Making and Experimenting with Voltaic Cells. I. Basic Concepts and Definitions (some ideas discussed in class are omitted here)

Transcription

1 Making xperimenting with Vltaic Cells I. Basic Cncepts Definitins (sme ideas discussed in class are mitted here) A. Directin f electrn flw psitiveness f electrdes. If ne electrde is mre psitive than anther, there will be a ptential difference between the electrdes, electrns (which are negative, right?!) will mve spntaneusly frm the mre negative ne t the mre psitive ne (if given the chance). If electrde A is mre psitive than electrde B, we will call electrde A the psitive electrde electrde B the negative electrde. Thus, if given the chance, electrns will mve spntaneusly frm electrde B t electrde A (because electrns have a negative charge, are attracted t the mre psitive). B. Deducing the verall reactin that is taking place in a vltaic. If yu knw which electrde is mre psitive in a given vltaic (yu can figure this ut using a vltmeter see C belw!), yu shuld be able t deduce the spntaneus chemical reactin that is taking place in the : (a) Knwing which ne is negative shuld tell yu which way the electrns are flwing (frm the negative ne t the psitive ne), then: (b) Once yu knw the directin f flw f electrns, yu shuld be able t deduce what (half) reactins are taking place at each electrde: if electrns are flwing frm an electrde, then xidatin must be ccurring there (t prduce the electrns that are flwing ut); if electrns are flwing int an electrde, then uctin must be ccurring there (the electrns are being added t a species in the slutin tuching the electrde). (c) Once bth half reactins are knwn, the equatin fr the verall reactin can be written ( balanced, if desi). C. Using a vltmeter t deduce experimentally which electrde is psitive which negative. If the reading n a vltmeter is psitive, that means that the (+) lead is attached t the (mre) psitive electrde ( the black (-) lead is attached t the (mre) negative ne)! If the reading n a vltmeter is negative, then the (+) lead is attached t the (mre) negative electrde ( technically yu shuld switch them because it is cnventin t reprt the ptential fr a vltaic as psitive s it (als) represents the ptential fr the spntaneus reactin that is ccurring in the ). ither way, the bttm line is that by knwing hw a vltmeter wrks, yu can use a vltmeter t tell yu which electrde is psitive which is negative in a vltaic (galvanic). D. Understing the meaning f relatin f xidatin) half reactins, hw ( is related t x ) values fr different uctin ( x. See xp. 32 discussin f this in lab manual. I'd like t add here that if, as is nted in the lab manual, the measu fr the in which ne electrde is Zn/Zn 2+ the ther is H2/H + /Pt is V, with the Zn electrde being the negative ne, then this means the fllwing: a) H + is a better xidizing agent than Zn 2+ (because Zn gets xidized in the, nt H2), which means that: b) The uctin f H + has a greater tendency t ccur than the uctin f Zn 2+, which means that: c) The stard uctin ptential f H + is mre psitive than the fr Zn 2+, d) The f H + is 0.76 V mre psitive than the fr Zn 2+.

2 A table f values is a ranking f the relative "strengths" f xidizing agents, it can be generated by measuring wh "wins the battle" fr the electrns ( by hw much!) in a series f pairwise "battles" (s) cntaining reactants prducts at stard state cnditins. This is what yu will be ding in Part A f xperiment 32 (Task 5 belw).. Using ( understing cnceptually) the Nernst quatin t pict ptentials under nnstard cnditins (e.g., when cncentratins f ins in the are varied frm stard). The Nernst quatin is directly derived frm the equatin: G = G + RTln Q, which, as discussed in class, relates the free energy change at a given T under any cnditins [cncentratins may be any values] t the free energy change at stard cnditins [cncentratins are all 1 M]. T help appreciate the abve equatin ( the Nernst equatin), ne needs t underst the cnceptual imprtance meaning f "G" "driving frce". S, a bit f review: 1. "Driving frce" G. Recall that "driving frce" refers t the tendency f a reactin t ccur (at any cncentratin cnditins; nt just stard state nes) that: If G < 0, frward reactin is spntaneus; there is a "driving frce" fr the frward reactin. If G > 0, frward reactin is nnspntaneus; there is a "driving frce" fr the reverse reactin. If G = 0, neither frward nr reverse reactin is spntaneus; there is n "driving frce" fr either reactin. THUS: *The "mre negative" the G, the greater the driving frce fr the frward reactin r the lesser the driving frce fr the reverse reactin.* *The "mre psitive" the G, the lesser the driving frce fr the frward reactin r the greater the driving frce fr the reverse reactin.* 2. "Driving frce". The relatin G = -nf qualitatively indicates that a psitive fr a given (hypthetical) reactin under a given set f cnditins is assciated with a negative G value. Thus (nte that signs are ppsite thse in bx in 1 abve!): If > 0, frward reactin is spntaneus; there is a "driving frce" fr the frward reactin. If < 0, frward reactin is nnspntaneus; there is a "driving frce" fr the reverse reactin. If = 0, neither frward nr reverse reactin is spntaneus; there is n "driving frce" fr either reactin. *The "mre psitive" the, the greater the driving frce fr the frward reactin r the lesser the driving frce fr the reverse reactin.* *The "mre negative" the, the lesser the driving frce fr the frward reactin r the greater the driving frce fr the reverse reactin.* 3. The Nernst quatin A rearranged versin f G = G + RTln Q Fr a slutin prcess represented by the generic balanced chemical equatin: aa + bb cc + dd 2

3 recall that Q, the reactin qutient, has the same frm as the equilibrium cnstant expressin, but describes a system under any cnditins, whereas K refers nly t a system at equilibrium: c d c d [ C] [ D] [ C] eq[ D] eq Q ; K a b a b [ A] [ B] [ A] eq[ B] eq Substituting -nf fr G -nf fr G int the equatin G = G + RTln Q, rearranging, using T = K, R = J/Kml, F = 96,485 C/ml e -, cnverting lnq = lgQ yields the mst cmmn frm f the Nernst quatin: V lgq n (T = 298 K; V is the unit "Vlt", it is nt a variable) Cmparing this equatin t: G = G + RTln Q it is clear that the 2nd term (the "lg Q" term) in each equatin represents a "crrectin" t accunt fr any "deviatin" frm stard state cnditins. That is, if Q is nt equal t 1, then the values f G will deviate frm their values at stard state cnditins. As Q gets smaller smaller (appraches zer), the will get mre mre psitive the G will get mre mre negative. A prcess gets a higher higher driving frce it has a greater greater tendency t ccur in the frward directin as Q gets smaller, G gets mre negative, gets mre psitive. I've summarized the imprtant relatinships belw, bth mathematical cnceptual, it wuld be well wrth yur time t study this table carefully until yu are cmfrtable with the ideas within: If Q = 1 If Q < 1** If Q << 1** (i.e, appraches 0) Mathematical Relatins then then then ln Q = 0 (lg Q = 0) ln Q < 0 (lg Q < 0) lnq << 0 (lg Q << 0) G = G = G < G > G < 0 > 0 (eventually) ** Remember that the smallest that Q can be is 0; Q cannt be negative! Cnceptual Meaning/Cmments G may be psitive r negative; it depends n G ( K). Same is true fr. There will be a driving frce smetimes fr the frward reactin smetimes fr the reverse reactin (at stard state). G may STILL be psitive r negative, but value is LSS than G ( may STILL be psitive r negative, but it is GRATR than ) the prcess has mre driving frce in the frward directin (r less driving frce in the reverse directin) than at stard state cncentratins If Q gets sufficiently small, G will be negative will be psitive the prcess will be spntaneus; there will be a "driving frce" fr the frward reactin [Table cntinues n next page ] 3

4 If Q > 1 then lnq > 0 If Q >> 1 (i.e, as it appraches infinity) then lnq >> 0 G > G < G > 0 < 0 (eventually) G may STILL be psitive r negative, but value is GRATR than G ( may STILL be psitive r negative, but it is LSS than ) the prcess has less driving frce in the frward directin (r mre driving frce in the reverse directin) than at stard state cncentratins If Q gets sufficiently large, G will be psitive will be negative the prcess will be nnspntaneus; there will be a "driving frce" fr the reverse reactin Practically, what this means is that if yu dilute the half cntaining a prduct (r decrease the cncentratin f a prduct in sme ther way), the system will have a greater tendency t have frward reactin ccur, this will manifest itself in a real way by increasing the! The actual ptential will becme mre psitive (i.e., larger). Cnversely, if yu dilute the half cntaining a reactant (r decrease the cncentratin f a reactant in sme ther way), the ptential becmes less psitive (i.e., it decreases) indicating less driving frce in the frward directin. Als, if yu were t increase the cncentratin f a prduct, yu d expect the ptential t decrease (less driving frce in frward directin), if yu were t increase the cncentratin f a reactant, I hpe yu can see that the ptential shuld increase (greater driving frce in the frward reactin). The specific value f the change in can be calculated theretically by the Nernst equatin (it depends n the frm f Q the value f n). II. Tasks xercises 1. Testing ut vltmeter peratin. Use the vltmeter (ask me hw t use it first!) n a stard battery t cnvince yurself that the statement in Part I.C. abve is true. That is, pretend that yu did nt knw which end f the battery was the psitive end, use the vltmeter reading the statement t figure it ut. 2. Observing a spntaneus chemical reactin directly. Drp the small (precut) piece f Zn(s) int a small beaker with a few ml f 1.0 M CuSO4(aq). Put a penny (Cu) int a bit f 1.0 M ZnSO4(aq) in a small beaker. Observe the tw beakers write yur bservatins in yur lab ntebk. Is there evidence fr reactin in either (r bth)? In yur prelab, yu wrte ut balanced chemical equatins t describe the pssible chemical reactin that culd be imagined t ccur in each case abve. Write the ne that describes the spntaneus chemical reactin yu are bserving n yur reprt frm. 3. Making a vltaic (frcing the electrns t g thrugh a wire). Cnstruct a vltaic that matches the equatin yu generated in questin #2 by using a strip f Zn, a strip f Cu, ~30 ml f each f the 1.0 M (slutins, tw 100 ml beakers Fr the salt bridge, yu will be using a strip f filter paper dipped in a slutin f KNO3 (ask me fr details). Be very careful abut which electrde yu put int which slutin!! Yu d NOT want direct reactin t ccur; yu want the electrns t flw thrugh the wire! Ask me if yu are uncertain abut this. 4. Measuring vltage figuring ut what s ging n in yur. Use the vltmeter t measure the ptential f the yu have made. Based n yur reading ( hw yu cnnected the leads!), which electrde is the psitive ne? What is the (spntaneus) reactin that is ccurring in the (see next sentence)? In rder t answer this questin, (a) sketch the, indicate the vltage reading, (b) write the half-reactins that are ccurring at each electrde, (c) label the 4

5 e cathde, (d) shw directin f electrn flw, (e) shw the sign f each electrde, lastly (f) write the balanced chemical equatin representing the spntaneus chemical reactin that must be ccurring in the. Is yur answer cnsistent with yur bservatins ( answers) in #2 abve? 5. Trying a different design, generating a series f values. The fllwing is a variatin f xperiment 32, Part A, but dn t read the lab manual nw. Just fllw the instructins here. Prepare the fllwing fur galvanic s, in the rder shwn, using the cup setup rather than the beakers setup. In the cup setup, yu d nt use a salt bridge the prsity f the clay vessel allws a tiny bit f cnnectin between the tw slutins (inside vs utside clay vessel). Yu d nt need t use the clamps n the uter cup t hld the electrdes in place (althugh yu can if yu want t). (1) Zn 2+ /Zn vs. I2/I - Put 20 ml f the 0.1 M I2/I - slutin in the uter (plastic) cup (using the graphite rd as electrde) 10. ml f the 0.1 M (nt 1 M!!) Zn 2+ slutin in the inner (clay) vessel (using the small Zn strip as electrde). Attach the leads frm the vltmeter recrd: a) the ptential (cnnect the leads s that the vltage reading is psitive), b) which electrde was attached t the lead which electrde was attached t the black lead when the vltmeter gave a psitive reading. *Dn t dump ut anything yet.* (2) Zn 2+ /Zn vs. Cu 2+ /Cu Remve the clay vessel, but keep the Zn 2+ slutin Zn strip in it! Rinse the utside f the clay vessel with DI water dry. Discard the cntents f the plastic cup, rinse with DI water, replace with 20. ml f the 0.1 M Cu 2+ slutin. Use the Cu wire fr the electrde). Return the clay vessel t the cup, cnnect the leads, recrd raw data as in (1). (3) Fe 3+ /Fe 2+ vs. Cu 2+ /Cu Remve the clay vessel, discard the Zn 2+ slutin. Keep the Cu 2+ slutin Cu wire in the plastic cup. Rinse bth the inside utside f the clay vessel with DI water. Replace with 10. ml f the Fe 3+ /Fe 2+ slutin, use the graphite rd as the electrde. Return the clay vessel t the cup, cnnect the leads, recrd raw data as in (1). (4) Ag + /Ag vs. Cu 2+ /Cu Remve the clay vessel, discard the Fe 3+ /Fe 2+ slutin. Keep the Cu 2+ slutin Cu wire in the plastic cup. Rinse bth the inside utside f the clay vessel with DI water. Replace with 10. ml f the Ag + slutin, use the silver wire as the electrde. Return the clay vessel t the cup, cnnect the leads, recrd raw data as in (1). **DO NOT DISMANTL THIS STUP. YOU WILL US IT FOR TASK 6!** 6. Testing the effect f cncentratin n ptential. Discnnect the vltmeter frm the last f the fur s yu created in Task 5, carefully carry the vltmeter t the hd. Reattach the leads verify that the ptential is clse t what yu measu earlier in Task 5. Recrd bth the ptential the clr f the slutin in the uter cup (shuld be light blue because f Cu 2+ (aq)). While the ptential is being measu, add abut 20 ml f 6 M NH3(aq) t the Cu 2+ slutin (uter cup), stirring carefully with yur stirring rd. Recrd bth the ptential the clr f the slutin. NOT: All data shuld be recrded in yur lab ntebk first. Fr Task 6, yur data will be transfer t the lab manual s reprt frm fr Part B Cmparisn f measu values theretical value (Pst-lab exercise). D as stated n the reprt frm. Take a mment t assess hw yur measu ptential values fr yur Zn 2+ /Zn vs. Cu 2+ /Cu galvanic s cmpare t the value calculated frm yur text s values fr the apprpriate stard uctin ptentials. 5

6 Data Questins Making xperimenting with Vltaic Cells Name(s): Task 1. What was the vltage reading (sign as well as value) when the lead f the vltmeter was attached t the psitive end f the test battery yu used? What was it when the leads were reversed? Task 2. (a) What, if anything, did yu bserve in each experiment (write bservatins, nt equatins!!!)? Penny in ZnSO4(aq): Zn in CuSO4(aq): (b) Write the balanced net inic equatin that yu think describes the reactin yu bserved ccurring: Tasks 3 & 4. (a)-(e) Sketch the, including experimentally bserved vltage as well as all the ther items asked fr in the text f the lab. Als write yur bserved vltage here: (beakers setup): (f) Balanced net inic equatin cnsistent with yur sketch abve fr what is ccurring in yur : (g) Des the spntaneus reactin that yu have deduced (f abve all was deduced simply frm the sign f the vltage reading f yur!) appear t be the same reactin as the ne yu bserved ccurring in Task 2? Describe what is ccurring verbally in this x reactin (wh is giving electrns t whm?): Task 5 (1) Use the lab manual s reprt frm fr xp 32 (Table A.1 Cell Ptentials) t reprt yur raw data fr Task 5 (the fur s yu made). Then cmplete the wrkup f the data in Table A.1 accrding t the instructins belw the table (als demnstrated in the class PwerPint). (2) In additin, fr each electrde system in Table A.1 circle the species (nt the whle electrde system!) that is the better xidizing agent. Briefly explain here yur reasning fr (3) nly: Please spend sme time n this exercise! Think f each experiment as a "battle fr electrns" between tw xidizing agents. Whichever ne is the better xidizing agent is the ne that will (a) pull electrns t its half /electrde, (b) be a reactant in the spntaneus reactin (appears n the left side f the equatin), (c) will have the greater stard uctin ptential ( ). Hpefully this will help yu relate the value f t the difference in the tw 's (f the tw ptential xidizing agents).

7 (3) Use the lab manual s reprt frm fr xp 32 (Table A.2 Table f lectrde Ptentials) t cmplete the analysis f yur data frm Task 5 as indicated. See the assciated PwerPint slides discussing this. (4) IN ADDITION, add a clumn with the heading frm text t the right f the right-mst clumn uctin in Table A.2, write the uctin values frm the text that crrespnd t the half reactins in yur Table. (i) Briefly cmment n hw well (verall) yur experimental Table f Reductin Ptentials matches the published ne (right-mst clumn): (ii) Are all f the half reactins the same rder (i.e., best t wrse xidizing agents) in bth tables? (iii) Are the values in pretty gd agreement? In answering, identify the uctin half reactin that is the clsest (ther than Ag + /Ag which is made t agree) farthest frm its crrespnding published value. Task 6. (1) Use the Reprt Frm frm the Lab Manual fr B.1 t reprt yur raw data nly. Then d the fllwing: (2) Write ut the balanced net inic equatin fr the reactin befre the additin f the NH3 (G back t Table A.1 t see the half reactins!). Als write the expressin fr Q t the right f the equatin. (3) What was the clr f the slutin after NH3 was added? This clr is due t: (chemical species?) (4) xplain cnceptually in detail why the ptential ges up when the NH3 is added. Hint: Reread the paragraph n p.4 f the pink hut just after the table! (5) Calculate the residual cncentratin f free Cu 2+ in. Use the Nernst quatin, assuming the [Ag + ] t be 1.0 M (even thugh it s nt), alng with yur raw data (measu values). Task 7. Calculate the stard ptential ( ) fr the Zn 2+ /Zn vs. Cu 2+ /Cu galvanic yu made in Task 3 in Task 5 ( #2). Use yur text s Table f Stard Reductin Ptentials shw the calculatin. (calculated using text s values): Yur experimental values: (beakers setup): ; ("cup" setup): Are yur experimental values reasnably clse t ne anther*? T the calculated value*? *Cnsider that the ptentials in the Table f Stard Reductin Ptentials span a range f nearly 6 vlts! As such, being within 0.1 V is fairly reasnable here.