Galvanic Cells and Cell Potentials Cu Ag Redox Reaction When a piece of Cu metal is added to Ag aq ions Cu s 2Ag aq Cu2 aq 2Ag s When run directly in a test tube Ag metal is deposited on the surface of the Cu strip Cu enters the solution as blue Cu2 ions Figure 16 2 Galvanic Cells Voltaic Cells In principle any spontaneous redox reaction can serve as a source of electrical energy in a galvanic cell This cell is designed to take advantage of the movement of electrons which occurs during a redox reaction The oxidation and reduction half reactions are compartmentalized into half cells Oxidation at one electrode anode Reduction at the other electrode cathode Electrons move through an external wire from the anode to the These moving electrons can be used as a source of electrical cathode energy The oxidation and reduction half reactions occur at separate electrodes Galvanic Cells The oxidation reaction occurs at the Anode Anions flow to the anode The reduction reaction occurs at the cathode Cations flow to the cathode Figure 16 3 A galvanic cell based on the spontaneous reaction between copper and silver I ions Cu s 2Ag aq Cu2 aq 2Ag s Cu Ag Galvanic Cell Cu s 2Ag aq Cu2 aq 2Ag s Cu metal anode dips into a solution of Cu2 ions Ag metal cathode dips into a solution of Ag ions The external wires are connected to a voltmeter At the Cu anode electrons are produced Cu s Cu2 aq 2e Electrons flow from the anode through the voltmeter to the Ag cathode the voltmeter indicates the cell s potential The electrons enter the Ag cathode at which 2Ag aq 2e Ag s Charge Balance Cu s 2Ag aq Cu2 aq 2Ag s As the oxidation occurs a surplus of positive ions builds up at the anode The area near the cathode becomes deficient in positive ions Cu s Cu2 aq 2e 2Ag aq 2e Ag s The salt bridge supplies additional cations and anions needed to balance out these charge differences and complete the circuit Salt Bridges The salt bridge is a porous material consisting of a concentrated salt solution NaNO3 or KNO3 are frequently used Anions flow toward the anode to neutralize the build up of positive Cations flow toward the cathode to replace the cations that are being charge consumed This flow of ions enables electrical neutrality to be achieved the circuit to be complete and current to flow Cell Notation Describes what happens in a galvanic cell Oxidation on the left Cu oxidized to Cu2 Reduction on the right Ag reduced to Ag Single vertical line represents a phase boundary Liquid metal or liquid gas etc Double line is the salt bridge Don t include spectator ions Sometimes the concentration of the ion s is included Cu s 2Ag aq Cu2 aq 2Ag s Cell Notation Cu s Cu2 aq Ag aq Ag s Anode Oxidation Cathode Reduction Galvanic Cell Summary Compartmentalizing the oxidation and reduction half reactions into half cells enables a spontaneous redox reaction to be a source of electrical energy In one half cell the anode oxidation occurs In the other half cell the cathode reduction occurs Each half cell consists of an electrode dipped into an aqueous solution The half cells are joined by an external wire electron movement and salt bridge ion movement Standard Reduction Potentials and Cell Potentials Electrode and Cell Potentials In a circuit the flow of charge is a result of an electrical potential difference between two points in the circuit Electrical potential is the ability of the electric field to do work on the charge In a galvanic cell this electrical potential difference is called cell potential The flow of charge in a galvanic cell results from the difference in the electrical potentials at each electrode Standard Potentials E The driving force of an electrochemical reaction is measured by the standard cell potential Depends on the nature of the redox reaction and on the concentration of species involved Standard potentials E are measured with All aqueous concentrations at 1M The pressure of all gases at 1 atm Temperature held constant usually at 25 C Units of E is V or mV E Cathode and Anode The E for a half reaction cannot be measured only E cell of the entire redox reaction can be measured A standard reduction potential E red has been set The value of E for the reduction of 2H ions to H2 g has been assigned as 0 000 V This is called the standard hydrogen electrode SHE A standard hydrogen electrode SHE 2H aq 1M 2e H2 g 1 atm E red 0 V If SHE is anode Pt H2 1 atm H 1 M cathode part If SHE is cathode anode part H 1 M H2 1 atm Pt The E red value of all other half reactions have been assigned relative to SHE Figure 16 6 A cell permitting experimental measurement of the standard electrode potential for the half reaction Cu2 aq 2e Cu s Table 16 1 Selected Standard Reduction Potentials at 25 C d e c u d e r y l i s a e e r o M s t n e g a g n z d x o i i i s a h t g n e r t s i g n s a e r c n I MnO4 Half Reaction Hg2 2 aq 2e 2Hg l Fe3 aq e Fe3 aq aq 2H2O l 3e MnO2 s 4OH aq I2 s 2e 2I aq NiO2 s 2H2O l 2e Ni OH 2 s 2OH aq Cu2 aq 2e Cu s Hg2Cl2 s 2e 2Hg l 2Cl aq AgCl s e Ag s Cl aq Sn4 aq 2e Sn2 aq 2H aq 2e H2 g Pb2 aq 2e Pb s Sn2 aq 2e Sn s Ni2 aq 2e Ni s Co2 aq 2e Co s as reduction E V 0 7973 0 771 0 558 0 5355 0 49 0 34 0 26808 0 22233 0 151 0 00 0 1262 0 1375 0 257 0 28 d e z i d i x o y l i s a e e r o M s t n e g a i g n c u d e r s a h t g n e r t s i g n s a e r c n I The half reaction that has the more positive reduction potential is the half reaction that will occur Standard Reduction Potentials Reactions with E red 0 are spontaneous reductions relative to the SHE Reactions with E red 0 are spontaneous oxidations relative to the SHE The larger the difference between E red values the larger E cell The more positive the value of E red the greater the driving force for reduction E cell E red cathode E red anode Calculating Standard Cell Potentials Compare reduction potentials to determine which species is oxidized reduced The half rxn with the more positive reduction potential occurs as a reduction rxn In a Galvanic cell this is the cathode The …