Name Chem 163 Sectin: Team Number: ALE 24. Vltaic Cells and Standard Cell Ptentials (Reference: 21.2 and 21.3 Silberberg 5 th editin) What des a vltmeter reading tell us? The Mdel: Standard Reductin and Oxidatin Ptentials In general, the uctin f a metal catin in aqueus slutin t the elemental state f the metal can be written as eqn 1. M m+ (aq) + m e - M(s) (1) The uctin half-reactin has a standard uctin ptential, E. (The superscript naught indicates that the cncentratin f metal catins in slutin is 1 M.) The ptential has units f vlts (V). The greater the desire f a metal t becme uced, the mre psitive the metal s uctin ptential is. As with mst reactins, uctin reactins are reversible. The reverse f eqn 1 is written as eqn 2. M(s) M m+ (aq) + m e - (2) Being an xidatin half-reactin, eqn 2 has a standard xidatin ptential, E. x E f eq 2 has the same magnitude but the ppsite sign as E f eqn 1. While it is cnvenient t think f a metal either underging uctin r xidatin, certainly ther species can be uced r xidized. Hydrgen has been chsen t serve as a reference fr all ther species. By definitin, the standard uctin ptential f hydrgen is 0 V. Key Questins 2 H + (aq) + 2 e - H 2 (g) E = 0 V 1. Use the table f standard uctin ptentials in yur textbk t rank the fllwing 3d blck transitin metal catins in rder frm having the lwest ptential t be uced t having the highest ptential t be uced t the elemental state. Cr 3+ Mn 2+ Fe 2+ C 2+ Ni 2+ Cu 2+ Zn 2+ x lwest ptential t be uced < < < < < < highest ptential t be uced 2a. If a piece f zinc metal is placed in an aqueus slutin f nickel(ii), will the reactin shwn as eqn 3 ccur? Ni 2+ (aq) + Zn(s) Ni(s) + Zn 2+ (aq) (3) Explain yur answer. Page 1 f 6
2b. If a piece f cpper metal is placed in an aqueus slutin f cbalt(ii), will the reactin shwn as eqn 4 ccur? Explain yur answer. C 2+ (aq) + Cu(s) C(s) + Cu 2+ (aq) (4) 3. Metallic nickel disslves in 1 M hydrchlric acid spntaneusly accrding t eqn 5. But metallic cpper des nt. Ni(s) + 2 HCl(aq) NiCl 2 (aq) + H 2 (g) (5) Cu(s) + 2 HCl(aq) CuCl 2 (aq) + H 2 (g) Use the table f standard uctin ptentials in yur textbk t explain these facts. The Mdel: Vltaic Cells and Standard Cell Ptentials Because electrns can neither be created nr destryed, a uctin cannt ccur withut an accmpanying xidatin. If tw substances which have different uctin ptentials are cnnected t each ther (via an electrically-cnducting wire), then electrns can flw frm the material which is underging xidatin t the secnd material. When this secnd material accepts the electrns, uctin will ccur. Cnsider the vltaic that is cnstructed by cnnecting the standard hydrgen electrde ( SHE ) t a secnd half- cnsisting f an aluminum electrde submerged in a 1 M Al 3+ (aq) slutin. A schematic f the Al-H + is shwn t the right. The tw slutins are cnnected by a salt bridge (e.g., an agar suspensin f ptassium nitrate, in which the ins are free t flw). The tw electrdes are cnnected by a wire. Between the tw electrdes may be placed a vltmeter. If the tw electrdes are perating at their standard states, then the vltmeter wuld read 1.66 V. Key Questins Al(s) 1 M Al 3+ (aq) 4a. Which species, aluminum r hydrgen, has the greater ptential t be uced? (Circle yur answer.) V KNO 3 salt bridge Pt(s) wire 1 atm H 2 (g) 1 M H + (aq) Page 2 f 6
4b. Which f the fllwing reactins ccur in the tw half-s f the Al-H +? (Circle yur answer.) half- n left half- n right i. Al 3+ (aq) + 3 e - Al(s) & 2 H + (aq) + 2 e - H 2 (g) ii. Al 3+ (aq) + 3 e - Al(s) & H 2 (g) 2 H + (aq) + 2 e - iii. Al(s) Al 3+ (aq) + 3 e - & 2 H + (aq) + 2 e - H 2 (g) iv. Al(s) Al 3+ (aq) + 3 e - & H 2 (g) 2 H + (aq) + 2 e - 5a. When the xidatin half-reactin and the uctin half-reactin ccur simultaneusly, by what number must yu multiply the xidatin reactin? By what number must yu multiply the uctin reactin s that the number f electrns is cnserved? b. Write the verall balanced reactin that ccurs in the Al-H +. 6a. What is the standard xidatin ptential (in V) f the species that is xidized in the Al-H +? b. What is the standard uctin ptential (in V) f the species that is uced? c. What is the sum f the standard xidatin ptential f the xidized species [the answer t part (a)] and the standard uctin ptential f the uced species [the answer t part (b)]? (Refer back t the Mdel t see what the vltmeter reads when cnnected t the Al-H +.) d. True OR False: When determining the standard ptential, befre yu add the standard xidatin and uctin ptentials tgether, yu need t multiply E x by the same number that yu multiplied the xidatin half-reactin and yu need t multiply E by the same number that yu multiplied the uctin half-reactin. (Circle yur answer.) 7. The slutin that the aluminum electrde is submerged in is labeled Al 3+ (aq), but there must be sme anin (e.g., nitrate) in slutin s that it is electrically neutral. The same is true f the slutin that is labeled H + (aq). When xidatin ccurs at the surface f the aluminum electrde, Al 3+ ges int slutin. When uctin ccurs at the surface f the platinum electrde, H + leaves the slutin (as hydrgen gas). a. The ptassium catins flw frm the salt bridge int what half-? The nitrate anins flw frm the salt bridge int what half-? K + flws t: NO 3 - flws t: b. The electrde t which catins flw is called the cathde. Smething that can help yu t remember that the uctin half-reactin ccurs at the cathde is that bth wrds begin with a cnsnant. By analgy, why is the secnd electrde called the ande? What d the wrds ande and xidatin have in cmmn that might help yu remember which type f halfreactin ccurs at the ande? Page 3 f 6
7c. D electrns flw frm the ande t the cathde r frm the cathde t the ande thrugh the wire? (Circle yur answer.) 8. A schematic drawing f the H 2 -Ag + is shwn t the right. wire 1 atm H 2 (g) KNO 3 salt bridge Ag(s) Pt(s) 1 M H + (aq) 1 M Ag + (aq) a. Having t draw a schematic like the abve every time yu want t infrm a reader f what yu re cnsidering is tedius. Instead, we can use a shrthand ntatin called a diagram. The diagram starts ff with the ande written n the left. As yu cntinue t write the diagram, yu write additinal species in the rder in which they d be encunte in the slutins in ging frm the ande t the cathde. The symbl is used t indicate mving frm ne phase t anther and the symbl is used t indicate the salt bridge. The diagram fr the Al-H 2 (See the Mdel.) in the standard state is: Al(s) Al 3+ (aq, 1 M) H + (aq, 1 M) H 2 (g) Pt(s) Write the diagram fr the H 2 -Ag +. b. Write the half-reactins that ccur at the ande and at the cathde, and write the balanced reactin (WITH NO LEFT OVER ELECTRONS!) that ccurs in the H 2 -Ag +. Half-reactin at the ande: Half-reactin at the cathde: Balanced reactin: c. What are the values f E f the species that is xidized, E f the species that is uced, and E f the H 2 -Ag + x? E E E x 9. The diagram f the Al-Ag + in its standard state is: Al(s) Al 3+ (aq, 1 M) Ag + (aq, 1 M) Ag(s) a. Write the balanced net reactin that ccurs in the Al-Ag +. b. What is the value f E f the Al-Ag +? E Page 4 f 6
10. Indicate belw if the statement is true r false. If false, crrect the statement t make it true. a.) T r F In a vltaic, the ande is negative relative t the cathde. b.) T r F Oxidatin ccurs at the ande in bth vltaic s and electrlytic s. c.) T r F Electrns flw int the cathde f an electrlytic. d.) T r F In a vltaic, the surrundings d the wrk n the system. e.) T r F If a metal is plated ut f an electrlytic, it appears n the cathde. f.) T r F The electrlyte prvides a slutin f mbile electrns. 11. Cnsider the vltaic belw. a.) In which directin d electrns flw in the external circuit? b.) In which half- des uctin ccur? c.) In which half- d the electrns leave the? d.) At which electrde are the electrns generated? e.) Which electrde is psitively charged? f.) Which electrde increases in mass? h.) Suggest a pair f ins fr the salt bridge. i.) Fr which electrde culd yu use an inactive metal? j.) In which directin d catins within the salt bridge mve t maintain charge neutrality? g.) Suggest a slutin fr the ande electrlyte 12. Write the balanced half-reactins and the verall spntaneus reactin fr the vltaic in the previus questin. Page 5 f 6
13. In acidic slutin, zne and the manganese (II) in react spntaneusly: O 3 (g) Mn (aq) H 2 O(l) O 2 (g) MnO 2 (s) 2H (aq) E = 0.84 V a.) Write the balanced uctin half-reactin. Hint: use xidatin numbers t determine what is xidized and what is uced and then use Appendix D in yur text t find the balanced xidatin and uctin half-reactins. b.) Write the balanced xidatin half-reactin. (See the hint in part a!) c.) Using Appendix D in yur textbk t find E zne, and then calculate E manganese. 14. When a clean irn nail is placed in an aqueus slutin f cpper (II) sulfate, the nail immediately begins t turn a brwn-black clr. In a few minutes, the nail is cmpletely cve with a material f this clr. a.) What is the material cating the nail? Explain. b.) What is the xidizing agent? What is the ucing agent? c.) Can this reactin be made int a vltaic? Explain. d.) Write the balanced equatin fr the reactin. e.) Calculate E fr the prcess. Page 6 f 6