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# 00 - APChem Exam 00 1 2H 2S(g)<=>2H 2(g S2(g A 3.40 g...

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AP Chem  Exam -  ‘00 1. 2  H 2 S (g)        <==>    2  H 2 (g)    +   S 2 (g) A   3.40   g   sample   of   H 2 S (g)   is   introduced   into   an   evacuated   rigid   1.25   L  container.  The sealed  container  is heated  to 483 K, and  3.72 x10 –2  mol of S 2 (g)  is present at equilibrium. (a) Write   the   equilibrium- constant,   K c ,   for   the   decomposition   reaction   represented  above. K c   =  [H 2 (g) ] 2 [S 2 (g) ]/[H 2 S (g) ] 2 (i) Calculate   the   equilibrium   concentration,   in   mol/L,   of   the  following  gases in the container at 483 K. H 2 (g)  =  (3.72 x10 –2  mol of S 2 )(2 mol H 2 /mol  S 2 )/1.25 L  =  0.0595 mol H 2 (g) /L H 2 S (g)   =  0.08 M - 0.00595 M =  0.0203 M H 2 S (g) (ii) Calculate  the  value  of  the  equilibrium  constant.  K c . For  the  decomposition  reaction at 483 K. K c   =  [0.0595] 2 [0.0372/1.25 L]/[0.0205] 2   =  0.256 (iii) Calculate   the   partial   pressure   of   S 2 (g)   in   the   container   at  equilibrium  at 483 K. P S 2   =  ()0.0372 mol)(0.0821 L-atm/mol- K)(483 K)/1.25 L  =  1.18 atm (iv) For the  reaction  H 2 (g)  + 1/2   S 2 (g)  <==> H 2 S (g)  at 483 K. calculate  the value of the equilibrium  constant, K c . K c1    =  1/K c      3.9 2. Answer  the following  questions that relate to electrochemical reactions.

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(a) Under  standard  conditions  at 25° C, Zn (s)  reacts with  Co  2+ (aq)  to produce   Co (s) . (i) Write the balanced  equation  for the oxidation  half reaction. Zn  <==>  Zn  2+ (aq)    +  2  e  - (ii) Write the balanced  net-ionic equation  for the overall reaction. Co  2+ (aq)   +  Zn  <==>  Zn  2+ (aq)   + Co (s) (iii) Calculate the standard  potential, E°, for the overall reaction at 25°  C. 0.76  +  (- 0.28)   =  0.48 V (b) At 25° C, H 2 O 2  decomposes according  to the following  reaction. 2  H 2 O 2 (aq)        ==>   2  H 2 O  (l)    +   O 2 (g)                              E°  =  0.55 V (i) Determine  the value  of the standard  free energy  change, ∆G°, for the  reaction at 25° C. ∆G°  =  -nFE°=   - (2)(96500 C/mol)(0.55 J/C)  =  - 110 kJ (ii) Determine  the value  of the equilibrium  constant, K eq , for the reaction  at 25° C. ∆G°/-  RT =  lnK eq   =  - (- 110 kJ)/(0.00831 kJ/mol- K)(298 K)  =  ln K eq   =    44.4 K eq   =  1.96 x10 19 (c) The standard  reduction  potential, E°, for the half-reaction O 2 (g)       +  4  H  +  (aq)        +  4  e-       2  H 2 O (l) has   a   value   of   1.23   V.     Using   this   information   in   addition   to   the  information  given  above, determine  the value  of the standard  reduction
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00 - APChem Exam 00 1 2H 2S(g)<=>2H 2(g S2(g A 3.40 g...

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