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. 19.1 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) 0 +2 +2 0 Dr. Al Saadi Oxidatin states 2 1
Assigning Oxidatin States in Redx Reactins 2Na(s) + Cl 2 (g) 2NaCl(s) 0 0 +1 11 19.1 CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) 4 0 +4 +1 4 2 2 2 +1 2 CH 4 CO 2 + 8e 4 2 4 +4 CH 4 is a reducing agent 2O 2 + 8e CO 2 + 2H 2 O 0 2 2 2 O 2 is an xidizing agent Dr. Al Saadi 3 Balancing Redx Reactins in Acidic Media 19.1 H + (aq) + Cr 2 3+ 2 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) +6 +3 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
Balancing Redx Reactins in Acidic Media Step 2: Balance all elements and charges fr each half reactin. Fr the reductin half reactin: 19.1 6e + 14H + + Cr 2 O 2 7 2 Cr 3+ + 7H 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. +14 2 +6 Similarly, fr the xidatin half reactin: C 2 H 5 OH + 3H 2 O 2CO 2 + 12H + + 12e 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 3+ + 14H 2 O 12e + 28H + + 2Cr 2 O 2 7 4Cr 3+ + 14H 2 O C 2 H 5 OH + 3H 2 O 2CO 2 + 12H + + 12e 16H + + 2Cr 2 O 7 2 +C 2 H 5 OH 4Cr 3+ + 11H 2 O + 2CO 2 19.1 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
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
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
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. 19.2 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
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). 19.2 Dr. Al Saadi 13 Galvanic Cell Ntatin 19.2 Ande Cathde Phase bundary Salt bridge Phase bundary Dr. Al Saadi 14 7
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
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
Measuring 2 Zn /Zn cell cathde ande Th d d d i cell H /H 2 Zn /Zn 2 0.76V 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
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 + 0.34 V. 0.34 V Dr. Al Saadi Cu 2 /Cu 21 Cmparisn f Values Cu 2+ (aq) + 2e Cu (s) = + 0.34 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
Cell f a Zinc Cpper Cell cell cathde ande 2 0.34V ( 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
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 2 0.80V ( 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
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
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 29 19.4 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 = 9.65 10 4 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
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 2 2 6 1.66 V Al 3 /Al ΔG = nf cell cell = cathde ande = 1.18 V ( 1.66 V) = + 0.48 ΔG = nf cell = (6 e )(9.65 10 4 J/V ml e )(0.48 V) = 2.78 10 5 J/ml = 2.78 10 2 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 0.0257 n V ln K 0.0592 n V lg K Dr. Al Saadi 32 16
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 0.0257 n ncell K e 0.0257V K e (6)(0.48V) 0.0257V V ln K 4.7 x 10 48 Remember that the prcess is spntaneus (large K c value) Dr. Al Saadi 33 17