond to the reaction: ' 6 Li+(g)+rr(g) —>LiH(s) AHm =LE By using your result from part a, along with the reaction below, determine the lattice energy LE for LiH(s). \ Lil-Ks) —>Lz'(g) + H (g) Aern = + 469 kJ/mol L%)+H'(5) L5 7mm) 67"? ~M’m= 44% thjme) 2444 Icy/M? m ‘ k7“ Lg = 444%} —%9 L5} = ~6le 7,43 d. The Li+ cation and the hydride anion H_ both have 2 electrons. Which ion is larger? Explain your response. mmuflzm M a 5mm))€n.mvx&1d4 (No-«MU an“? lacs/3264”; lewd +8.»; gleam ‘ l9“ @ WM)“; H" WNQLQ km "Kw. YA&3MS. ...
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CHEM31EX1key_2011 - N 1:ng l A B C Which of the responses...

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