Fernando O. Raineri Office Hours: MWF 9:30-10:30 AM Room 519 Tue. 3:00-5:00 CLC (lobby). P1) What is the reduction potential of the hydrogen electrode g bar H O aq Pt(s) H,1 2 3 when the aqueous solution is at ph = 7 and the temperature is T = 298.15 K? A. 0.82 V B. 0.0 V C. 0.82V D. 0.41 V Answer: D 2 1
Electrochemical Cell Kinetics The electric power of an electrochemical cell, depends on both E cell and the rate at which the charge is being transferred between electrodes as measured by the electric current i. Power i E A V W cell The magnitude of the electric current i is determined by the rate of the chemical reactions taking place in the half cells. rateof the reaction in an dq electric current ; C i A electrochemical cell dt s The rate of reaction in an electrochemical cell depends on the various factors listed below: a) Temperature of the system b) The activation energy of the reaction c) The porosity of the electrodes d) The solvation of ions in solution e) The concentration of ions at the interface f) The rate of ion diffusion in solution Discuss and explain how each of these factors may affect reaction rate and, thus, the magnitude of the electric current generated by an electrochemical cell. 2
Electric Current At the beginning, the reactants can be expected to be plentiful and the electric current will be determined by the rate at which electrons are transferred between metal atoms and ions at the electrode/solution interface. As the reaction proceeds and reactants are consumed, the availability of ions at the electrode/solution interface will decrease, slowing down the overall process Change in E cell A A n+ + n e- B n+ + n e- B Anode (-) Cathode (+) As electrons move from anode to cathode: A n+ accumulates making the anode more + B n+ depletes making the cathode more - The electric current causes effective electric potential at each electrode to change, and thus E cell = E red (cathode) E red (anode) changes. electrical power delivered by the Power i Ecell A V W electrochemical cell 3
2 mol mol Cu s Cu aq,1.0 Ag aq,1.0 Ag L L Experimental behavior for a cell connected to an apparatus where a current i 1 is established How would you interpret this graph? s By convention: i anodic > 0 i cathodic < 0 i-e cell Data 4
Let s Think Discuss and explain which set of i-e cell data corresponds to: a) A cell working at high or low temperature; b) A cell with more porous versus less porous electrodes; c) A cell in which ions diffuse fastly versus slowly. Low T Low Porosity Slow Diffusion Electrolysis Electrons are forced into the electrochemical system from an external power source (a battery) so that an unfavorable reaction takes place. Like in voltaic cells, electrolysis cells have two electrodes connected externally to a power source. A simple example is the electrolysis of a molten salt. 10 5
Electrolysis Voltaic Cell Electrolytic Cell reduction, cathode negative, cations migrate to it oxidation, anode positive, anions migrate to it 12 6
Electrolysis Electrons are forced into the electrochemical system from an external power source (a battery) so that an unfavorable reaction takes place. Like in voltaic cells, electrolysis cells have two electrodes connected externally to a power source. A simple example is the electrolysis of a molten salt. reduction, cathode 2Na 2e 2Na negative, cations migrate to it oxidation, anode 2Cl Cl2 g 2e positive, anions migrate to it 2Cl melt a +C g 2Na melt 2N l 2 Electrolysis The situation is more complicated in an electrolyte solution, as we have to determine which are the species that are reduced and oxidized. Consider the electrolysis of an aqueous solution of KI. The species in solution are: K +, I, H 2 O. In principle two species can undergo oxidation at the anode: 2 e E 2 2I aq I s 2 I / I 0.535 V or e E 6H O O g 4H O aq 4 O /H O,pH=7 0.82V 2 2 3 2 2 The less positive the reduction potential, the easier that the oxidation reaction will occur. I will be oxidized. 7
Electrolysis There are two possible species that can be reduced: e E K aq aq K s K / K 2.925 V 2e aq 2H O aq H g 2OH aq E H O / H,pH 7 0.41V 2 2 2 2 The more positive the reduction potential, the more easily the substance on the left-side of the halfreaction can be reduced. H 2 O will be reduced. The net reaction in the electrolysis is 2H O H g 2OH aq 2I aq I s E E 0 cell 0 cell 0 cathode 2 2 2 0 anode 5V H2O/H2 EI 2 /I E E E 0.41V 0.53 = 0.945 V Electrolysis When electrolysis is carried out in an aqueous solution, the oxidation and reduction half-reactions that occur are those that require the least voltage to be applied (the half-reactions that give the least negative cell voltage). A metal ion or other species can be reduced if it has a standard reduction potential that is more positive than E H2O / H 2,pH 7 0.41V 2e aq 2H O H g 2OH aq 2 2 16 8
Electrolysis A species can be oxidized in aqueous solution if its reduction potential is less positive than 2 2 E O / H O, ph 7 0.82V 6H O O g 4H O aq 4e 2 2 3 The voltage that needs to be applied to an electrolysis cell is always larger than the estimate found with the standard reduction potential or with the Nernst equation. An overvoltage needs to be applied to overcome the activation barrier each of the electrode reactions. 17 Quantification of the extent of reaction From electricity and magnetism we know that dq electric current i dt number of electrons that circulate per unit time If the current is independent of time: q i t charge circulated constant current time charge Coulombs current Ampere time second 18 9
Electrolysis If when we apply a voltage to an electrolytic cell the reaction that occurs in the cathode is (M is a metal) ne n M aq aq M s then: (a) if we know the current i and the time t for the electrolysis process we can find the number of moles of metal M deposited in the cathode. (b) If we know the mass of metal M deposited in the cathode we can find the number of electrons that have circulated in the process. 19 Quantification of the extent of reaction F We recall that C charge of a mol 96485 mol of protons In the reaction ne n M aq aq M s for n moles of electrons that circulate (with a charge of -nf coulombs) a mol of metal M is deposited in the cathode. 20 10
Quantification of the extent of reaction When a constant current i circulates for a time t: number of moles q of metal M deposited nf in the cathode i t nf number of moles of M deposited in the cathode mass of M deposited M M 21 Electrolysis A current of 0.015 A is passed through a solution of AgNO 3 for 155 minutes. What is the Ag mass deposited in the cathode? e Ag aq aq Ag s mass of it M Ag silver nf s 0.015 A 155min 60 mass of g min 107.8682 silver mol C 96485 mol mass of 0.156 g silver 22 11
P2) A CuSO 4 solution is electrolyzed for 7.00 min with a current of 0.60 A. How many moles of Cu are deposited in the cathode? 3 A. 1.3110 mol 5 B. 2.1810 mol 3 C. 2.6210 mol 2 D. 1.2610 mol 4 E. 2.2810 mol Answer: A 23 P3) Electrolysis of a solution of CuSO 4 (aq) to give copper metal is carried out using a current of 2.12 A. How long should the electrolysis continue to produce 0.50 g of metallic copper? M Cu 63.546g mol 1 (1): 1.22 hours (2): 66.9 days (3): 12.6 hours (4): 716 s (5): 358 s Answer: (4) 24 12
Electrolysis V(II) can be produced by electrolysis of V(III) in solution. How long must an electrolysis last if you wish to convert completely 0.125 L of a 0.015 mol L 1 aqueous solution of V(III) to V(II) using a current of 0.268 A? e 3 2 V aq aq V aq 3 3 mol 3 nv V Vsolution 0.015 0.125L 1.87510 mol L 3 C 3 nv nf 1.87510 mol 96485 mol t 675.0 s i 0.268A 25 Let s apply! Assess what you know 13
Hydrogen Fuel Cell Fuel cells are electrochemical devices in which reactants are continuously supplied to the system to sustain the redox reaction. In a hydrogen fuel cell, hydrogen gas and oxygen gas from air are used to produce energy. Let s apply! Redox Process Consider the overall chemical reaction in a hydrogen fuel cell: H 2 (g) + 1/2 O 2 (g) H 2 O(l) Determine the oxidation state of each atom in the reactants and products. Identify the oxidized and reduced atoms in the process. Identify the oxidizing agent and reducing agent in the reaction. Determine the number of electrons exchanged per molecule of H 2 consumed. 14
Let s apply! Thermodynamics H 2 (g) + 1/2 O 2 (g) H 2 O(l) Qualitatively predict the signs of DH rxn and DS rxn for the overall reaction in a hydrogen fuel cell. Predict the effect of changing temperature T on the extent of the reaction. Verify your predictions using thermodynamic data to calculate DH o rxn and DS o rxn. Let s apply! Thermodynamics H 2 (g) + 1/2 O 2 (g) H 2 O(l) Calculate the DG o rxn and 25 o C and 70 o C (typical temperature at which a hydrogen fuel cell operates). Build a graph showing how DG o rxn changes as a function of T. If DG o rxn = nfe o cell Calculate the value of E o cell at 25 o C and 70 o C. Build a plausible explanation for the change of E o cell with temperature. 15
Let s apply! Cell Potential H 2 (g) + 1/2 O 2 (g) H 2 O(l) Express the half reactions taking place in the anode and the cathode of hydrogen fuel cell working under acidic conditions. Determine E o cell for the fuel cell and compare your result with the value obtained using the DG o rxn at 25 o C in the previous activity. Let s apply! Energy Output H 2 (g) + 1/2 O 2 (g) H 2 O(l) Estimate the maximum amount of energy that the fuel cell produces per mol of hydrogen consumed under standard conditions. Express the Nernst equation for the fuel cell in terms of the temperature of the system and the pressure of the gases used as reactants. Qualitatively analyze this relationship and determine the best conditions to maximize energy output. 16
Let s apply! Fuel Cell Kinetics The graph represents i-e cell data for hydrogen fuel cells with two different types of electrodes: Which type of electrode would be best to use based on the data? When a current is established, the change in Ered is greater in the cathode than in the anode. Propose a plausible explanation for this result. Let s apply! Fuel Cell Kinetics How would you expect temperature to affect the value of E cell when an electric current flows through the fuel cell? 17
Sol. Hydrogen electrode at ph = 7 Sunday, December 4, 2016 2:39 PM Class39 Page 1
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P2S Sunday, December 4, 2016 3:41 PM Class39 Page 1
P3S Sunday, December 4, 2016 3:50 PM Class39 Page 1