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2 Chem 16 Cmmn 3 Part I April, 18, 8 ANSWERS 1. The Haber prcess is at equilibrium at 7 K with sme nitrgen, hydrgen, and ammnia: N (g) 3H (g) NH 3 (g) H = -9 k Which f the fllwing cnditins DOES NOT increase the equilibrium amunt f ammnia? (a) The additin f nitrgen (b) The additin f hydrgen (c) Increasing the pressure by reducing the vlume (d) Increasing the temperature (e) Decreasing the temperature. Fr which f the fllwing prcesses is the change in internal energy, E,? (a) Steam at 1 ºC is cnverted t water at 1 ºC. E decreases (b) An aluminum bar melts at 65 ºC. E increases (c) A gas expands adiabatically (q = ) against a pressure f 1 atm. E decreases (w is dne n surrundings) (d) The heat added at cnstant vlume (q v ) is. E = q w= q V = (PV wrk = at cnstant V) (e) The heat added at cnstant pressure (q p ) is. H is. E may be -,, r. 3. What is the change in E, when f heat is added and the system expands frm 5. t 5. L against an external pressure f.85 atm. (1 L atm = 11.3 ) (a), (b), w=pext V =(.85atm)( 5 5L) =17L atm =1,7 (c) 6,1 L atm (d) -,7 E = q w=, 17 =, 7 (e) -6,1. A prcess takes place at cnstant pressure. The vlume changes and the temperature decreases by 9 C. The heat capacity f the system at cnstant vlume, C V, is 315 k/ C and C P, is 35 k/ C. What is E in k? (a) 9, k (b) -9, k 315k E = C (c) -91, k ( 9 V T = C) =91,k C (d) -18, k (e) Insufficient infrmatin t determine answer 5. Fr which f the fllwing reactins is the enthalpy change in the reactin, (a) N 3H NH3 N g 3H g NH g (b) ( ) ( ) 3 ( ) (c).5n( g) 1.5H( g) NH3( g) (d).5n( g) 1.5H( g) NH3( g).75( g) (e).5n ( g) 1.5H ( l) NH ( g).75 ( g) 3 6. Which f the fllwing mlecules has the largest entrpy? (a) C Br 6 (b) C F 6 (c) CH (d) CF 3 CH 3 (e) CF 3 CBr 3 H, equal t H f fr NH 3 (g)? ( ) ( ) ( ) ( ) H = H NH.5 H N 1.5 H H = H NH f 3 f f f 3

3 7. Calculate the free energy f reactin fr the fllwing reactin: Fe 3 C(s) 3.5O (g) Fe O 3 (s) CO (g) G k/ ml : (a) 1,558 k (b) 1,98 k (c) -1,19 k (d) -1,98 k (e) -1,558 k f ( ) 8. Fr the reactin the abve reactin (prblem 7), H <. Is this reactin exthermic r endthermic and what is the expected change in entrpy? (a) Exthermic and entrpy decreases (because there are mre mles f reactant gas than prduct gas) (b) Exthermic and entrpy increases (c) Endthermic and entrpy decreases (d) Endthermic and entrpy increases (e) Endthermic and n change in entrpy 9. Fr HOl ( ) HO( g) at 7 ºC, G 3.18 k/ ml. Calculate the apprximate vapr pressure f water at this temperature. (Careful: k and.) (a).39 atm (b).38 atm G =RTln Keq (c).5 atm (d).999 atm 3,18 G (e).38 atm ln Keq = = ml =1.115 RT 8.31 ( 33K ) ml K P = K = e =.38atm 1. Fr isprpyl alchl the H 5.8 k/ ml and S 8.6 / ( ml K ) = =. Calculate the freezing pint f isprpyl alchl. ( = in r melting. Hint cnsider the reactin : isprpyl alchl(s) isprpyl alchl(l). Here H = H. Careful: k and.) (a) -9 ºC (b) -88 ºC (c) -86 ºC (d) -8 ºC (e) -8 ºC HO eq ( ) ( ) ( ) G prducts = Gf Gf reactants = =1,558k G = H T S = at equilibrium (freezing pint) H 5, 8 ml / T = = = 185K = =88 C S 8.6 / ml K ( ) 11. Fr the reactin A(s) B (aq) A (aq) B(s), G = 1. k at 98 K. What is G if [B ] is 3.5 M, [A ] is. M, and sme f each slid is present? A. G = G RTln Q where Q = = =.196 (a) -3. k 3.5 (b) k B (c) 7. k k G = 1.k 8.31 ( 98K) ln (.196) 3.k (d) -.3 k 3 = K 1 (e) nne f the abve

4 1. What is the xidatin state f sulfur in HS O - 3? (a) - (b) 1 (c) (d) 3 (e) H = 1, O=, S = x: 1 x 3( ) = 1; x = ; x = 13. Fr a spntaneus reactin frm left t right, which f the fllwing is INCORRECT? (a) Q < K (b) H T S < (c) G > (d) E > (where E is the vltage) (e) S universe > 1. What mass f Cu(II) ins is plated frm an average current f. A fr 5.5 minutes? (a).17 g C 6s (b).3 g q = It = 5.5min x = 6,6C s min (c) 1.9 g (d).36 g 1ml e ml e = 6, 6C =.68ml e (e) nne f the abve 96,5C 1ml Cu 63.5g Cu.68ml e.17g Cu = ml e ml Cu 15. Fr the galvanic cell Cd Cd Cu Cu the vltage fr standard cnditins is.7 vlts. What is the vltage at 98 K if [Cd ] is changed t.1 M? (a).71 V Frm the cell diagram, Cd is xidized and Cu(II) ins are reduced. Thus, the reactin is: (b).73 V (c).7 V Cd ( s) Cu ( aq) Cd ( aq) Cu ( s) (d).75 V Cd.1 (e).77 V Q = = =.1; n = (because t electrns needed fr reductin r xidatin) Cu E = E lg Q =.7 lg.1 =.77V n 16. Which irn nail is mst likely t immediately start rusting? (a) A nail in the Sahara desert (b) A partially galvanized (cated with Zn) nail drpped in dilute acid (c) A partially galvanized nail expsed t air and salt water (d) A nail carefully cated with a layer f paint expsed t air and salt water (e) A nail that has been bent and straightened several times expsed t air and salt water.

5 Part II (36 pints) I.. ml f. M Ammnium chlride, NH Cl, is titrated with.15 M sdium hydrxide. K a = 5.7 x 1-1 M. (a) ( pts) Write the neutralizatin reactin include charges, aqueus, etc. where apprpriate. ( ) ( ) HO ( l) NH3 ( aq) net inic r ( ) NaOH ( aq) HOl ( ) NH ( aq) NaCl( aq) NH aq OH aq NH Cl aq (b) (3 pts) What is the ph after 1 ml f sdium hydrxide have been added? NH ( aq) OH ( aq) H O ( l) NH3( aq) ( ) (.ml ) = I ml mml mml L C E [ NH ] 1.5 ph = pka = ( x ) = = 3 1 lg lg lg NH 1.5 (c) ( pts) What is the ph at the midpint pint? At the midpint, [ NH ] [ NH ] = NH 3 ph = pka = ( x ) () = 3 1 lg lg lg 1 9. NH (d) ( pts) Hw many ml f sdium hydrxide are added t reach the equivalence pint? At the equivalence pint, mml OH = mml initial NH = 16 mml (all the acid.15ml.15mml has been neutralized)..15 M = = and L ml ml 16 17mL =.15mmlOH ( mml OH ) (e) (3 pts) What is the ph after 13 ml f sdium hydrxide have been added? ( ) ( ) ( ) ( ) NH aq OH aq H O l NH aq 3 I 16 mml 19.5 mml C E ph determined by the cncentratin f the strng base. 3.5mml poh = lg = mL ph = 1 poh = 1.31

6 II. The fllwing reactin will be balanced in a basic slutin: S ( aq) Cl ( g ) Cl ( aq) SO ( aq) (a) (pts) What is the balanced reductin half-reactin? (The half reactins need t crrect fr basic slutin.) ( ) ( ) Cl g e Cl aq (b) (pts) What is the balanced xidatin half-reactin? ( ) ( ) ( ) 8 ( ) 8 ( ) ( ) 8OH ( aq) ( ) ( ) 8OH ( aq) ( ) HO( aq) 8 H ( aq) SO ( aq) 8H O( aq) 8e S aq H O aq SO aq H aq e S aq H O aq SO aq 8H aq 8e S aq O ( ) 8 ( ) ( ) ( ) S aq OH aq SO aq H O aq 8e (c) (pts) What is the balanced reactin? ( ) ( ) Cl g 8e 8Cl aq ( ) ( ) ( ) ( ) S aq 8OH aq SO aq H O aq 8e ( ) ( ) ( ) ( ) ( ) ( ) Cl g S aq 8OH aq 8Cl aq SO aq H O aq (d) (pts) What is xidized? S (lses electrns) (e) (pts) What is the xidizing agent? Cl (allws smething t be xidized by taking electrns, that is, it is reduced)

7 III. (pts each part) The silver-zinc battery has the fllwing half-cells: Ag O s H O l e Ag s OH aq ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) Zn s OH aq Zn OH s e (a) Circle the crrect respnse: (i) Reductin takes place at the zinc electrde and it is psitive. (ii) Reductin takes place at the zinc electrde and it is negative. (iii) Oxidatin takes place at the zinc electrde and it is psitive. (iv) Oxidatin takes place at the zinc electrde and it is negative. ( k ml) 1 1 ( K ml ) ( ) ( ) ( ) ( ) ( ) ( ) H / S f (b) What is H? Zn s Ag O s H O l Zn OH s Ag s ( ) H = H prducts H reactants = =35k f f (c) What is S? S = S prducts S reactants = ml =6K ml K ( ) ( ) -1 (d) What is G? 6 1k G = H T S =35k 98 =37k 3 K 1 (e) What is E, that is, the vltage? G 37, G = nfe; E = = = 1.59 = 1.59V nf 96,5C C ( ml e ) ml e (f) What is the maximum wrk prduced frm a silver-zinc battery cnsisting f 3.7 g Zn, 116 g Ag O, and 9.1 g H O (these are equal mlar amunts)? 3.7g mles = =.5 ml Zn, etc. 65. gmlzn / max max = = (.5 )( 37 / ) =15 ( This is wrk dne n system) ( ) = ( ) = 15 w n G ml k ml k w surrundings w system k max