Chapter 19. Electrochemistry. Dr. Al Saadi. Electrochemistry

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1 Chapter 19 lectrchemistry Part I Dr. Al Saadi 1 lectrchemistry What is electrchemistry? It is a branch f chemistry that studies chemical reactins called redx reactins which invlve electrn transfer Oxidatin f zinc in a slutin f cpper sulfate is an example f xidatin reductin redx reactins. Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) Dr. Al Saadi Oxidatin states 2 1

2 Assigning Oxidatin States in Redx Reactins 2Na(s) + Cl 2 (g) 2NaCl(s) CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) CH 4 CO 2 + 8e CH 4 is a reducing agent 2O 2 + 8e CO 2 + 2H 2 O O 2 is an xidizing agent Dr. Al Saadi 3 Balancing Redx Reactins in Acidic Media 19.1 H + (aq) + Cr O 7 (aq) + C 2 H 5 OH(l) Cr (aq) + CO 2 (g) + H 2 O(l) Step 1: Write half equatins (xidatin reductin). Cr 2 O 7 2 (aq) Cr 3+ (aq) (reductin) C 2 H 5 OH(l) CO 2 (g) (xidatin) 2 +4 Ntice that we didn t deal with H + and H 2 O because they are ging t be added at a later stage t balance the redx reactin. Dr. Al Saadi 4 2

3 Balancing Redx Reactins in Acidic Media Step 2: Balance all elements and charges fr each half reactin. Fr the reductin half reactin: e + 14H + + Cr 2 O Cr H 2 O a) Balance all elements except xygen and hydrgen atms. b) Balance xygen with H 2 O. c) Balance hydrgen with H +. d) Balance charges using electrns Similarly, fr the xidatin half reactin: C 2 H 5 OH + 3H 2 O 2CO H e Dr. Al Saadi 5 Balancing Redx Reactins in Acidic Media Step 3: qualize the number f electrns in bth balanced half reactins. 12e + 28H + + 2Cr 2 O 7 2 Step 4: Add up the tw half reactins: 4Cr H 2 O 12e + 28H + + 2Cr 2 O 2 7 4Cr H 2 O C 2 H 5 OH + 3H 2 O 2CO H e 16H + + 2Cr 2 O 7 2 +C 2 H 5 OH 4Cr H 2 O + 2CO The reductin half reactin after being multiplied by 2 Step 5: Check that elements and charges in the final reactin are balanced. Dr. Al Saadi 6 3

4 Balancing Redx Reactins in Basic Media 19.1 basic Ag(s) + CN (aq) + O 2 (g) Ag(CN) 2 (aq) Step 1: Ag + CN Ag(CN) 2 (xidatin) Step 2: O 2 Ag + 2CN Ag(CN) 2 + e (reductin) O 2 + 4H + + 4e 2H 2 O Step 3: 4Ag + 8CN 4Ag(CN) 2 + 4e O 2 + 4H + + 4e 2H 2 O Dr. Al Saadi 7 Balancing Redx Reactins in Basic Media 19.1 Step 4: 4Ag + 8CN + O 2 + 4H + 4Ag(CN) 2 + 2H 2 O + 4OH + 4OH Step 5: 4Ag + 8CN + O 2 + 4H 4H + + 4OH 2 O 4Ag(CN) 2 +2H 2 O + 4OH Step 6: liminate H 2 O mlecules frm bth sides: 4Ag + 8CN + O 2 + 2H 2 O 4Ag(CN) 2 + 4OH Step 7: Recheck the balance f elements and charges. Dr. Al Saadi 8 4

5 Galvanic Cells Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) 19.2 The tw half reactins can physically be separated in tw beakers. As a result, the electrns lst by Zn travel thrugh an external wire in rder t be gained dby Cu 2+ ins. This generates electricity. Dr. Al Saadi 9 Galvanic Cells 19.2 A galvanic cell is the experimental apparatus fr generating electricity thrugh the use f a spntaneus reactin. Salt bridge e e Ande ( ) Cathde (+) Aqueus slutin Aqueus slutin Oxidatin half reactin Reductin half reactin Dr. Al Saadi lectrdes 10 5

6 Galvanic Cells A galvanic cell (als called vltaic cell) is the experimental apparatus fr generating electricity thrugh the use f a spntaneus reactin. Oxidatin lectrdes Reductin An electric current flws frm ande t cathde because there is a difference in electrical ptential energy between the tw electrdes Oxidatin half reactin Reductin half reactin Dr. Al Saadi 11 Salt Bridge 19.2 A salt bridge is an essential cnducting medium thrugh which the catins and anins can mve frm ne half cell t the ther. It maintains i bl balance f the charge between the tw half cells. In migratin: Catins migrate tward the cathde Anins migrate tward the ande Dr. Al Saadi 12 6

7 Cell Ptential The cell ptential ( cell ) is the difference in electrical ptential between the ande and cathde half cells. The cell ptential is: cncentratin dependent. temperature dependent. determined by nature f reactants. The cell ptential is measured using a vltmeter with units f vlts (V) Dr. Al Saadi 13 Galvanic Cell Ntatin 19.2 Ande Cathde Phase bundary Salt bridge Phase bundary Dr. Al Saadi 14 7

8 Half Cell Ptential Is it pssible fr the Zn xidatin halfreactin t ccur by itself? Is it pssible fr the Cu 2+ reductin half reactin t ccur by itself? The answer is NO. A half reactin can nt take place independently. Thus, a half cell ptential is impssible t be measured. We can nly measure a full cell ptential that invlves ande and cathde half reactins. If xidatin (r reductin) f H 2 is taken t be a reference half reactin and if it is assigned a half cell ptential equal t exactly zer: H 2 2H + +2e Then: Dr. Al Saadi 15 Standard Hydrgen lectrde Then, the hydrgen electrde is used as a reference fr ther electrdes. 2H + (aq) +2e H 2 (g) The platinum (Pt) electrde is used t: prvide a surface n which the xidatin f H 2 can take place. serve as an electrical cnductr. Under standard state cnditin at 25C, the reductin ptential f H + is defined as exactly zer. The standard hydrgen electrde (SH) By definitin Dr. Al Saadi 16 8

9 Standard Reductin Ptential The standard reductin ptential () is the ptential assciated with a reductin half reactin at an electrde when the in cncentratin is 1 M and the gas pressure is 1 atm. The standard reductin ptential () fr the hydrgen electrde (SH) is assigned the value 0 V. H /H2 0 V shwn as reductin The relative standard reductin ptentials f ther halfreactins (all are represented as reductin reactins t avid unclearness) are measured relative t the SH ptential. cell cathde ande Dr. Al Saadi 17 Measuring 2 Zn /Zn standard reductin ptential fr zinc Dr. Al Saadi 18 9

10 Measuring 2 Zn /Zn cell cathde ande Th d d d i cell H /H 2 Zn /Zn V 0 V 0.76V Zn 2 /Zn The standard reductin ptential f zinc is 0.76 V. The standard reductin ptential f zinc is negative, which means, when cnnected with hydrgen half cell, zinc Zn 2 /Zn tends t be xidized, nt reduced. Dr. Al Saadi 19 Measuring 2 Cu /Cu standard reductin ptential fr cpper Dr. Al Saadi 20 10

11 Measuring 2 Cu /Cu cell cell cathde Cu 0.34 V /Cu ande H /H 2 Cu 2 /Cu 2 0 V The standard reductin ptential f cpper is V V Dr. Al Saadi Cu 2 /Cu 21 Cmparisn f Values Cu 2+ (aq) + 2e Cu (s) = V Cathde 2H + (aq) + 2e H 2 (g) = 0 V asi er t be reduced er t be xidized asi Ande Zn 2+ (aq) + 2e Zn (s) = 0.76 V Dr. Al Saadi 22 11

12 Cell f a Zinc Cpper Cell cell cathde ande V ( 0.76V) cell Cu /Cu 2 Zn /Zn cell 1.10 V Dr. Al Saadi 23 Standard reductin ptentials () f sme half reactins at 25C Inc creasing ease f reduct tin Increasin ng strength as an xidi zing agent Inc creasing ease f xidat tin Increas ing strength as a reduc cing agent Dr. Al Saadi 24 12

13 Calculating the Standard Cell Ptential ( ) cell xample: Calculate the standard cell ptential f the fllwing cell: Zn(s) Zn 2+ (1 M) Ag + (1 M) Ag(s) Frm the table f standard reductin ptentials: ccurs as reductin ccurs as xidatin cell cathde ande cell V ( 0.76V) Ag /Ag cell 1.56 V Zn /Zn Dr. Al Saadi 25 Calculating the Standard Cell Ptential ( ) cell xample: Write the balanced equatin f the verall cell reactin. Zn(s) Zn 2+ (1 M) Ag + (1 M) Ag(s) ccurs as reductin ccurs as xidatin 2Ag + (aq) + 2e 2Ag (s) Standard reductin Zn (s) 2e + Zn 2+ (aq) Zn (s) + 2Ag + (aq) Zn 2+ (aq) + 2Ag (s) ptential is an intensive prperty (like temperature and density), and it is nt an extensive prperty (like mass and vlume). Dr. Al Saadi 26 13

14 Predicting the Occurrence f Redx Reactins xample: Determine what redx reactin, if any, ccurs at 25C when lead metal (Pb) is added t: (a) 1.0 M slutin f NiCl 2. The prpsed reactin is: Pb(s) + Ni 2+ (aq) Pb 2+ (aq) + Ni (s) xidized reduced The prpsed reactin will ccur nly if Ni 2 /Ni Pb 2 /Pb 0.25 V 0.13V Ni /Ni 2 Pb /Pb 2 Dr. Al Saadi 27 Thus, the redx reactin will NOT ccur. Predicting the Occurrence f Redx Reactins xample: Determine what redx reactin, if any, ccurs at 25C when lead metal (Pb) is added t: (b) 1.0 M slutin f HCl. The prpsed reactin is: Pb(s) + 2H + (aq) Pb 2+ (aq) + H 2 (g) xidized reduced The prpsed reactin will ccur nly if /H H 2 Pb 2 /Pb 0.00 V 0.13V H /H 2 Pb /Pb Dr. Al Saadi 28 2 Thus, the redx reactin will ccur 14

15 19.4 Relatinship between cell and ΔG Is there a relatinship between thermdynamics and electrchemistry? In a galvanic cell, chemical (ptential) energy is cnverted t electric energy. electric energy = (ttal electric charge) (cell ptential) Jules = Culmbs Vlts Since and then: w max = w electrical = nf cell w max = ΔG ΔG = nf cell Dr. Al Saadi Relatinship between cell and ΔG Fr reactants and prducts at their standard states: ΔG = nf cell ΔG : Standard free energy change n: Number f mles f electrns that pass thrugh the circuit. F: Faraday s cnstant (the electric charge cntained in 1 ml f electrns) 1 F = J / V ml e cell : Standard d cell ptential. ti When ΔG is ve, the value f cell is +ve. (the prcess is spntaneus). When ΔG is +ve, the value f cell is ve. (the prcess is nnspntaneus). Dr. Al Saadi 30 15

16 19.4 Relatinship between cell and ΔG xample: Give the balancedequatin equatin and determine the standardfree energy change fr the fllwing reactin at 25C: 2 Al (s) + 3Mn 2+ (aq) 2 Al 3+ (aq) + 3Mn (s) The half cell reactins: reductin: Mn 2+ (aq) + 2e 3 6 3Mn(s) 1.18 V Mn 2 /Mn xidatin: Al (s) Al 3+ (aq) + 3e V Al 3 /Al ΔG = nf cell cell = cathde ande = 1.18 V ( 1.66 V) = ΔG = nf cell = (6 e )( J/V ml e )(0.48 V) = J/ml = kj/ml (spntaneus) Dr. Al Saadi 31 Relatinship between cell and K c 19.4 ΔG = RT lnk (frm Chapter 18) ΔG = nf cell Cmbining the abve tw equatins: RT lnk = nf cell Slving fr cell : At 25C: cell cell RT ln K nf n V ln K n V lg K Dr. Al Saadi 32 16

17 Relatinship between cell and K c 19.4 xample: Calculatethe the equilibrium cnstant fr the fllwing reactin at 25C : 2 Al (s) + 3Mn 2+ (aq) 2 Al 3+ (aq) + 3Mn (s) cell n ncell K e V K e (6)(0.48V) V V ln K 4.7 x Remember that the prcess is spntaneus (large K c value) Dr. Al Saadi 33 17