10.7. Electrolysis. Electrolytic Cells

Transcription

1 Elctrolysis As discussd in Sction 10.6, corrosion is th outcom of a spontanous rdox raction. An imprssd currnt (on mthod that is usd to provid cathodic protction) can stop corrosion by rvrsing th spontanous corrosion raction and driving it in th non-spontanous dirction. An imprssd currnt passs an lctric currnt through th structur bing protctd, such as a piplin. An xtrnal lctrical sourc supplis th currnt. Othr non-spontanous rdox ractions can b forcd to occur in a similar way. On xampl is rcharging th scondary clls in a battry. A battry uss a spontanous rdox raction to produc an lctric currnt. Rcharging a battry involvs applying an xtrnal nrgy sourc to driv th rdox raction in th rvrs, nonspontanous dirction. For xampl, a lad storag battry in a car can b rchargd by applying a currnt to it (Figur 1). In this sction, you will larn how som usful non-spontanous ractions can b forcd to occur. Many of ths ractions hav important consumr and industrial applications Figur 1 A car with a dad battry can b rstartd by conncting th dad battry to a working battry. It is important to follow th manufacturr s dirctions whn stting up jumpr cabls. Othrwis, a spark might caus th hydrogn gas that is producd to xplod. Elctrolytic Clls A galvanic cll producs a currnt whn an oxidation rduction raction happns spontanously. Anothr kind of cll, calld an lctrolytic cll, uss a currnt from an outsid sourc to mak an lctrolytic raction occur. In a procss calld lctrolysis, th currnt is forcd through th cll to produc a chmical chang for which th cll potntial is ngativ. That is, lctrical nrgy causs an othrwis non-spontanous chmical raction to occur. Elctrolysis has grat practical importanc. For xampl, lctrolysis is usd to rcharg a battry, produc aluminum mtal, and covr objcts with a layr of chrom. lctrolytic cll a cll that uss lctrical nrgy to produc a chmical chang that would not occur spontanously lctrolysis th application of currnt through a cll to produc a chmical chang 10.7 Elctrolysis 663

2 Figur 2 shows th diffrncs btwn a galvanic cll and an lctrolytic cll. Th galvanic cll (Figur 2(a)) runs spontanously to produc 1.10 V. Anod half-raction quation: Zn 1s2 S Zn21 1aq Cathod half-raction quation: Cu21 1aq S Cu 1s2 Figur 2(b) shows an xtrnal nrgy sourc forcing lctrons through th cll in th dirction opposit to that shown in Figur 2(a). This rquirs an xtrnal potntial gratr than 1.10 V, bcaus th xtrnal potntial must b applid in opposition to th natural cll potntial. Th dvic in Figur 2(b) is an lctrolytic cll. Notic that th anod and th cathod ar rvrsd in Figurs 2(a) and 2(b). In a galvanic cll, zinc is th anod. In an lctrolytic cll, coppr is th anod. Also, ions flow through th salt bridg in opposit dirctions in th two clls. powr supply gratr than 1.10 V cations Zn(s) (a) cations Cu(s) anions Zn(s) Cu(s) anions Zn2 (aq) SO42 (aq) Cu2 (aq) SO42 (aq) Zn2 (aq) SO42 (aq) Cu2 (aq) SO42 (aq) 1.0 mol/l Zn2 solution 1.0 mol/l Cu2 solution 1.0 mol/l Zn2 solution 1.0 mol/l Cu2 solution anod cathod Zn(s) h Zn2 (aq) + 2 Cu2 (aq) + 2 h Cu(s) (oxidation) (rduction) Galvanic cll cathod anod (b) Zn2 (aq) + 2 h Zn(s) Cu(s) h Cu2 (aq) + 2 (rduction) (oxidation) Elctrolytic cll Figur 2 (a) A galvanic cll producs a currnt from a spontanous rdox raction: Zn 1s2 1 Cu21 1aq2 S Zn21 1aq2 1 Cu 1s2 (b) An lctrolytic cll rquirs a powr sourc to driv th opposit, non-spontanous raction: Cu 1s2 1 Zn21 1aq2 S Cu21 1aq2 1 Zn 1s2 Elctrolysis of watr In Sction 10.3, you larnd that th raction in which hydrogn and oxygn combin spontanously to form watr can b usd to produc an lctric currnt in a ful cll. Th rvrs raction is non-spontanous and is an xampl of lctrolysis. Elctric currnt must b supplid to carry out th lctrolysis of watr. During lctrolysis, watr is oxidizd at th anod and rducd at th cathod. Thrfor, watr is both th oxidizing agnt and th rducing agnt in this raction. Only two quations in Tabl 1, Appndix B7, involv just watr as an oxidizing agnt or a rducing agnt: O2(g) 1 4 H1(aq) S 2 H2O(l) E r V 2 H2O(l) S H2(g) 1 2 OH2(aq) E r V Not that th watr in ths two quations forms an upward diagonal to th right. This mans that th lctrolysis raction is non-spontanous. Howvr, w can forc it to occur by applying a currnt. Th raction quations can b writtn as follows: 664 Chaptr 10 Elctrochmical Clls 7924_Chm_CH10.indd 664 5/4/12 3:50 PM

3 or Anod half-raction quation: 2 H 2 O(l) S O 2 (g) 1 4 H 1 (aq) Cathod half-raction quation: 4 H 2 O(l) S 2 H 2 (g) 1 4 OH 2 (aq) [multiply by 2 to balanc lctrons] Nt ionic quation: 6 H 2 O1l2 S 2 H 2 1g2 1 O 2 1g2 1 4 H 1 1aq2 1 4 OH 2 1aq2 2 H 2 O1l2 S 2 H 2 1g2 1 O 2 1g2 4 H 2 O1l2 Th standard cll potntial for th lctrolysis of watr can b dtrmind using th sam quation that is usd for galvanic clls: g E r (cll) 5 E r (cathod) 2 E r (anod) V2 (11.23 V) E r (cll) V If you trid to lctrolyz pur watr by using platinum lctrods connctd to a 6 V battry, you would obsrv no raction. This is bcaus pur watr contains so fw ions that almost no currnt can flow. Howvr, dissolving vn a small amount of an lctrolyt, such as a solubl salt or a strong acid, in th watr causs th immdiat production of bubbls of hydrogn and oxygn (Figur 3). Th lctrolysis of watr supplis oxygn to submarin crws and to astronauts in th Intrnational Spac Station. Figur 3 Th lctrolysis of watr producs hydrogn gas at th cathod (on th right) and oxygn at th anod (on th lft). Not that twic as much hydrogn as oxygn is producd. Comparing Galvanic and Elctrolytic Clls A galvanic cll and an lctrolytic cll ar both typs of lctrochmical cll. Thy hav som charactristics in common and som diffrncs. Th main charactristics of galvanic clls and lctrolytic clls ar compard in Tabl 1. A galvanic cll convrts chmical nrgy into lctrical nrgy through a spontanous chmical raction that taks plac at th lctrods. Oxidation occurs at th anod, giving th anod a ngativ charg. Rduction occurs at th cathod, giving th cathod a positiv charg. A wir conncts th anod and th cathod, allowing lctrons to mov from th anod to th cathod. Th lctrods ar also connctd by a salt bridg or a similar ionic conductor, which prmits ions to flow btwn thm. Th flow of ions prvnts a charg from building up in on of th lctrods. Sinc th rdox raction that taks plac in a galvanic cll is spontanous, a galvanic cll can b usd as an nrgy sourc for an lctrical dvic. In contrast, an lctrolytic cll rquirs an xtrnal nrgy sourc to oprat. Th basis for an lctrolytic cll is a non-spontanous rdox raction. Thrfor, nrgy must b providd to driv th raction. Lik a galvanic cll, an lctrolytic cll has an anod (th sit of oxidation) and a cathod (th sit of rduction). Elctrons lav th anod along th wir, pass through th sourc of lctrical nrgy, and travl to th cathod. lctrochmical cll a gnral trm that is usd to rfr to both a galvanic cll and an lctrolytic cll Tabl 1 Comparison of Galvanic and Elctrolytic Clls Galvanic cll involvs a spontanous rdox raction producs lctrical nrgy uss a salt bridg to prvnt a charg buildup Elctrolytic cll involvs a non-spontanous rdox raction rquirs lctrical nrgy to driv th raction may or may not us a salt bridg rsults in oxidation at th anod and rduction at th cathod rsults in oxidation at th anod and rduction at th cathod Th lctrod that would b th anod of a galvanic cll is th cathod of an lctrolytic cll. Th lctrod that would b th cathod of a galvanic cll is th anod of an lctrolytic cll Elctrolysis 665

4 powr supply Commrcial Elctrolytic Procsss An important charactristic of mtals is thir ability to donat lctrons (bcom oxidizd) to form ions. Sinc mtals ar usually good rducing agnts, most ar found in natur in ors. Ors ar mixturs of ionic compounds, which oftn contain oxid, sulfid, and silicat anions. Th nobl mtals, such as gold, silvr, and platinum, ar mor difficult to oxidiz and ar mor oftn found as pur mtals. anod (graphit) anions cations cathod (iron) solution containing Zn 2 (aq) ions Figur 4 A modl of th lctrolytic cll that is usd industrially to lctroplat zinc onto iron Elctrolysis of Aquous Solutions Many industris us lctrolytic clls. On important application of lctrolysis involvs plating on mtal onto anothr mtal. For xampl, in Sction 10.6 you larnd that an ffctiv way to prvnt iron from corroding is to coat it with zinc. In this procss, calld galvanizing, th mtal objct to b platd is mad th cathod of th lctrolytic cll by attaching it to th positiv trminal of th lctrical nrgy supply (Figur 4). An inrt lctrod, such as graphit, is oftn usd as th anod. Th lctrods ar thn placd in an aquous solution that contains cations of th mtal to b platd onto th cathod. For galvanizing, a zinc sulfat solution is usd. As th cll oprats, a shiny layr of zinc prcipitats onto th iron cathod. Bubbls of oxygn ris from th anod, suggsting that th watr is oxidizd. Th raction quations ar givn blow: Anod half-raction quation: 2 H 2 O(l) S O 2 (g) 1 4 H 1 (aq) Cathod half-raction quation: 2 Zn 21 (aq) S 2 Zn(s) [multiply by 2 to balanc lctrons] Nt ionic quation: 2 H 2 O(l) 1 2 Zn 21 (aq) S 2 Zn(s) 1 O 2 (g) 1 4 H 1 (aq) 4 H 2 O1l2 Th standard cll potntial for this cll is E r (cll) V2 (11.23 V) E r (cll) V In practic, a potntial diffrnc of mor than 1.99 V must b applid for zinc to plat onto iron. mini Invstigation Pncil Elctrolysis mini Invstigation Skills: Prforming, Obsrving, Analyzing SKILLS HANDBOOK A1 In this invstigation, you will us lctrical nrgy to mak a nonspontanous raction occur. Th graphit in pncil lad srvs as th lctrod for both anod and cathod half-ractions. Equipmnt and Matrials: chmical safty goggls; lab apron; Ptri dish; 2 small pncils sharpnd at both nds; 2 conncting wirs with alligator clips; 9 V battry; 0.5 mol/l potassium iodid, KI(aq); droppr bottl of phnolphthalin indicator 1. Put on your chmical safty goggls and lab apron. 2. Half-fi ll th Ptri dish with th potassium iodid solution. 3. Add about 10 drops of phnolphthalin to th solution. 4. Plac on nd of ach pncil in th dish. Kp th pncils far apart from ach othr. 5. Us th conncting wirs to connct th dry nd of ach pncil to a trminal on th battry. 6. Aftr about 30 s, disconnct th pncils from th battry. 7. Look for vidnc of chmical chang. Rmov ach pncil, and chck for an odour. 8. Dispos of th contnts of th dish according to your tachr s instructions. A. Idntify th anod and th cathod of this cll. T/I B. What vidnc of chmical chang did you obsrv whil th pncils wr connctd? T/I C. Basd on your vidnc, prdict th idntity of on substanc that was producd at th anod and on substanc that was producd at th cathod. Justify your prdictions. T/I 666 Chaptr 10 Elctrochmical Clls

5 Aluminum Production Sinc aluminum is a vry activ mtal, it occurs naturally as an oxid in an or calld bauxit. Th production of aluminum mtal from its or provd to b mor difficult than th production of most othr mtals. In 1854, a procss for producing mtallic aluminum using sodium was discovrd. Howvr, aluminum rmaind xtrmly xpnsiv. In 1886, Charls M. Hall in th Unitd Stats and Paul Héroult in Franc almost simultanously discovrd a practical lctrolytic procss for producing aluminum. Th ky factor in th Hall Héroult procss is th us of moltn cryolit, Na 3 AlF 6 (l), as th solvnt for th aluminum oxid (also calld alumina). Today, aluminum is rlativly inxpnsiv and has a wid varity of applications, such as building matrials (Figur 5). Elctrolysis is possibl only if ions can mov to th lctrods. A common mthod for making ions mobil is to dissolv th substanc to b lctrolyzd in watr. Howvr, watr cannot b usd for th lctrolysis of th aluminum salt bcaus watr is mor asily rducd than aluminum ions: 2 H 2 O1l S H 2 1g2 1 OH 2 1aq2 E r V Al 31 1aq S Al1s2 E r V Thus, aluminum mtal cannot b platd out of an aquous solution of aluminum ions. Gnrally, ions can b mad mobil by mlting th salt. Howvr, th mlting point of aluminum oxid, Al 2 O 3, is much too high (2050 C) for lctrolysis of th moltn oxid to b practical. A mixtur of aluminum oxid and cryolit, howvr, has a mlting point of 1000 C, and th rsulting moltn mixtur can b lctrolyzd to obtain aluminum mtal. As a rsult of th Hall Héroult discovry, th pric of aluminum plungd and its widsprad us bcam conomically fasibl. Th industrial production of aluminum still uss th Hall Héroult procss. Bauxit is not pur aluminum oxid. It also contains oxids of iron, silicon, and titanium, along with various silicat matrials. To obtain th pur hydratd aluminum oxid, Al 2 O 3 # nh 2 O, th bauxit is tratd with aquous sodium hydroxid. Aluminum oxid is amphotric and thus racts in a basic solution: Al 2 O 3 1s2 1 2 OH 2 1aq2 S 2 AlO 22 1aq2 1 H 2 O1l2 Th othr mtal oxids, which ar basic, rmain as solids. (Rcall Sction 8.6.) Th solution that contains th aluminat ion, AlO 2 2 (aq), is sparatd from th sludg of th othr oxids and is acidifid with carbon dioxid gas, causing th hydratd alumina to r-form: 2 CO 2 1g2 1 2 AlO 22 1aq2 1 1n 1 12H 2 O1l2 S 2 HCO 32 1aq2 1 Al 2 O 3 # nh 2 O(s) Thn, th purifid aluminum oxid is mixd with cryolit, Na 3 AlF 6, and mltd at about 1000 C. Th dissociatd aluminum ions ar rducd to aluminum mtal in an lctrolytic cll (Figur 6). Figur 5 Th low mass, th lustr, and th corrosion rsistanc of aluminum mak it a usful matrial for cladding th outsids of buildings. anods (graphit rods) carbon dioxid formd at anods carbon-lind iron tank moltn aluminum to xtrnal powr supply moltn Al 2 O 3 /Na 3 AlF 6 (l) mixtur plug Figur 6 This lctrolytic cll is usd to produc aluminum by th Hall Héroult procss. Sinc moltn aluminum is dnsr than th moltn mixtur of cryolit and alumina, th mtal sttls to th bottom of th cll and is drawn off priodically Elctrolysis 667

6 Th lctrolyt solution in th Hall-Héroult cll includs many diffrnt ions that contain aluminum, so th chmistry is not compltly clar. Howvr, aluminum oxid probably racts with th cryolit anion: Al 2 O 3 1s2 1 4 AlF l2 S 3 Al 2 OF l2 1 6 F 2 1l2 Th following half-cll ractions ar also thought to occur: Cathod half-raction quation: AlF l S Al1s2 1 6 F 2 1l2 Anod half-raction quation: 2 Al 2 OF l F 2 1l2 1 C1s2 S 4 AlF l2 1 CO 2 1g Th ovrall cll raction quation can b writtn as 2 Al 2 O 3 1s2 1 3 C1s2 S 4 Al1s2 1 3 CO 2 1g2 Th aluminum producd in this lctrolytic procss is 99.5 % pur. Most applications of aluminum us alloys that includ othr mtals. For xampl, to b usful as a structural matrial, aluminum is alloyd with mtals such as zinc (for trailr and aircraft construction) and mangans (for cooking utnsils, storag tanks, and highway signs). Unfortunatly, th production of aluminum has ngativ ffcts on th nvironmnt. Sinc a grat dal of nrgy is consumd during th procss of aluminum rfining, aluminum production contributs to grnhous gas missions, such as prfluorocarbons (PFCs), polycyclic aromatic hydrocarbon (PAH), fluorid, F (aq), sulfur dioxid, SO 2 (g), and carbon dioxid, CO 2 (g). Placing aluminum smlting plants nar rnwabl sourcs of lctrical nrgy, such as watrfalls, rducs th missions from lctricity production. Howvr, th consumption of th carbon lctrods during th lctrolytic procss also producs PAH and carbon dioxid. Rcycling usd aluminum rquirs only about 5 % of th nrgy that is usd to mak nw aluminum. Effctiv rcycling programs not only rduc manufacturing costs, but also rduc th harmful missions associatd with producing aluminum. Elctrorfining Mtals Th purification of mtals is anothr important application of lctrolysis. Impur coppr from th chmical rduction of coppr or is cast into larg slabs that srv as th anods for lctrolytic clls. Aquous coppr sulfat is th lctrolyt. Thin shts of ultrapur coppr function as th cathods (Figur 7). Whn a potntial diffrnc is applid across th lctrods, only coppr is dpositd on ths cathods. All th Figur 7 Ultrapur coppr shts, which srv as th cathods of an lctrolytic cll, ar lowrd btwn slabs of impur coppr, which srv as th anods. It taks about four wks for th anods to dissolv and for th pur coppr to b dpositd on th cathods. 668 Chaptr 10 Elctrochmical Clls

7 othr impuritis ar lft as sludg in th cll. Ultrapur coppr is important for th production of smiconductors and othr lctronic componnts. Th main anod half-raction quation can b writtn as follows: powr supply Cu1s2 S Cu 21 1aq Othr mtals, such as zinc and iron, ar also oxidizd from th impur anod: Zn1s2 S Zn 21 1aq and F1s2 S F Nobl mtal impuritis in th anod, such as silvr, gold, and platinum, ar not oxidizd at th voltag usd. Thy fall to th bottom of th cll to form a sludg, which is thn procssd to rmov thm. Th coppr(ii) ions from th solution ar rducd and dpositd onto th cathod: Anod half-raction quation: Cu 21 1aq S Cu1s2 This procss producs coppr mtal that is % pur. High-purity gold is also producd by lctrorfining. Elctroplating Mtals Mtals that radily corrod can oftn b protctd by applying a thin coating, or plating, of a mtal that rsists corrosion. Exampls ar tin cans (which ar actually stl cans with a thin coating of tin), chrom-platd stl car bumprs, and silvrplatd jwllry and dcorativ itms. An objct can b platd by making it th cathod in a tank that contains ions of th plating mtal. Silvr plating is an xampl of this lctrolytic procss (Figur 8). Th silvr plating solution contains ions that form complxs with th silvr ion. By lowring th concntration of availabl silvr ions, a smooth, vn coating of silvr forms ovr th surfac of th objct (Figur 9). CAREER LINK Ag anod Ag Ag Ag cathod Figur 8 In an lctroplating cll, th itm to b platd (th spoon) is th cathod and a silvr bar is th anod. Silvr is platd out at th cathod: Ag 1 1aq2 1 2 S Ag1s2 Not that a salt bridg is not ndd bcaus silvr ions ar involvd at both lctrods. Figur 9 Silvr plating is oftn usd to bautify and protct cutlry and tablwar, such as this lgant ta st. Mtal plating procsss hav brought many bnfits but also pos a rang of nvironmntal concrns. Havy mtals, such as chromium, lad, and cadmium, ar usd in ths procsss. Cyanid is a poisonous compound that is usd in plating baths. Th rlas of any of ths toxic mtals or thir ions into th nvironmnt can caus srious halth problms for organisms and prsistnt damag to cosystms Elctrolysis 669

8 10.7 Rviw Summary Elctrolysis involvs forcing a currnt through a cll to caus a nonspontanous rdox raction to occur. Elctrolytic and galvanic clls ar both lctrochmical clls. Thy contain an anod whr oxidation occurs and a cathod whr rduction occurs. An lctrolytic cll, unlik a galvanic cll, rquirs an xtrnal nrgy sourc and involvs a non-spontanous rdox raction rathr than a spontanous rdox raction. Industrial applications of lctrolysis includ th lctrolysis of watr and othr aquous solutions, as wll as rcharging battris, producing aluminum, lctrorfining mtals, and lctroplating. Qustions 1. Explain, using a graphic organizr of your choic, th diffrncs btwn a galvanic cll and an lctrolytic cll. Rfr to typ of raction and nrgy flow. K/U 2. Dscrib a consumr product that contains a galvanic cll or battry and is somtims usd as part of an lctrolytic cll. A 3. Prdict th half-raction that will tak plac at th cathod and anod whn ach salt is mltd and lctrolyzd. T/I (a) NiBr 2 (l) (b) AlF 3 (l) (c) MnI 2 (l) 4. Explain why aluminum rcycling can b good for both businss and th nvironmnt. T/I 5. A stl ring can b lctroplatd with gold in an aquous solution that contains gold(iii) ions. Sktch and labl a diagram of a cll that could b usd to produc gold-platd stl. Idntify th anod and th cathod in th cll. Indicat th dirction of lctron flow. T/I 6. Draw a diagram of a procss that can b usd to lctroplat coppr onto a stl objct, such as a coin. Includ in your diagram suggstd lctrods, th ions in solution, powr supply and connctors, th sign of th lctrods, and th dirction of lctron and ion flow. T/I C 7. During th lctrolysis of a tin(ii) chlorid solution, SnCl 2 (aq), a silvr-colourd mtal, is dpositd onto th cathod. Bubbls of a gas ar obsrvd at th anod. A blach-lik odour is dtctd. Us a rdox tabl to prdict th anod, th cathod, and th nt ionic quation. T/I C 8. An lctrolytic cll is st up using inrt lctrods in an aquous coppr(ii) sulfat solution. Aftr an hour of lctrolysis, th solution is tstd. Th concntration of coppr(ii) ions has dcrasd, whil th concntration of hydrogn ions has incrasd. Th mass of on lctrod has also incrasd. Explain ths obsrvations using th half-raction quation for ach lctrod. T/I 9. Considr th quations for th anod and cathod half-ractions for th Hall Héroult procss, givn in this sction. T/I A (a) Combin ths two quations to dtrmin th ovrall cll raction quation. (b) Combin your answr in (a) with th quation for th raction of aluminum oxid with th cryolit ion, AlF 32 6 (l), to giv th ovrall quation for th production of aluminum: 2 Al 2 O 3 (s) 1 3 C(s) S 4 Al(s) 1 3 CO 2 (g) 10. Rsarch and xplain th stps involvd in rmoving tarnish from a silvr objct using aluminum foil and baking soda. Includ th quation for th chmical raction. T/I 11. Elctroplating companis us a varity of hazardous substancs. Rsarch answrs to th following qustions, and summariz your findings in a brif rport: T/I A C What nvironmntal hazards ar associatd with th lctroplating industry? What halth and safty hazards ar associatd with th wast that is gnratd by lctroplating companis? What stps do lctroplating companis tak to minimiz any ngativ halth or nvironmntal ffcts of thir industry? WEB LINK 670 Chaptr 10 Elctrochmical Clls