3TRANSFER REACTIONS Atom/Group transfer Electron transfer HCl + H2O ---> Cl H3O+Electron transferCu(s) Ag+(aq) ---> Cu2+(aq) Ag(s)
4Electron 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.
5Review 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.
6OXIDATION-REDUCTION REACTIONS Direct Redox ReactionOxidizing and reducing agents in direct contact.Cu(s) Ag+(aq) ---> Cu2+(aq) Ag(s)
7Cu + Ag+ --give--> Cu2+ + Ag Balancing EquationsCu + Ag give--> Cu Ag
8Balancing 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
9Balancing 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.
10OXIDATION-REDUCTION REACTIONS Indirect Redox ReactionA battery functions by transferring electrons through an external wire from the reducing agent to the oxidizing agent.
11ElectrochemistryAlessandro Volta, , Italian scientist and inventor.Luigi Galvani, , Italian scientist and inventor.
12CHEMICAL 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)
13CHEMICAL 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.
14Fe --> Fe2+ + 2e-Cu2+ + 2e- --> CuOxidationAnodeNegativeReductionCathodePositiveFe<--AnionsCations-->Fe•Electrons travel through external wire.Salt bridge allows anions and cations to move between electrode compartments.
18CELL 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)
19CELL 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.
20Calculating 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.
212 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
22Supplier 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
24Overall 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.
25Zn/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
26Uses 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!
28Potential Ladder for Reduction Half-Reactions Figure 20.14Best oxidizing agentsBest reducing agentsPotential Ladder for Reduction Half-Reactions
29TABLE OF STANDARD REDUCTION POTENTIALS oxidizingability of ionEo(V)Cu2++ 2e Cu+0.342 H++ 2e H0.00Zn+ 2e Zn-0.76reducing abilityof element2
30Using 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? ____________________
31Standard 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+.
32Standard 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
34Cu(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.
35Using 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?
36Eo 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.
37E 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
38Eo 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
39Eo 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
45Fuel 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.