2. Before we answer the question, here are four important terms relating to redox reactions and galvanic cells.

Transcription

1 CHAPTER SEVENTEEN ELECTROCHEMISTRY Fr Review 1. Electrchemistry is the study f the interchange f chemical and electrical energy. A redx (xidatin-reductin) reactin is a reactin in which ne r mre electrns are transferred. In a galvanic cell, a spntaneus redx reactin ccurs which prduces an electric current. In an electrlytic cell, electricity is used t frce a nnspntaneus redx reactin t ccur. 2. Befre we answer the questin, here are fur imprtant terms relating t redx reactins and galvanic cells. a. Cathde: The electrde at which reductin ccurs. b. Ande: The electrde at which xidatin ccurs. c. Oxidatin half-reactin: The half-reactin in which electrns are prducts. In a galvanic cell, the xidatin half-reactin always ccurs at the ande. d. Reductin half-reactin: The half-reactin in which electrns are reactants. In a galvanic cell, the reductin half-reactin always ccurs at the cathde. See Figures 17.2 and 17.3 fr designs f galvanic cells. The electrde cmpartment in which reductin ccurs is called the cathde and the electrde cmpartment in which xidatin ccurs is called the ande. These cmpartments have electrdes (a slid surface) immersed in a slutin. Fr a standard cell, the slutin cntains the reactant and prduct slutes and gases that are in the balanced half-reactins. The slute cncentratins are all 1 M and gas partial pressures are all 1 atm fr a standard cell. The electrdes are cnnected via a wire and a saltbridge cnnects the tw slutins. The purpse f the electrdes is t prvide a slid surface fr electrn transfer t ccur in the tw cmpartments. Electrns always flw frm the ande (where they are prduced) t the cathde (where they are reactants). The salt bridge allws cunter ins t flw int the tw cell cmpartments t maintain electrical neutrality. Withut a salt bridge, n sustained electrn flw can ccur. In the salt bridge, anins flw int the ande t replenish the lss f negative charge as electrns are lst; catins flw int the cathde t balance the negative charge as electrns are transferred int the cathde. The pull r driving frce n the electrns is called the cell ptential (E cell ) r the electrmtive frce. The unit f electrical ptential is the vlt (V) which is defined as 1 jule f wrk per culmb f charge transferred. It is the cell ptential that can be used t d useful wrk. We harness the spntaneus redx reactin t prduce a cell ptential which can d useful wrk.

2 2 CHAPTER 17 ELECTROCHEMISTRY 3. The zer pint fr standard reductin ptentials (EΕ) is the standard hydrgen electrde. The half-reactin is: 2 H e H 2. This half-reactin is assigned a standard ptential f zer, and all ther reductin half-reactins are measured relative t this zer pint. Substances less easily reduced than H + have negative standard reductin ptentials (EΕ < 0), while substances mre easily reduced than H + have psitive standard reductin ptentials (EΕ > 0). The species mst easily reduced has the mst psitive EΕ value; this is F 2. The least easily reduced species is Li + with the mst negative EΕ value. When a reductin half-reactin is reversed t btain an xidatin half-reactin, the sign f the reductin ptential is reversed t give the ptential fr the xidatin half-reactin (E x =!Ered). The species xidized are n the prduct side f the reductin half-reactins listed in Table Li will have the mst psitive xidatin ptential [E x =!Ered =! (!3.05 V) = 3.05 V], s Li is the mst easily xidized f the species. The species mst easily xidized is the best reducing agent. The wrst reducing agent is F because it has the mst negative xidatin ptential (E =!2.87 V). x Fr a spntaneus reactin at standard cnditins, E must be psitive (E = E + E cell cell red x > 0). Fr any tw half-reactins, there is nly ne way t manipulate them t cme up with a psitive E cell (a spntaneus reactin). Because the half-reactins d nt depend n hw many times the reactin ccurs, half-reactins are an intensive prperty. This means that the value f E red r E x is nt changed when the half-reactins are multiplied by integers t get the electrns t crss ff. The line ntatin f the standard galvanic cell illustrated in Figure 17.5 wuld be: Zn(s) Zn 2+ (aq) H 2 (g) H + (aq) Pt r Zn(s) Zn 2+ (1.0 M) H 2 (1.0 atm) H + (1.0 M) Pt The duble line represents the salt-bridge separating the ande and cathde cmpartments. T the left f the duble line are the pertinent ande cmpartment cntents and t the right are the pertinent cathde cmpartment cntents. At each end, the electrdes are listed; t the inside, the slutin cntents are listed. A single line is used t separate the cntents f each cmpartment whenever there is a phase change. Here in the cathde cmpartment, a single line is used t separate H 2 (g) frm H + (aq) and t separate H + (aq) frm Pt (the electrde). When cncentratins and partial pressures are nt listed, they are assumed t be standard (1.0 M fr slutes and 1.0 atm fr gases). Fr cells having nnstandard cncentratins and pressures, we always include the actual cncentratins and pressures in the line ntatin instead f the phases. 4. GΕ =!nfeε; GΕ is the standard free energy change fr the verall balanced reactin, n is the number f electrns transferred in the verall balanced reactin, F is called the Faraday cnstant (1 F = 96,485 culmbs f charge transferred per mle f electrns), and EΕ is the standard cell ptential fr the reactin. Fr a spntaneus redx reactin, E cell is psitive while G rxn is negative. The negative sign is necessary t cnvert the psitive E cell value fr a spntaneus reactin int a negative G rxn. The superscript Ε indicates standard cnditins. These are T = 25ΕC, slute cncentratins f 1.0 M, and gas partial pressures f

3 CHAPTER 17 ELECTROCHEMISTRY 3 5. E = EΕ 1.0 atm. Nte that n is necessary in rder t cnvert the intensive prperty EΕ int the extensive prperty, GΕ. RT ln Q; At 25ΕC, the Nernst equatin is: E = EΕ lg Q nf n Nnstandard cnditins are when slutes are nt all 1.0 M and/r partial pressures f gases are nt all 1.0 atm. Nnstandard cnditins als ccur when T 25ΕC, Fr mst prblem slving, T = 25ΕC is usually assumed, hence the secnd versin f the Nernst equatin is mst ften used. E = cell ptential at the cnditins f the cell; EΕ = standard cell ptential; n = number f electrns transferred in the verall reactin, and Q is the reactin qutient determined at the cncentratins and partial pressures f the cell cntents. At equilibrium, E = 0 and Q = K. At 25ΕC, EΕ = (0.0591/n) lg K. The standard cell ptential allws calculatin f the equilibrium cnstant fr a reactin. When K < 1, the lg K term is negative, s E cell is negative and GΕ is psitive. When K > 1, the lg K term is psitive, s E is psitive and GΕ is negative. cell Frm the equatin EΕ = (0.0591/n) lg K, the value f EΕ allws calculatin f the equilibrium cnstant K. We say that EΕ gives the equilibrium psitin fr a reactin. E is the actual cell ptential at the cnditins f the cell reactin. If E is psitive, then the cell reactin is spntaneus as written (the frward reactin can be used t make a galvanic cell t prduce a vltage). If E is negative, the frward reactin is nt spntaneus at the cnditins f cell, but the reverse reactin is spntaneus. The reverse reactin can be used t frm a galvanic cell. EΕ can nly be used t determine spntaneity when all reactants and prducts are at standard cnditins (T = 25ΕC, [ ] = 1.0 M, P = 1.0 atm). 6. Cncentratin cell: a galvanic cell in which bth cmpartments cntain the same cmpnents, but at different cncentratins. All cncentratin cells have E cell = 0 because bth cmpartments cntain the same cntents. The driving frce fr the cell is the different in cncentratins at the ande and cathde. The cell prduces a vltage as lng as the in cncentratins are different. Equilibrium fr a cncentratin cell is reached (E = 0) when the in cncentratins in the tw cmpartments are equal. The net reactin in a cncentratin cell is: M a+ (cathde, x M) M a+ (ande, y M) E cell = 0 and the Nernst equatin is: E = EΕ lg Q = n a [M (ande)] lg where a is the number f a+ a [M (cathde)] electrns transferred.

4 4 CHAPTER 17 ELECTROCHEMISTRY T register a ptential (E > 0), the lg Q term must be a negative value. This ccurs when M a+ (cathde) > M a+ (ande). The higher in cncentratin is always at the cathde and the lwer in cncentratin is always at the ande. The magnitude f the cell ptential depends n the magnitude f the differences in in cncentratins between the ande and cathde. The larger the difference in in cncentratins, the mre negative the lg Q term and the mre psitive the cell ptential. Thus, as the difference in in cncentratins between the ande and cathde cmpartments increase, the cell ptential increases. This can be accmplished by decreasing the in cncentratin at the ande and/r by increasing the in cncentratin at the cathde. When NaCl is added t the ande cmpartment, Ag + reacts with Cl t frm AgCl(s). Adding Cl, lwers the Ag + cncentratin which causes an increase in the cell ptential. T determine K sp fr AgCl (K sp = [Ag + ][Cl ]), we must knw the equilibrium Ag + and Cl cncentratins. Here, [Cl ] is given and we use the Nernst equatin t calculate the [Ag + ] at the ande. 7. As a battery discharges, E cell decreases, eventually reaching zer. A charged battery is nt at equilibrium. At equilibrium, E cell = 0 and G = 0. We get n wrk ut f an equilibrium system. A battery is useful t us because it can d wrk as it appraches equilibrium. Bth fuel cells and batteries are galvanic cells that prduce cell ptentials t d useful wrk. Hwever, fuel cells, unlike batteries, have the reactants cntinuusly supplied and can prduce a current indefinitely. The verall reactin in the hydrgen-xygen fuel cell is 2 H 2 (g) + O 2 (g) 2 H 2 O(l). The half-reactins are: 4 e + O H 2 O 4 OH cathde 2 H OH 4 H 2 O + 4 e ande Utilizing the standard ptentials in Table 17.1, E cell = 0.40 V V = 1.23 V fr the hydrgen-xygen fuel cell. As with all fuel cells, the H2(g) and O 2 (g) reactants are cntinuusly supplied. See Figure fr a schematic f this fuel cell. 8. The crrsin f a metal can be viewed as the prcess f returning metals t their natural state. The natural state f metals is t have psitive xidatin numbers. This crrsin is the xidatin f a pure metal (xidatin number = 0) int its ins. Fr crrsin f irn t take place, yu must have: a. expsed irn surface a reactant b. O 2 (g) a reactant c. H 2 O(l) a reactant, but als prvides a medium fr in flw (it prvides the salt bridge)

5 CHAPTER 17 ELECTROCHEMISTRY 5 d. ins t cmplete the salt bridge Because water is a reactant and acts as a salt bridge fr crrsin, cars d nt rust in dry air climates, while crrsin is a big prblem in humid climates. Salting rads in the winter als increases the severity f crrsin. The disslutin f the salt int ins n the surface f a metal increases the cnductivity f the aqueus slutin and accelerates the crrsin prcess. Sme f the ways metals (irn) are prtected frm crrsin are listed belw. a. Paint: Cvers the metal surface s n cntact ccurs between the metal and air. This nly wrks as lng as the painted surface is nt scratched. b. Durable xide catings: Cvers the metal surface s n cntact ccurs between the metal and air. c. Galvanizing: Cating steel with zinc; Zn frms an effective xide cating ver steel; als, zinc is mre easily xidized than the irn in the steel. d. Sacrificial metal: Attaching a mre easily xidized metal t an irn surface; the mre active metal is preferentially xidized instead f irn. e. Allying: Adding chrmium and nickel t steel; the added Cr and Ni frm xide catings n the steel surface. f. Cathdic prtectin: A mre easily xidized metal is placed in electrical cntact with the metal we are trying t prtect. It is xidized in preference t the prtected metal. The prtected metal becmes the cathde electrde, thus, cathdic prtectin. 9. An electrlytic cell uses electrical energy t prduce a chemical change. The prcess f electrlysis invlves frcing a current thrugh a cell t prduce a chemical change fr which the cell ptential is negative. Electrical wrk is used t frce a nnspntaneus reactin t ccur. The units fr current are amperes (A) which equal 1 culmb f charge per sec. current (A) time (s) = culmbs f charge passed We use Faraday s cnstant (F = 96,485 culmbs f charge per mle) t cnvert culmbs f charge passed int mles f electrns passed. The half-reactin gives the mle rati between mles f electrns and mles f metal prduced (r plated ut). Plating means depsiting the neutral metal n the electrde by reducing the metal ins in slutin. In electrlysis, as with any redx reactin, the reactin that ccurs first is the ne mst favred thermdynamically. The reductin reactin mst favred thermdynamically has the largest, mst psitive E red value. The xidatin reactin mst likely t ccur is the ne with the largest, mst psitive E value. Nte that fr electrlytic cells that E cell < 0, s the E x red and E x values are cmmnly negative. The half-reactins that ccur first as a current is applied are the nes with the least negative ptentials (which are the mst psitive ptentials).

6 6 CHAPTER 17 ELECTROCHEMISTRY T predict the cathde half-reactin, write dwn the half-reactin and E red value fr all species present that can be reduced. The cathde reactin that ccurs has the least negative (mst psitive) E red value. The same thing is dne fr the ande; write dwn everything present that can be xidized; the species xidized has the least negative (mst psitive) E x value. Nte that we cmmnly assume standard cnditins when predicting which halfreactins ccur, s we can use the standard ptentials in Table When mlten salts are electrlyzed, there is nly ne species present that can be xidized (the anin in simple salts) and there is nly ne species that can be reduced (the catin in simple salts). When H 2 O is present as is the case when aqueus slutins are electrlyzed, we must cnsider the xidatin and reductin f water as ptential reactins that can ccur. When water is present, mre reactins can take place, making predictins mre difficult. When the vltage required t frce a chemical reactin t ccur is larger than expected, this is called vervltage. The amunt f vervltage necessary t frce a reactin t ccur varies with the type f substance present. Because f this, EΕ values must be used cautiusly when predicting the half-reactins that ccur. 10. Electrlysis is used t prduce many pure metals and pure elements fr cmmercial use. It als is used t purify metals as well as t plate ut thin catings n substances t prvide prtectin frm crrsin and t beautify bjects. Anther applicatin f electrlysis is the charging f batteries. When aqueus NaCl is electrlyzed, water, with its less negative reductin ptential is preferentially reduced ver Na + ins. Thus, the presence f water desn t allw Na + ins t be reduced t Na. In mlten NaCl, water is nt present, s Na + can be reduced t Na. Purificatin by electrlysis is called electrrefining. See the text fr a discussin f the electrrefining f cpper. Electrrefining is pssible because f the selectivity f electrde reactins. The ande is made up f the impure metal. A ptential is applied s just the metal f interest and all mre easily xidized metals are xidized at the ande. The metal f interest is the nly metal plated at the cathde due t the careful cntrl f the ptential applied. The metal ins that culd plate ut at the cathde in preference t the metal we are purifying will nt be in slutin, because these metals were nt xidized at the ande.