3 TRANSFER REACTIONS Atom/Group transfer Electron transfer HCl + H2O ---> Cl H3O+Electron transferCu(s) Ag+(aq) ---> Cu2+(aq) Ag(s)
4 Electron Transfer Reactions Electron transfer reactions are oxidation-reduction or redox reactions.Redox reactions can result in the generation of an electric current or be caused by imposing an electric current.Therefore, this field of chemistry is often called ELECTROCHEMISTRY.
5 Review of Terminology for Redox Reactions OXIDATION—loss of electron(s) by a species; increase in oxidation number.REDUCTION—gain of electron(s); decrease in oxidation number.OXIDIZING AGENT—electron acceptor; species is reduced.REDUCING AGENT—electron donor; species is oxidized.
6 OXIDATION-REDUCTION REACTIONS Direct Redox ReactionOxidizing and reducing agents in direct contact.Cu(s) Ag+(aq) ---> Cu2+(aq) Ag(s)
7 Cu + Ag+ --give--> Cu2+ + Ag Balancing EquationsCu + Ag give--> Cu Ag
8 Balancing EquationsStep 1: Divide the reaction into half-reactions, one for oxidation and the other for reduction.Ox Cu ---> Cu2+Red Ag+ ---> AgStep 2: Balance each for mass. Already done in this case.Step 3: Balance each half-reaction for charge by adding electrons.Ox Cu ---> Cu e-Red Ag+ + e- ---> Ag
9 Balancing EquationsStep 4: Multiply each half-reaction by a factor so that the reducing agent supplies as many electrons as the oxidizing agent requires.Reducing agent Cu ---> Cu e-Oxidizing agent 2 Ag e- ---> 2 AgStep 5: Add half-reactions to give the overall equation.Cu Ag > Cu AgThe equation is now balanced for both charge and mass.
10 OXIDATION-REDUCTION REACTIONS Indirect Redox ReactionA battery functions by transferring electrons through an external wire from the reducing agent to the oxidizing agent.
11 ElectrochemistryAlessandro Volta, , Italian scientist and inventor.Luigi Galvani, , Italian scientist and inventor.
12 CHEMICAL CHANGE ---> ELECTRIC CURRENT With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.”Electrons are transferred from Zn to Cu2+, but there is no useful electric current.Oxidation: Zn(s) ---> Zn2+(aq) + 2e-Reduction: Cu2+(aq) + 2e- ---> Cu(s)Cu2+(aq) + Zn(s) ---> Zn2+(aq) + Cu(s)
13 CHEMICAL CHANGE ---> ELECTRIC CURRENT To obtain a useful current, we separate the oxidizing and reducing agents so that electron transfer occurs through an external wire.This is accomplished in a GALVANIC or VOLTAIC cell.A group of such cells is called a battery.
14 Fe --> Fe2+ + 2e-Cu2+ + 2e- --> CuOxidationAnodeNegativeReductionCathodePositiveFe<--AnionsCations-->Fe•Electrons travel through external wire.Salt bridge allows anions and cations to move between electrode compartments.
18 CELL POTENTIAL, E 1.10 V Cu and Cu2+, Zn and Zn2+, cathode anode 1.0 MCu and Cu2+,cathodeZn and Zn2+,anodeElectrons are “driven” from anode to cathode by an electromotive force or emf.For Zn/Cu cell, this is indicated by a voltage of 1.10 V at 25 ˚C and when [Zn2+] and [Cu2+] = 1.0 M.Standard reduction potentials are measured at standard conditions (1 M, 25oC)
19 CELL POTENTIAL, EFor Zn/Cu cell, potential is V at 25 ˚C and when [Zn2+] and [Cu2+] = 1.0 M.This is the STANDARD CELL POTENTIAL, Eo—a quantitative measure of the tendency of reactants to proceed to products when all are in their standard states at 25 ˚C.
20 Calculating Cell Voltage Balanced half-reactions can be added together to get overall, balanced equation.Zn(s) ---> Zn2+(aq) + 2e-Cu2+(aq) + 2e- ---> Cu(s)Cu2+(aq) + Zn(s) ---> Zn2+(aq) + Cu(s)If we know Eo for each half-reaction, we could get Eo for net reaction.
21 2 H+(aq, 1 M) + 2e- <----> H2(g, 1 atm) CELL POTENTIALS, EoCan’t measure 1/2 reaction Eo directly. Therefore, measure it relative to a STANDARD HYDROGEN CELL2 H+(aq, 1 M) e- <----> H2(g, 1 atm)Eo = 0.0 V
22 Supplier of electrons Acceptor of electrons Zn/Zn2+ half-cell hooked to a SHE.Eo for the cell = VNegative electrodePositive electrodeSupplier of electronsAcceptor of electronsZn --> Zn2+ + 2e-OxidationAnode2 H+ + 2e- --> H2ReductionCathode
24 Overall reaction is reduction of H+ by Zn metal. Zn(s) + 2 H+ (aq) --> Zn2+ + H2(g) Eo = VTherefore, Eo for Zn ---> Zn2+ (aq) + 2e- is VZn is a better reducing agent than H2.
25 Zn/Cu Electrochemical Cell +Anode, negative, source of electronsCathode, positive, sink for electronsZn(s) ---> Zn2+(aq) + 2e- Eo = VCu2+(aq) + 2e- ---> Cu(s) Eo = VCu2+(aq) + Zn(s) ---> Zn2+(aq) + Cu(s)Eo (calc’d) = V
26 Uses of Eo ValuesOrganize half-reactions by relative ability to act as oxidizing agentsUse this to predict direction of redox reactions and cell potentials.Cu2+(aq) + 2e- ---> Cu(s) Eo = VZn2+(aq) + 2e- ---> Zn(s) Eo = –0.76 VNote that when a reaction is reversed the sign of E˚ is reversed!
28 Potential Ladder for Reduction Half-Reactions Figure 20.14Best oxidizing agentsBest reducing agentsPotential Ladder for Reduction Half-Reactions
29 TABLE OF STANDARD REDUCTION POTENTIALS oxidizingability of ionEo(V)Cu2++ 2e Cu+0.342 H++ 2e H0.00Zn+ 2e Zn-0.76reducing abilityof element2
30 Using Standard Potentials, Eo Table 20.1 Which is the best oxidizing agent: O2, H2O2, or Cl2? _________________Which is the best reducing agent: Hg, Al, or Sn? ____________________
31 Standard Redox Potentials, Eo Any substance on the right will reduce any substance higher than it on the left.Zn can reduce H+ and Cu2+.H2 can reduce Cu2+ but not Zn2+Cu cannot reduce H+ or Zn2+.
32 Standard Redox Potentials, Eo Ox. agentCu2++ 2e- --> Cu+0.34+2 H+ 2e- --> H20.00Zn+ 2e- --> Zn-0.76Red. agentAny substance on the right will reduce any substance higher than it on the left.Northwest-southeast rule: product-favored reactions occur betweenreducing agent at southeast corneroxidizing agent at northwest corner
34 Cu(s) | Cu2+(aq) || H+(aq) | H2(g) CathodePositiveAnodeNegativeElectrons<Cu e- --> CuH2 --> 2 H+ + 2 e-The sign of the electrode in Table 20.1 is the polarity when hooked to the H+/H2 half-cell.
35 Using Standard Potentials, Eo In which direction do the following reactions go?Cu(s) Ag+(aq) ---> Cu2+(aq) Ag(s)Goes right as written2 Fe2+(aq) + Sn2+(aq) ---> 2 Fe3+(aq) + Sn(s)Goes LEFT opposite to direction writtenWhat is Eonet for the overall reaction?
36 Eo for a Voltaic Cell Cd --> Cd2+ + 2e- or Cd2+ + 2e- --> Cd Fe --> Fe2+ + 2e-orFe2+ + 2e- --> FeAll ingredients are present. Which way does reaction proceed? Calculate Eo for this cell.
37 E at Nonstandard Conditions The NERNST EQUATIONE = potential under nonstandard conditionsn = no. of electrons exchangedF = Faraday’s constantR = gas constantT = temp in Kelvinsln = “natural log”Q = reaction quotient
38 Eo and Thermodynamics ∆Go = -nFEo Eo is related to ∆Go, the free energy change for the reaction.∆G˚ is proportional to –nE˚∆Go = -nFEowhere F = Faraday constant = x 104 J/V•mol of e-(or x 104 coulombs/mol)and n is the number of moles of electrons transferred
39 Eo and ∆Go ∆Go = - n F Eo For a product-favored reaction Reactants ----> Products∆Go < 0 and so Eo > 0Eo is positiveFor a reactant-favored reactionReactants <---- Products∆Go > 0 and so Eo < 0Eo is negative
40 Eo and Equilibrium Constant DGo = -RT ln KDGo = -nFEo
45 Fuel Cells: H2 as a FuelFuel cell - reactants are supplied continuously from an external source.Cars can use electricity generated by H2/O2 fuel cells.H2 carried in tanks or generated from hydrocarbons.