Ch 18 Electrochemistry 電化學

Transcription

1 Ch 18 Electrchemistry 電化學 Electrchemistry is the study f the intercnversin f electrical and chemical energy. This cnversin takes place in an electrchemical cell. 1. Vltaic cell. ( 伏打電池 ). Electrlytic cell. ( 電解電池 ) Redx reactin: cntains uctin and xidatin reactin. Reductin: electrns are cnsumed. Oxidatin: electrns are prduced. The number f electrns cnsumed in uctin must be exactly equal t the number (f electrns) prduced in xidatin half-reactin. Electrchemical cell: Cathde 陰極 : uctin reactin. + Cu ( aq) + e Cu(s) Ande 陽極 : xidatin reactin. Zn() s Zn + ( aq) + e Anins ( 陰離子 ) mves t ande, catins ( 陽離子 ) mves t cathde. Cathde 陰極 : 發生還原反應, 氧化數減少, 被氧化. Ande 陽極 : 發生氧化反應, 氧化數增加, 被還原. Vltage ( 電壓 電位 ): is a measure f reactin spntaneity. Cell vltages depend n the nature f the half-reactins ccurring at the electrdes and n RT the cncentratins f species invlved. E = E - lnq nf - 1 -

2 Frm the vltage measu at standard cncentratins, it is pssible t calculate the standard free energy change ( G = - nfe ) and the equilibrium cnstant ( G = - RT lnk) f the reactin invlved. Cntent: 18-1 Vltaic cells 18- Standard vltages 18-3 Relatins between E, G, and K 18- Effect f cncentratin n vltage 18-5 Electrlytic cells 18-6 Cmmercial cells 18-1 Vltaic Cell ( 伏打電池 ) Any spntaneus x reactin can serve as a surce f energy in a vltaic cell. The cell must designed that xidatin ccurs at ande with uctin at cathde. The electrns prduced at the ande must be transfer t the cathde, where they are cnsumed. The Zn Cu + cell: When a piece f zinc is added t a water slutin cntaining Cu + ins, the fllwing x reactin takes place: + + Zn() s + Cu ( aq) Zn ( aq) + Cu(s) Cu metal plates ut n the surface f the zinc. The blue clr f the aqueus Cu + ins fades as it is replaced by the clrless aqueus Zn + in. Fig. 18.1: Zn Cu + spntaneus x reactin. - -

3 Using the Zn Cu + reactin as a surce f electrical energy, the electrn transfer must ccur indirectly; that is the electrns given ff by zinc atms must be made t pass thrugh an external circuit befre they uce Cu + t cpper atms. + Fig. 18.: A Zn Cu vltaic cell. The vltaic cell cnsists tw half-cells: g a zinc ande dipping int a slutin cntaining Zn + ins. (n the right f Fig. 18.) g a Cu cathde dipping int a slutin cntaining Cu + ins. (n the left f Fig. 18.) g The external circuit cnsist f a vltmeter with leads t the ande and cathde. The Zn P Cu Zn + + Cu vltaic cell: 1. At the Zn ande: electrns are prduced by the xidatin half-reactin. Zn() s Zn + ( aq) + e pumps electrns int the external circuit, is rdinarily marked as the negative ple f the cell.. Electrns generated at the ande mve thrugh the external circuit t the cpper cathde. At the cathde, the electrns are cnsumed, ucing Cu + ins present in the slutin arund the electrde. + Cu ( aq ) + e Cu(s) - 3 -

4 The electrde, which pull electrns frm the external circuit, is cnside t be the psitive ple f the cell. 3. A surplus f psitive ins ( Zn + ) tends t build up arund the zinc electrde. The regin arund the cpper electrde tends t becme deficient in psitive ins as ( Cu + ) are cnsumed. T maintain electrical neutrality, catins must mve tward the cpper cathde r, alternatively, anins must mve tward zinc ande. In practice, bth migratins ccur. Salt bridge: The mvement f ins ccurs thrugh a salt bridge cnnecting the tw beakers. An inverted glass U-tube, plugged with glass wl at each end. The tube is filled with a slutin f that takes n part in the electrde reactins; ptassium nitrate KNO 3, is frequently used. K + ins mve frm the salt bridge int the cathde half-cell. NO 3 ins mve int the ande half-cell. Electrical neutrality is maintained withut Cu + ins cming in cntact with the zinc electrde, which wuld shrt-circuit the cell. Fig. 18. is ften abbreviated as: Zn P Cu Zn + + Cu In this ntatin: g The ande reactin (xidatin) is shwn at the left. Zn atms xidized t Zn + ins. g The salt bridge is indicated by the symbl P. g The cathde reactin (uctin) is shwn at the right. Cu + ins are uced t Cu atms. g A single vertical line indicates a phase bundary, such as that between a slid cathde and an aqueus slutin. - -

5 Other salt bridge cells: Example 1: + + Ni() s + Cu ( aq) Ni ( aq) + Cu(s) Ande: Ni; slutin: NiCl r NiSO Cathde: Cu; slutin: CuSO The cell ntatin: Ni Ni + P Cu + Cu Example : Zn() s + C ( aq) Zn ( aq) + C (aq) Ande: Zn; slutin: ZnSO Cathde: because n metal invlved in the cathde half-reactin, an inert electrde that cnducts an electric current is used. platinum Slutin: C( NO 3) 3 + ande: Zn() s Zn ( aq) + e 3+ + Cathde: C ( aq) + e C ( aq) The cell ntatin: Zn P C 3, C Zn Pt ]indicate the presence f an inert electrde as cathde. Ex 18.1: When chlrine gas is bubbled thrugh an aqueus slutin f NaBr, chlride ins and liquid brmine are the prducts f the spntaneus reactin. Fr the cell, a). Draw a sketch f the cell, labeling the ande, the cathde, and the directin f electrn flw. b). Write the half-reactin that takes place at the ande and at the cathde. c). Write a balanced equatin fr the cell reactin. d). Write a abbreviated ntatin fr the cell. Ans: - 5 -

6 a). Fig. 18.: A Cl Br vltaic cell. - - ). cathde: Cl ( ) + e Cl ( ) (uctin) b g aq ande: Br ( aq) Br () l + e (xidatin) - - c). Cl ( g) + Br ( aq) Cl ( aq) + Br () l - - d). Pt Br, Br P Cl,Cl Pt Summarizatin: g a vltaic cell cnsists f tw half-cells. They are jined by an external electrical circuit thrugh which electrns mve and a salt bridge thrugh which ins mve. g each half-cell cnsists f an electrde dipping int a water slutin. g in ne half-cell, xidatin ccurs at the ande, in anther, uctin takes place at the cathde. 18- Standard Vltages, E. 標準電位 The driving frce behind the spntaneus reactin in a vltaic cell is measu by the cell vltage

7 An intensive prperty. Independent f the number f electrns passing thrugh the cell. Cell vltage depends n the nature f the x reactin and the cncentratins f the species invlved. The standard vltage fr a given cell is that measu when the current flw is essentially zer, all ins and mlecules in slutin are at a cncentratin f 1 M, and all gases are at a pressure f 1 atm. Ex: Zn H + cell ; Zn Zn + P H, + H Pt + Zn + and H are bth 1 M, and the pressure f H ( ) g is 1 atm. The cell vltage at very lw current flw is V. The standard vltage, E. + + Zn() s + H ( aq, 1 M) Zn ( aq, 1 M) + H ( g, 1 atm) E = V and E x E x x E : : standard uctin vltage. E : standard xidatin vltage. E = E + E (18-1) Fr Zn Zn + P H, + H Pt cell: V = E (H H ) + E x (Zn Zn ) There is n way t measure the standard vltage fr a half-reactin, nly E can be measu directly. T btain values fr E x and E, the value zer is arbitrarily assigned t the standard vltage fr uctin f H + ins t H gas: Q H ( aq, 1 M) + e H ( g, 1 atm) E ( H H ) = V E = E + E x Zn(s) Zn ( aq, 1 M) + e E (Zn Zn ) = V x As sn as ne half-reactin vltage is established, thers can be calculated

8 Ex: Zn P Cu Zn + + Cu cell: E = V; E (Zn Zn ) = V Q + x + E = E (Cu Cu) + E (Zn Zn ) + + x V = E (Cu Cu) + (+0.76 V) + E (Cu Cu) = V Standard E is given in Table Standard ptential = E 獲得電子之能力 Zn ( aq, 1 M) + e Zn(s) E = V + - E x: standard xidatin vltage T btain the standard vltage fr an xidatin half-reactin, all yu have t d is change the sign f the standard ptential listed in Table 18.1 E x = -E Zn(s) Zn ( aq, 1 M) + e E = - E = V + - x Standard vltages fr frward and reverse half-reactin are equal in magnitude but ppsite in sign. Strength f Oxidizing and Reducing agents The strength f an xidizing agent is directly related t the standard vltage fr its uctin, E. E xidizing agent strength The mre psitive E is, the strnger the xidizing agent. The - 8 -

9 strngest xidizing agents are thse at the bttm f the left clumn. CrO 7 E = V Cl E = V F E = V The strngest ucing agents are listed in the tp f the right clumn. Li E = V x K E = V x The mre psitive E x is, the strnger the ucing agent. Reducing agent: K, Ba, Sr, Ca, Na, Mg, Al, Mn, Zn, Cr, Fe, C, Ni, Sn, Pb, H, Cu, Hg, Ag, Pt, Au. Ex. 18.: Cnsider the fllwing species in acidic slutin: MnO, I, NO 3, + HS and Fe, Using Table a). classify each f thse as an xidizing agent and/r ucing agent? b). arranging the xidizing agents in rder f increasing strength? c). d the same with the ucing agents? Ans: Frm Table 18.1: MnO : MnO ( aq) + 8H (aq) + 5e Mn (aq) + H O E = V + + I : I ( aq) I(s) + e E x = V NO : NO ( aq) + H (aq) + 3e NO(g) + H O E = V + H S: H Saq ( ) S(s) + H (aq) + e E = -0.1 V + + Fe : Fe ( aq) + e Fe(s) E = V Fe : Fe ( aq) Fe ( aq) + e E x a). xidizing agents: MnO E = V 3 NO E = V + Fe E + = V ucing agents: I E = V H S E = -0.1 V Fe + b). the strength f xidizing agents: Fe < NO 3 < MnO + c). the strength f ucing agents: Fe < I H x x E = V x < S x = V

10 Calculatin f E frm E and E x E = E + E x Ex. 18.3: Cnsider the vltaic cell in which the reactin is Ag + ( aq) + Cd(s) Ag(s) + Cd + ( aq) a). Use Table 18.1 t calculate E fr the vltaic cell. b). If the value zer is arbitrarily assigned t the standard vltage fr the uctin f Ag + ins t Ag, what is E fr the uctin f Cd ins t Cd? Ans: + a). [Ag ( aq) + e Ag(s)] E = V + Cd(s) Cd ( ) + e E x = +0.0 V Ag aq + + ( aq) + Cd(s) Ag(s) + Cd ( aq) E = E + Ex b). E = E + E ; if E = 0 V E = 0 + E x x E = V x E = V = V Cnclusins: 1. The calculated vltage, E, is s always a psitive quantity fr a reactin taking place in a vltaic cell.. The quantities E, E x and E are independent f hw the equatin fr the cell reactin is written. Intensive prperty. Spntaneity f x reactins If the calculated vltage fr a x reactin is a psitive quantity, the reactin will be spntaneus. E > 0 spntaneus reactin Ex. 18.: Using standard ptentials listed in Table 18.1, decide whether at

11 standard cncentratins a). the reactin 3 Fe + ( aq) + I ( aq) Fe + ( aq) + I () s will ccur. b). Fe(s) will be xidized t Fe + by treatment with hydrchlric acid. c). a x reactin will ccur when the fllwing species are mixed in acidic slutin: Cl, Fe +, Cr +, I. Ans: a aq aq 3+ + ). [Fe ( ) + e Fe ( )] E = V I ( aq) I() s + e E = V 3+ Fe ( aq) + I ( aq) Fe + ( aq) + I() s E = ( V) + (-0.53 V) = V Q E > 0 spntaneus b). xidatin half-reactin f Fe(s): Fe(s) Fe + ( aq) + e E x = V hydrlric acid: uctin reactin: H (aq) + e + H(g) E = V + + Fe(s) + H (aq) Fe ( ) + H (g) E = E + E = ( V) + (+0.09 V) x = V spntaneus c). Cl : Cl ( aq) Cl ( g) + e E = V aq + + Fe : Fe (aq) + e Fe(s) E = V aq aq + 3+ Fe ( ) Fe ( ) + e E x = Cr : Cr ( aq) Cr ( aq) + e I : I() s + e I ( aq) E = E + E > 0 x x x 9 V E = V x E = V + The nly cmbinatin reactin is: I react with Cr : I() s + e I ( aq) E = V + 3+ ( Cr ( aq) Cr ( aq) + e 3 I() s + Cr + ( aq) I ( aq) + Cr + ( aq) E = +0.9 V E = V x 18-3 Relatins Between E, G, and K

12 G : the standard free energy change. K: equilibrium cnstant. E and G : Frm thermdynamics: G = - n F E (18-) n: the number f mles f electrns transfer in the reactin. F: Faraday cnstant; 9. 68x10 J/ml V G < 0 spntaneus reactin E and K: Frm thermdynamics (Ch. 17): G = -RTlnK G = - n F E (18-) -RTlnK = - n F E RT E = ln K nf if T = 5 C = 98 K RT 8.31 J/ml K 98 K = = V F 9.68x10 J/ ml V E = ln K (T= 98 K) (18-3) n if E > 0 K > 1 E < 0 K < 1 Ex. 18.5: Fr the reactin: Ans: Ag() s + NO 3 ( aq) + H ( aq) 3Ag ( aq) + NO(g) + HO Use Table 18.1 t calculate, at 5 a). G b). K - 1 -

13 + a). 3 [Ag( s) Ag (aq) + e ] E = V NO (aq) + H ( aq) + 3e NO( g) + H O x E = V 3Ag( s) + NO (aq) + H ( aq) 3Ag (aq) + NO( g) + H O E = E + E G = -nfe = x = -.78x10 J = -7.8 kj b). G = -RTlnK -.78x10 J = lnk lnk = 19.3 K =.39x r E = ln K n = ln K 3 8 K =.39x10 8 x x = V 18- Effect f Cncentratin n vltage Standard vltage: gas: 1 atm Aqueus slutin: 1 M When the cncentratin f a reactant r prduct changes, the vltage changes as well. 1. Vltage will increase if the cncentratin f a reactant is increased r that f a prduct is decreased. [ reactan t] E [ prduct] E. Vltage will decrease if the cncentratin f a reactant is decreased r that f a prduct is increased. [ reactan t] E [ prduct] E When a vltaic cell perates, supplying electrical energy, the

14 cncentratin f reactants decreases and that f the prducts increases. As time passes, the vltage drps steadily. Eventually it becmes zer. The x reactin taking place within the cell is at equilibrium. Nernst equatin Frm Ch. 17: G = G + RTlnQ G = -nfe ; G = -nfe 代入 : -nfe = -nfe + RTlnQ RT E = E - lnq nf T= 98K; Nernst equatin: E = E - lnq n Q: reactin qutient aa + bb cc + dd c [ C] [ D] Q = [ ] a A [ B ] d b (18-) Discussin: 1. If Q > 1, which means that the cncentratins f prducts are high relative t thse f reactants, lnq is psitive and E < E.. If Q < 1 (cncentratins f prduct lw relative t reactants), lnq is negatives and E > E. 3. If Q = 1, lnq = 0 E = E. aa(s) + bb(aq) c [ C] ( Pd) Q = b [ B] d cc(aq) + dd(g) Ex. 18.6: Cnsider a vltaic cell in which the fllwing reactin ccurs O ( g,0.98atm) + H ( aq, ph= 1.) + Br ( aq, 0.15M) H O + Br () l + a). Calculate E fr the cell at 5. b). When the vltaic cell is at 35, E is measu t be V

15 Ans: What is E a 35. a). O ( g) + H ( aq) + e H O E = +1.9 V + Br ( aq) Br () l + e E = V + O ( g) + H ( aq) + Br ( aq) H O + Br () l E = V E = E + E x = (+1.9) + (-1.077) = V E = E - ln Q n = (+0.15) - ln + P [ H ] [ Br ] 1 O 1 = (+0.15) ln (0.98) (0.58) (0.15) 8 = (+0.15) ln(1.8x10 ) = (+0.15) = V b). E = E - ln Q n E= E + ln Q = ln(1.8x10 ) n = V The Nernst equatin can be als used t determine the effect f change in cncentratin n the vltage f an individual half-cell, E r E. x x Use f the Nernst equatin t determine in cncentratin Ex: 18.7: Cnsider a vltaic cell at 5 in which the reactin is Ans: Zn s aq aq g + + () + H ( ) Zn ( ) + H ( ) It is fund that the vltage is V when [Zn + ] = 0.85 M, P = atm. What is the ph in the H + H half-cell? H + H ( aq) + e H( g) E = V aq + Zn(s) Zn ( ) + e E x = V H aq g aq) E = V + + ( ) + Zn(s) H( ) + Zn (

16 0.057 E = E - ln Q n [ Zn ] (P ) ( ) = (+0.76) - ln [ H ] (0.85) (0.988) -0.0 = - ln + [ H ] 0.8 (0.0) ln = = [ H ] [ H ] = [H ] = ph = 3.5 Usage: ph meter H Electrlytic Cells 電解電池 A spntaneus x reactin is made t ccur by pumping electrical energy int the system. Fig. 18.8: Diagram f an electrlytic cell. Oxidatin ccurs at the ande, uctin at the cathde, in bth vltaic and electrlytic cells. Quantitative relatinships

17 Quantity f electrical charge: 1 mle electrns = 9.68x10 C. Rate f current flw: Ampere, A. 1 A = 1 C/s Amunt f electrical energy: J When a charge f ne culmb (C) mves thrugh a ptential difference f ne vlt (V), it acquires an energy f 1 J. 1 J = 1 CV Electrical energy: kwh 1 kwh = 3.600x10 6 J = 3.600x10 3 kj Ex 18.8: Chrmium metal can be electrplated frm a water slutin f ptassium dichrmate. The uctin half-reactin is: Ans: CrO aq aq O 7 ( ) + 1H + ( ) + 1e Cr(s) + 7H A current f 6.00 A and a vltage f.5 V are used in the electrplating. a). Hw many grams f chrmium can be plated by if the current is run fr 8 minutes? b). Hw lng will it take t cmpletely cnvert 15 ml f 1.5 M K Cr O 7 t elemental chrmium? c). Hw many kilwatt hurs (kwh) f electrical energy are requi t plate 1.00 g f chrmium?

18 6.00 A (8 60) s a). (5.00 g/ml) = 1.55 g C/ml b) L 1.5 M/L C/ml = C 5 charge C t = = = s = 1. h current 6.00 A c). ml C/ml.5 V = C V = J J = 0.01 kwh J/kWh Sme electrical energy is wasted in side reactins at the electrdes and in the frming f heat. The actual yield f prducts is less than the theretical yield. Cell reactins (water slutin) Electrlysis f water slutin A uctin half-reactin ccurs at the cathde f an electrlytic cell. This half-reactin will be: Cathde: g the uctin f a catin t the crrespnding metal. + Ag ( aq) + e Ag(s) E = V Cu ( aq) + e Cu(s) E = V + This is an electrplating prcesses, in which a metal bject serve as the cathde. g the uctin f a water mlecule t hydrgen gas. HO + e H( g) + OH ( aq) E = V

19 This half-reactin ccurs when the catin in the slutin is very difficult t uce. Ex. K + ins E = V r Na + ins E = -.71 V. Ande: g the xidatin f an anin t the crrespnding nnmetal. I ( aq) I() s + e E x = V g the xidatin f a water mlecule t xygen gas. + HO O( g) + H ( aq) + e E = V x This half-reactin ccurs when the anin cannt be xidized. Ex. NO 3, SO : the nnmetal present is already in its highest xidatin state. (+5 fr N, +6 fr S) 18-6 Cmmercial Cells Applicatins f electrlytic cells in electrplating and electrsynthesis are f particular imprtance. Electrlysis f aqueus NaCl The mst imprtant electrlysis carried ut in water slutin is that f sdium chlride. Fig : Electrlysis f aqueus NaCl

20 Ande: Cl ( aq) Cl ( g) + e E = V Cathd: HO + e H( g) + OH ( aq) E = V Cl ( aq) +HO Cl ( g) + H ( g) +OH ( aq) E = V ande cathde x Usage: Cl : 1). is used t purify drinking water. ). raw material fr PVC. H : synthesis f ammnia. NaOH: 1). prcessing pulp and paper. ). purificatin f aluminum re. 3). manufacture f glass and textiles. Primary (nnrechargeable) vltaic cells Dry cell. (Leclanche cell) Ande : zinc wall f the cell. + Zn(s) Zn ( aq) + e Cathde: graphite rd. MnO () s + NH ( aq) + e Mn O () s + NH ( aq) + H O Re dx : Zn(s) + MnO () s + NH ( aq) Zn ( aq) + Mn O () s + NH ( aq) + H O E = V If t large a current is drawn frm a Leclanche cell, the ammnia frms a gaseus insulating layer arund the carbn cathde. When this happens, the vltage drps sharply and then returns slwly t its nrmal f 1.50 V. Alkaline dry cell: The paste between the electrdes cntains KOH rather than NH Cl

21 Ande : Zn + KOH Cathde : MnO Zn(s) + OH ( aq) ZnO(s) + H O + e MnO (s) + H O+ e Mn(OH) () s + OH (aq) Re dx: Zn(s) + MnO (s) + H O ZnO(s) + Mn(OH) () s 優點 : lnger shelf life and prvide mre current. 缺點 : mre expansive. E = V Mercury cell: Ande : Zinc-mercury amalgan Zn(s) + OH ( aq) Zn(OH) (s) + e Cathde : a plate made up f mercury(ii) xide, HgO Electrlyte: a paste cntaining HgO and sdium r patassium hrdrxide. HgO() s + H O + e Hg( l ) + OH ( aq) Zn(s) + HgO() s + H O Zn(OH) (s) + Hg( l) E = +1.3 V 優點 : 1. small aize.. the verall reactin des nt invlve any ins in slutin, s there are n cncentratin changes when current is drawn. the battery maintains a cnstant vltage f 1.3 V thrughut its life. 缺點 : 1. Expansive.. nt envirnmental friendly. Strage (rechargeable) vltaic cells can be recharged repeatedly. This can be accmplished because the prducts f the reactin are depsited directly n the electrdes. By passing thrugh a strage cell, it is pssible t reverse the electrde reactins and restre the cell t its riginal - 1 -

22 cnditin. Lead strage battery Ande : Pb plates. Pb(s) + HSO ( aq) PbSO () s + H ( aq) +e Cathde : lead xide PbO. Electrlyte: H SO. + PbO () s + 3H ( aq) + HSO ( aq) + e PbSO () s + H O + + discharg e recharg e Pb(s) + PbO () s + H ( aq) + HSO ( aq) PbSO () s + H O G = kj E = +.10 V Recharge: a direct current is passed thrugh it in the prper directin s as t reverse the abve reactin. 何時 recharge: H SO (aq) 之比重 < 1.05 時. Fuel cells 燃料電池 A fuel, usually hydrgen, is xidized at the ande. Ande : Graphite. Cathde : 碳棒. H ( g) + OH ( aq) H O + e O ( g) + H O + e OH ( aq) H ( g) + O ( g) H O G = -7. kj E = V The deterrents t the use f fuel cells: 1. The cst per kiljule f energy is ten times that with a gasline r disel pwe engine.. The strage f hydrgen in a vehicle is a serius prblem. - -