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2 Chem 16 Cmmn 3 Part I April 8, 011 ANSWERS 1. What is the xidatin state f chrmium in the dichrmate in, CrO 7? (a) 3 (b) 4 (c) 5 (d) 6 (e) 7 O =, Cr = x: x+ 7 = ; x = 6. Which f the fllwing cmpunds has the largest entrpy? (a) CH3CHBr (g) (b) CH3CHBr (s) Gases generally have larger entrpies than liquids which have larger entrpies than (c) CH3CHFBr (g) slids. The larger the mass and the mre cmplicated the mlecule the larger (d) CH3CHFBr (l) the entrpy. (e) CH3CH 3 (s) 3. What heat is added t the system if E = -1. kj and the system the system expands frm.0 t 8.0 L against an external pressure f 3.0atm. (1 L atm = J) (a) -0.6 kj (b) 0.6 kj 101.3J w=pext V =( 3.0atm)( 8 L) =18L atm = 1,800J =1.8kJ (c) -3.0 kj L atm (d) 3.0 kj E = q+ w; q = E w= kJ = 0.6kJ (e) 1.8 kj 4. Which f the fllwing statements is INCORRECT fr a cnstant pressure prcess? (a) If H sys > 0 (endthermic) and S sys < 0, then prcess is NOT spntaneus at all temperatures. (b) If H sys > 0 (endthermic) and S sys > 0, then prcess is NOT spntaneus at lw temperatures. (c) If H sys < 0 (exthermic) and S sys > 0, then prcess is spntaneus at all temperatures. (d) If H sys < 0 (exthermic) and S sys < 0, then prcess is spntaneus at high temperatures. Gsys = Hsys T Ssys < 0 fr spntaneus prcess 5. Hw many jules f heat are added t 7.0 g f zinc t increase its temperature frm ºC t 35ºC? Hint, use the law f Dulng et Petit t find the specific heat f Zn, that is, the mlar heat capacity f all metals is abut 5 J/(mlᴼC). Aside, it cmes frm the fact that metals all take up heat r energy the same way, namely, the atms vibrate mre strngly. (a) 40 J (b) 5 J (c) 94 J (d) 59 J (e) 35 J c 5J 1ml 0.38J = x = ml C 65.4g g C s 0.38J q = mcs T = ( 7g) ( 13 C) = 35J gc

3 6. Calculate the change in entrpy when 55 g f water is heated frm 7ºC t 67ºC. Sme f the fllwing prperties f water may be useful: H = 6,000 J/ mland the specific heat is 4.184J/(g. K). fusin (a) 9 J/K (b) 10 J/K (c) 1.6 J/K (d) 1 J/K (e) 18,000 J/K q = mc T. S, dq = mc dt s rev s dq T rev dt T 4.184J 340 S = = mcs mcsln ( 55g) ln 9JK T T = = = 1 T T 1 g K at 5 C : H kj/ml -18 S f f -1 1 ( K ) + CCl l C s Cl g 4 Jml Calculate the change in entrpy fr the abve reactin fr 1 mle f reactant at 5 C. (a) 36 J/K (b) -13 J/K 1 S prducts rxn = Sf Sf reactants = x = 36JK (c) -36 J/K (d) 13J/K (e) 4 J/K 8. Calculate (a) 1 kj (b) 90. kj (c) 6 kj (d) -37,600 J (e) -10 kj G rxn at 160 C fr the same reactin. H = H prducts H reactants = = 18kJ rxn f f 36J 1kJ G = H T S = 18kJ ( 433K) = 5.8kJ K 1000J 9. Still fr the same reactin, Calculate the temperature range fr which the reactin is spntaneus. (a) > 540 K (b) < 540 K (c) >3.0 x 10 4 K (d) < 3.0 x 10 4 K (e) > 0.54 K H 18000J G = 0 = H T S ; T = = = K S 36 J/ K fr T > 540 K, G < 0. S, reactin is spntane us. Frm the previus prblem, we see that fr T < 540 K, G > 0. S, fr T > 540 K, G < 0. Anther way: S > 0. S, T S < 0. As T gets larger this term becmes mre rxn negative and eventual G becmes negative. rxn rxn

4 10. What is the equilibrium cnstant fr a reactin at 300ºC if G rxn = 0.35 kj/ml at 300ºC. Careful. a b G 350 J / ml c. 1.1 G = RTln ( K) ; ln ( K) = = = RT ( 8.31 J / ml/ K )( 573K ) d e. nne f the abve K = e = If the reactin fr the abve prblem is A( g) B( g) C( s) D( g) Careful. a. atm b. atm -1 c. atm - d. atm -3 e. atm -4 K eq + 3 +, what are the units f K? P atm = = = atm PP D 3 5 A B atm 4 1. In the reactin, CH ( g) 3.5O ( g) CO ( g) 3HO( l) (a) C in carbn dixide (b) C in ethane, CH 6 (c) O in mlecular xygen (d) O in carbn dixide (e) H in ethane + +, which element is xidized? 6 In CH H is +1 (frm rules); s, C is assigned In CO O is - (frm rules); s C is assigned + 4. This means each C in ethane lses 7 electrns r is xidized. 13. In any reactin, including xidatin and reductin, bth material and charge must be balanced. Which f the fllwing reactins is crrectly balanced? a. Cr(s) + Br (aq) Cr 3+ (aq) +Br - (aq) b. Cr(s) + Br (aq) Cr 3+ (aq) +Br - (aq) c. Cr(s) + 3Br (aq) Cr 3+ (aq) +6Br - (aq) d. Cr(s) + Br (aq) Cr 3+ (aq) +3Br - (aq) 14. Calculate the standard galvanic cell ptential, Eº, fr the half reactins given t the right. (a) 4.69 V (b) V (c) 0.77 V (d) V (e) 1.05 V C C e V F + e F.87V C + F C + F 1.05V + 3+ Half reactin Eº (V) C 3+ + e - C F + e - F -.87

5 15. Fr the fllwing reactin, the standard cell ptential is V. What is Gº? (a) 81 kj (b) -81kJ (c) 160 kj (d) -160 kj (e) 30 kj Cr aq Cu s Cr aq Cu aq 3 Cr + + e Cr +. S, n =. ( ) 96,500 C 0.84 J G = nfe = ml e = 16, 000J ml e C 16. Fr the fllwing reactin, the standard cell ptential is V. What is the cell ptential fr the given cnditins? Careful Cr aq + Cu s Cr aq + Cu aq (a) -0.7 V (b) V (c) V (d) V (e) -1.1 V 0.03 M 7.g 1.6M 3.7M x3.7 E = E lgq = 0.84 lg 0.96V n = 0.03

6 Part II (36 pints) I (4 pints each part) (a) Frm 1st cmmn. The decmpsitin f "A" is given by the equatin: A(g) 3B(g) + C(g). The initial pressure f "A" is 1.0 atm and there are n prducts. What is the apprximate pressure f "A" when the ttal pressure is 1.5 atm? Cmplete the ICE table fr full credit. A(g) 3B(g) + C(g) Initial 1 atm 0 0 Change: -x +3x +x End: 1-x 3x x P = P + P + P tt A B C 1.5 = 1 x + 3x+ x = 1+ 3x x = 0.5 / 3 = 0.17atm P = 1 x = 0.83atm A (b) Frm nd cmmn. Calculate the ph f 0.90 M HCN. Include the apprpriate reactin and ICE table. K a is 6. x Reactin: Initial: + HCN aq + H O l H O aq + CN M Change: - x +x +x Equilibrium: 0.9 x x x 10 x 10 5 Ka = 6.x10 = ; x = 6.x10 ( 0.9 x) =.4x10 M 0.9 x 0 ph. ph 5 ( x ) = lg.4 10 = 4.63 (c) What is the vltage fr the fllwing reactin at 98 K and with the given cnditins: ( g) r MnO + 4 H +e Mn + H O. S, n = MnO s + H aq + H g Mn aq + H O l E = 1.1V sme.5m 14atm 0.33M sme Each H in H is assigned 0; in water it is +1. S e are lst. S, n =. r H + H H O; H + H + H O H O; H + H 4 H ; H H +e. S, n = Mn The Nerst equatin: E = E lg ( Q) where Q= = 1.1 lg 1.8V = n + H P.5 x14 H

7 II. The fllwing reactin will be balanced in a acidic slutin: P ( s) + NO ( aq) H PO ( aq) + NO ( g) (a) (3 pts) What is ne f the balanced half-reactins? Circle: xidatin r reductin half-reactin NO aq NO g 3 NO aq NO g H O 3 NO aq H NO g H O + 3 NO aq + H + e NO g + H O + 3 (b) (3 pts) What is the ther balanced half-reactin? Circle: xidatin r reductin half-reactin P s H PO aq P s H O H PO aq P s H O H PO aq H aq P s + 16H O 4H PO aq + 0H aq + 0e (c) ( pts) What is the balanced reactin? NO aq 40H 0e 0NO g 0H O 3 P s + 16H O 4H PO aq + 0H aq + 0e P s + 0NO aq + 0H aq 4H PO aq + 0NO g + 4H O (d) (4 pts) If the reactin were balanced in a basic slutin, what wuld be the verall balanced reactin? HPO is a triprtic acid. What happens when a base is added t an acid? Careful, phsphric acid OH ( aq) OH ( aq) HO 4 + 0NO ( g) + 4H O + 0OH ( aq) PTS HO HO + 1HO + 8OH ( aq) HO OH ( aq) PTS P s NO aq H aq H PO aq NO g H O P s NO aq H PO aq P s NO aq PO aq NO g 4 3 P s NO aq PO aq NO g 4 3 4

8 III. ( pts each part) The Ag-Cr galvanic cell is shwn belw. The reductin ptentials are: ( a) What is the net reactin and vltage fr the galvanic cell? Ag + Cr 3Ag + Cr E = 1.53V + Ag aq + e Ag s E = 0.80V 3+ Cr aq + 3 e Cr s E =0.73V (b) What is the reactin at the ande: 3 Cr s Cr + aq + 3e (b) Which electrde gets heavier: Silver (d) Which electrde is psitive? Silver (e) In which directin t the electrns flw thrugh the light bulb: left t right r right t left (circle) (f) Int which slutin d the K + ins mve frm the salt bridge: T maintain charge neutrality AgNO 3 r Cr(NO 3 ) 3 (circle) Why? (The nitrate ins, f curse mve int the ther slutin.)