Chemistry 132 NT If yu g flying back thrugh time, and yu see smebdy else flying frward int the future, it s prbably best t avid eye cntact. Jack Handey 1 Chem 132 NT Electrchemistry Mdule 3 Vltaic Cells Dependence f emf n Cncentratin Sme Cmmercial Vltaic Cells Electrlytic Cells Electrlysis f Mlten Salts Aqueus Electrlysis Stichimetry f Electrlysis An experiment in crrsin f an irn nail and magnesium ribbn. Crrsin is an electrchemical prcess. 2 Review Determining the relative strengths f xidizing and reducing agents Determining the directin f spntaneity frm electrde ptentials Calculating the emf frm standard ptentials Calculating the freeenergy change frm electrde ptentials Calculating the cell emf frm freeenergy change Calculating the equilibrium cnstant frm cell emf 3 1
Dependence f emf n Cncentratin Recall that the free energy change, DG, is related t the standard free energy change, DG, by the fllwing equatin. DG = DG + RTlnQ Here Q is the thermdynamic reactin qutient. 4 Dependence f emf n Cncentratin Recall that the free energy change, DG, is related t the standard free energy change, DG, by the fllwing equatin. DG = DG + RTlnQ If we substitute DG = nfe cell and DG = nfe cell int this equatin, we get nfecell = nfecell+ RT lnq 5 Dependence f emf n Cncentratin The result rearranges t give the Nernst equatin, an equatin relating the cell emf t its standard emf and the reactin qutient. Ecell = Ecell 2.303RT lgq nf 6 2
Dependence f emf n Cncentratin The result rearranges t give the Nernst equatin, an equatin relating the cell emf t its standard emf and the reactin qutient. Substituting values fr R and F at 25 C, we get Ecell = Ecell 0.0592 lgq n (values in vlts at 25 C) 7 Dependence f emf n Cncentratin The result rearranges t give the Nernst equatin, an equatin relating the cell emf t its standard emf and the reactin qutient. The Nernst equatin illustrates why cell emf decreases as the cell reactin prceeds. As reactant cncentratins decrease and prduct cncentratins increase, Q increases, thus increasing lg Q which in turn decreases the cell emf. 8 A Prblem T Cnsider What is the emf f the fllwing vltaic cell at 25 C? 2 + 5 2 + Zn (s) Zn (1 10 M) Cu (0.100M) Cu(s) The standard emf f the cell is 1.10 V. The cell reactin is + 2 + Zn (s) + Cu (aq) Zn (aq) 2 + Cu(s) The number f electrns transferred is 2; hence n = 2. The reactin qutient is 2+ 5 [Zn ] 1.00 10 Q = = 2+ [Cu ] 0.100 = 1.00 10 4 9 3
A Prblem T Cnsider What is the emf f the fllwing vltaic cell at 25 C? 2 + 5 2 + Zn (s) Zn (1 10 M) Cu (0.100M) Cu(s) The standard emf f the cell is 1.10 V. The standard emf is 1.10 V, s the Nernst equatin becmes Ecell = Ecell 0.0592 lgq n 10 A Prblem T Cnsider What is the emf f the fllwing vltaic cell at 25 C? 2 + 5 2 + Zn (s) Zn (1 10 M) Cu (0.100M) Cu(s) The standard emf f the cell is 1.10 V. The standard emf is 1.10 V, s the Nernst equatin becmes E cell 0.0592 = 1.10V lg(1.00 10 2 4 ) 11 A Prblem T Cnsider What is the emf f the fllwing vltaic cell at 25 C? 2 + 5 2 + Zn (s) Zn (1 10 M) Cu (0.100M) Cu(s) The standard emf f the cell is 1.10 V. The standard emf is 1.10 V, s the Nernst equatin becmes E cell = 1.10V (0.12) = 1.22 V The cell emf is 1.22 V. 12 4
Sme Cmmercial Vltaic Cells The Leclanché dry cell, r zinccarbn dry cell, is a vltaic cell with a zinc can as the ande and a graphite rd in the center surrunded by a paste f manganese dixide, ammnium and zinc chlrides, and carbn black, as the cathde. The electrde reactins are ande 2+ Zn (s) fizn (aq) + 2e cathde (aq) + 2MnO (s) + 2e fimn 2 O 3(s) + H2O(l) + 2NH3(aq + 2NH4 2 ) 13 Leclanché dry cell. 14 Sme Cmmercial Vltaic Cells The alkaline dry cell, is similar t the Leclanché cell, but it has ptassium hydrxide in place f ammnium chlride. The electrde reactins are ande Zn (s) fizn 2+ cathde (aq) + 2e MnO 2 (s) + H2O(l) + 2e fi Mn 2O3(s) + 2OH (aq) 15 5
A small alkaline dry cell. 16 Sme Cmmercial Vltaic Cells The lithiumidine battery is a slid state battery in which the ande is lithium metal and the cathde is an I 2 cmplex. The slid state electrdes are separated by a thin crystalline layer f lithium idide. Althugh it prduces a lw current, it is very reliable and is used t pwer pacemakers. 17 A slidstate lithiumidine battery. 18 6
Sme Cmmercial Vltaic Cells The lead strage cell (a rechargeable cell) cnsists f electrdes f lead ally grids; ne electrde is packed with a spngy lead t frm the ande, and the ther electrde is packed with lead dixide t frm the cathde. The electrde reactins are ande + Pb(s) + HSO (aq) fipbso (s) + H (aq) + 2e 4 4 cathde + PbO (s) + 3H (aq) + HSO (aq) + 2e fi PbSO (s) + 2H O(l) 2 4 4 2 19 A lead strage battery. 20 Sme Cmmercial Vltaic Cells The nickelcadmium cell (nicadcell)cnsists f an ande f cadmium and a cathde f hydrated nickel xide n nickel; the electrlyte is ptassium hydrxide. The electrde reactins are ande Cd(s) + 2OH (aq) fi Cd(OH) 2(s) + 2e cathde NiOOH(s) + H O(l) + e fi Ni(OH) (s) OH (aq) 2 2 + 21 7
Nicad strage batteries 22 Sme Cmmercial Vltaic Cells A fuel cell is essentially a battery, but differs by perating with a cntinuus supply f energetic reactants, r fuel. Fr a hydrgenxygen fuel cell, the electrde reactins are ande 2 H 2 ( g) + 4OH (aq) 4 H 2 O (l ) + 4e cathde O 2 (g ) + 2 H 2 O (l ) + 4e 4 OH (aq ) 23 A hydrgenxygen fuel cell. 24 8
Electrlytic Cells An electrlytic cell is an electrchemical cell in which an electric current drives an therwise nnspntaneus reactin. The prcess f prducing a chemical change in an electrlytic cell is called electrlysis. Many imprtant substances, such as aluminum metal and chlrine gas are prduced cmmercially by electrlysis. 25 Electrlysis f mlten sdium chlride 26 Electrlysis f Mlten Salts A Dwns cell is a cmmercial electrchemical cell used t btain sdium metal by electrlysis f mlten NaCl. A number f ther reactive metals are btained by the electrlysis f a mlten salt. Lithium, magnesium, and calcium metals are all btained by the electrlysis f the chlrides. 27 9
A Dwns cell fr the preparatin f sdium metal. 28 Aqueus Electrlysis In the electrlysis f a mlten salt, the nly pssible halfreactins invlve the ins f the salt. Hwever, in aqueus slutin, yu must cnsider the pssibility that water might be xidized r reduced at ne r bth electrdes. The xidatin f water fllws the mechanism belw. + 2H2O(l) fi O2(g) + 4H (aq) + 4e E = 1.23 V 29 Aqueus Electrlysis In the electrlysis f a mlten salt, the nly pssible halfreactins invlve the ins f the salt. Hwever, in aqueus slutin, yu must cnsider the pssibility that water might be xidized r reduced at ne r bth electrdes. The reductin f waterfllws the mechanism belw. 2H2O(l) + 2e fi H2(g) + 2OH (aq) E = 0.83V 30 10
Aqueus Electrlysis In the electrlysis f a mlten salt, the nly pssible halfreactins invlve the ins f the salt. Water may xidize at the ande if its xidatin ptential (1.23 V) is less negative than the xidatin ptential f the cmpeting anin frm the salt. 31 Aqueus Electrlysis In the electrlysis f a mlten salt, the nly pssible halfreactins invlve the ins f the salt. Water may be reduced at the cathde if its reductin ptential (0.83 V) is less negative than the reductin ptential f the cmpeting catin frm the salt. 32 Stichimetry f Electrlysis What is new in this type f stichimetric prblem is the measurement f numbers f electrns. Yu d nt weigh them as yu d substances. Rather, yu measure the quantity f electric charge that has passed thrugh a circuit. Yu use the fllwing fact: 4 OneFaraday(9.65 10 C) 1ml electrns 33 11
Stichimetry f Electrlysis What is new in this type f stichimetric prblem is the measurement f numbers f electrns. T determine this we must knw the current and the length f time it has been flwing. Electric current is measured in amperes. An ampere (A) is the base SI unit f current equivalent t 1 culmb/secnd. 34 Stichimetry f Electrlysis What is new in this type f stichimetric prblem is the measurement f numbers f electrns. The quantity f electric charge passing thrugh a circuit in a given amunt f time is given by Electric charge(cul) = electric current (cul/sec) time lapse(sec) culmbs (C) = amps(cul/sec) time(sec) 35 A Prblem T Cnsider When an aqueus slutin f ptassium idide is electrlyzed using platinum electrdes, the halfreactins are 2I (aq) fi I 2(aq) + 2e 2H2O(l) + 2e fi H2 (g) + 2OH (aq) Hw many grams f idine are prduced when a current f 8.52 ma flws thrugh the cell fr 10.0 min? When the current flws fr 6.00 x 10 2 s (10.0 min), the amunt f charge is ( 2 8.52 10 3 A) (6.00 10 s) = 5.11C 36 12
A Prblem T Cnsider When an aqueus slutin f ptassium idide is electrlyzed using platinum electrdes, the halfreactins are 2I (aq) fi I 2(aq) + 2e 2H2O(l) + 2e fi H2 (g) + 2OH (aq) Hw many grams f idine are prduced when a current f 8.52 ma flws thrugh the cell fr 10.0 min? Nte that tw mles f electrns are equivalent t ne mle f I 2. Hence, 1 ml e 1 ml I2 254 g I 2 3 5.11 C = 6.73 10 g I 9.65 10 C 2 ml e 1 ml I 4 2 2 37 Operatinal Skills Calculating the cell emf fr nnstandard cnditins (Nernst Equatin) Predicting the halfreactins in an aqueus electrlysis Relating the amunts f prduct and charge in an electrlysis 38 wmax = nfe cell DG = nfe Key Equatins cell 0.0592 E cell = lg K n 0.0592 Ecell = E cell lgq n Time fr a few review questins 39 13
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