clr,p AP@ chemrsrry 2003 SCORING GUIDELINES (Form B) Question 6 Total Score 9 points l Answer the following questions about electrochemistry. (a) Several different electrochemical cells can be constructed using the materials shown below. Write the balanced net-ionic equation for the reaction that occurs in the cell that would have the greatest Hil positive value of E rn. 1.0 M Al(Nol)1 1.0 M Cu(NOr)z 1.0 M Fe(NO-,)2 n Al Metal Strip n Cu Metal Strip n Fe Metal Strip mmaterials fbr Salt Bridge HSolution to Fill Salt Bridge Voltmeter with Wire Al(s) -+ Cuz+(aq) 2 Al(s) + Al3*(aq) + 3 e- +2e- + Cu(s) 3 Cuz+(aq) --> 2 Al3+(aq) + 3 Cu(s) 1 point for selection of correct two redox couples I point for correctly balanced net ionic equation (b) Calculate the standard cell potential, Eo rrl, for the reaction written in part (a). E =-1.66Y Al3+(aq)+3e- + Al(s) Cuz+(aq)*Te- -+ Cu(s) E" : +0.34V E ru : Elanoa, - Ekoa" : +0.34 V-(-1.66V) = +2.00V 1 point for correct E"""n (Must be consistent with part (a)) Copyright @ 2003 by College Entrance Examination Board. A1l rights reserved. Available at apcentral.collegeboard.com. {3
2003 SCORING GUIDELINES (Fornn B) Question 6 (cont'd.) (c) A cell is constructed based on the reaction in part (a) above. Labellthe metal used for the anode on the cell shown in the figure below. Voltmeter Salt Bridge Cathode (Anode Metal) The metal is aluminum solid. I point for correct metal (Must be consistent vrith part (a)) (d) Of the compounds NaOH, CuS, and NaNO3, which one is appropriate to use in a salt bridge? Briefly explain your answer, and for each of the other compounds, include a reason why it is not appropriate. NaOH is not appropriate. The anion, OH-, would migrate towards the anode. The OH- would react with the Al3+ ion in solution. CUS is not appropriate. It is insoluble in water, so no ions would be available to migrate to the anode and cathode comparhnent to balance the charge. NaNO3 is appropriate. It is soluble in water, and neither the cation nor the anion will react with the ions in the anode or cathode compartment. i point for correctly whether each compound is appropriate, along with an explanation (3 points total) Coppight O 2003 by College Entrance Examination Board. All rights reserved. Available at apcentral.collegeboard.com. 14
2003 SCORING GUIDELINES (Form B) Question 6 (cont'd.) (e) Another standard cell is based on the following reaction. zn + Pt2+ -> znz* + Pb If the concentration of h?+ is decreased from 1.0 M to 0.25 M,whateffect does this have on the cell potential? Justiff your answer. E,,, : ELu -0.0s91n (rtr#j rf fznz+1is reduced, then the *," (t3^o11od < t, therefore h(tf#i) < 0' rhus 8""11 increases (becomes more positive). I point for correctly indicating how E"nn is affected I point for explanation in terms ofnernst equation and Q Copyright O 2003 by College Entrance Examination Board. All rights reserved. Available at apcentral,collegeboard.com. l5
2004 SCORING GUIDELINES (Form B) Question 6 Voltmeter f,lr lrlr : - MAgNO.., : -i MZn(NO.*), a The following questions refer to the electrochemical cell shown in the diagram above. (a) Write a balanced net ionic equation for the spontaneous reaction that takes place in the cell. Znz+(aq) * 2 e- --> Zn(s) Ag*(aq) + e- -+ Ag(s) Zn(s) + ZAg+(aq) -+ Zr?*@q; + 2Ag(s) I point for the balanced net-ionic equation 1 point for correct direction (reactants and products) (b) Calculate the standard cell potential, E, for the reaction in part (a). Znz+(aq) + 2e -+ Zn(s) Ag*(oq) + e- -+ Ag(s) E""ru = Elo*oa" - Ekoa" = tr : -0.76Y E" = *0.80 V +0.80V - (-0.76 V) : +1.56V 1 point for the correct, positive 8"""n (c) In the diagram above, (i) label the anode and the cathode on the dotted lines provided, and The anode is the zinc metal electrode, the cathode is the silver metal electrode. I point for the correct metal for anode and cathode I Copyright A 2n4 by College Entrance Examination Board. All rights reserved. Visit apcenhal.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for AP students and parents). 12
AP@ GHEMISTRY 2004 SCORING GUIDELINES (Form B) Question 6 (cont'd.) (ii) indicate, in the boxes below the half-cells, the concentration of AgNO3 and the concentration of Zn(NO3)2 that are needed to generate E" [Ag].IOrl : [Zn(NO3)rl = I M 1 I point for the correct concentration for the anodic chamber point for the correct concentration for the cathodic chamber (d) How will the cell potential be affected if KI is added to the silver half-cell? Justify your answer. A precipitate will form as I- ions react with in solution in cathode compartment. [Ag*] reduced, causing cell potential to decrease. Ag' rons will be I point for correctly indicating a reaction occurs I point for indicatingthe cell potential decreases l { Copyright @ 2004 by College Entrance Examination Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for AP students and parents). 13
2007 SCORING GUIDELINES (Form B) Question 3 ZH2G) + Oz(s) *t 2IjIO(I) In a hydrogen-oxygen fuel cell, energy is produced by the overall reaction represented above. (a) When the fuel cell operates at25"c and 1.00 atm for 78.0 minutes, 0.0746 mol of Oz(S) is consumed. Calculate the volume of H2(S) consumed during the same time period. Express your answer in liters measured at25oc and 1.00 atm. (0.0746mo1o).#A = 0.14e mol H, One point is earned for calculation of moles of H,, n- RT P = 3.65LHz (0.149 mol H2X0.0821L atm mol-r K-1X298 K) 1.00atm One point is earned for substitution into PV = nrt. One point is earned for the answer. (b) Given that the fuel cell reaction takes place in an acidic medium, (i) write the two half reactions that occur as the cell operates, o2 + 4H* * 4e- --+ zhzo Hz -) 2H+ + 2e- One point is earned for each of the two half reactions. \\ (ii) identify the half reaction that takes place at the cathode, and O, + 4H* + 4 e- -+ 2H,zO One point is eamed for either the equation of the correct half reaction, or for indicating "the reduction half reaction" if the correct equation is given in (bxi). (iii) determine the value of the standard potential, E., of the cell. E= 1.23Y +0.00V = 1.23V One point is eamed for the standard potential. A 2Wi The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.corr/apstudents (for students and parents).
2A07 SCORING GUIDELINES (Form B) Question 3 (continued) (c) Calculate the charge, in coulombs, that passes through the cell during the 78.0 minutes of operation as described in part (a). 4 mole- Q.0746 mol O" ) x x ' lmolo, 96,500 C = 2.88xtdC lmol e- One point is earned for the stoichiometry. One point is earned for the answer. @ 2007 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents).