Principles of Reactivity: Electron Transfer Reactions Chemistry and Chemical Reactivity 6th Edition John C. Kotz Paul M. Treichel Gabriela C. Weaver Principles of Reactivity: Electron Transfer Reactions Lectures written by John Kotz © 2006 Brooks/Cole Thomson

ELECTROCHEMISTRY Chapter 19

TRANSFER REACTIONS Atom/Group transfer Electron transfer HCl + H2O ---> Cl- + H3O+ Electron transfer Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s)

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.

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.

OXIDATION-REDUCTION REACTIONS Direct Redox Reaction Oxidizing and reducing agents in direct contact. Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s)

Cu + Ag+ --give--> Cu2+ + Ag Balancing Equations Cu + Ag+ --give--> Cu2+ + Ag

Balancing Equations Step 1: Divide the reaction into half-reactions, one for oxidation and the other for reduction. Ox Cu ---> Cu2+ Red Ag+ ---> Ag Step 2: Balance each for mass. Already done in this case. Step 3: Balance each half-reaction for charge by adding electrons. Ox Cu ---> Cu2+ + 2e- Red Ag+ + e- ---> Ag

Balancing Equations Step 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 ---> Cu2+ + 2e- Oxidizing agent 2 Ag+ + 2 e- ---> 2 Ag Step 5: Add half-reactions to give the overall equation. Cu + 2 Ag+ ---> Cu2+ + 2Ag The equation is now balanced for both charge and mass.

OXIDATION-REDUCTION REACTIONS Indirect Redox Reaction A battery functions by transferring electrons through an external wire from the reducing agent to the oxidizing agent.

Electrochemistry Alessandro Volta, 1745-1827, Italian scientist and inventor. Luigi Galvani, 1737-1798, Italian scientist and inventor.

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)

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.

Fe --> Fe2+ + 2e- Cu2+ + 2e- --> Cu Oxidation Anode Negative Reduction Cathode Positive Fe <--Anions Cations--> Fe •Electrons travel through external wire. Salt bridge allows anions and cations to move between electrode compartments.

The Cu|Cu2+ and Ag|Ag+ Cell

Electrochemical Cell Electrons move from anode to cathode in the wire. Anions & cations move thru the salt bridge. Electrochemical Cell

Terms Used for Voltaic Cells Figure 20.6

CELL POTENTIAL, E 1.10 V Cu and Cu2+, Zn and Zn2+, cathode anode 1.0 M Cu and Cu2+, cathode Zn and Zn2+, anode Electrons 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)

CELL POTENTIAL, E For Zn/Cu cell, potential is +1.10 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.

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.

2 H+(aq, 1 M) + 2e- <----> H2(g, 1 atm) CELL POTENTIALS, Eo Can’t measure 1/2 reaction Eo directly. Therefore, measure it relative to a STANDARD HYDROGEN CELL 2 H+(aq, 1 M) + 2e- <----> H2(g, 1 atm) Eo = 0.0 V

Supplier of electrons Acceptor of electrons Zn/Zn2+ half-cell hooked to a SHE. Eo for the cell = +0.76 V Negative electrode Positive electrode Supplier of electrons Acceptor of electrons Zn --> Zn2+ + 2e- Oxidation Anode 2 H+ + 2e- --> H2 Reduction Cathode

Reduction of H+ by Zn Active Figure 20.13

Overall reaction is reduction of H+ by Zn metal. Zn(s) + 2 H+ (aq) --> Zn2+ + H2(g) Eo = +0.76 V Therefore, Eo for Zn ---> Zn2+ (aq) + 2e- is +0.76 V Zn is a better reducing agent than H2.

Zn/Cu Electrochemical Cell + Anode, negative, source of electrons Cathode, positive, sink for electrons Zn(s) ---> Zn2+(aq) + 2e- Eo = +0.76 V Cu2+(aq) + 2e- ---> Cu(s) Eo = +0.34 V --------------------------------------------------------------- Cu2+(aq) + Zn(s) ---> Zn2+(aq) + Cu(s) Eo (calc’d) = +1.10 V

Uses of Eo Values Organize half-reactions by relative ability to act as oxidizing agents Use this to predict direction of redox reactions and cell potentials. Cu2+(aq) + 2e- ---> Cu(s) Eo = +0.34 V Zn2+(aq) + 2e- ---> Zn(s) Eo = –0.76 V Note that when a reaction is reversed the sign of E˚ is reversed!

Potential Ladder for Reduction Half-Reactions Figure 20.14 Best oxidizing agents Best reducing agents Potential Ladder for Reduction Half-Reactions

TABLE OF STANDARD REDUCTION POTENTIALS oxidizing ability of ion E o (V) Cu 2+ + 2e- Cu +0.34 2 H + + 2e- H 0.00 Zn + 2e- Zn -0.76 reducing ability of element 2

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? ____________________

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+.

Standard Redox Potentials, Eo Ox. agent Cu 2+ + 2e- --> Cu +0.34 + 2 H + 2e- --> H2 0.00 Zn + 2e- --> Zn -0.76 Red. agent Any substance on the right will reduce any substance higher than it on the left. Northwest-southeast rule: product-favored reactions occur between reducing agent at southeast corner oxidizing agent at northwest corner

Cu(s) | Cu2+(aq) || H+(aq) | H2(g) Cathode Positive Anode Negative Electrons <---------- Cu2+ + 2e- --> Cu Or Cu --> Cu2+ + 2 e- H2 --> 2 H+ + 2 e- or 2 H+ + 2e- --> H2

Cu(s) | Cu2+(aq) || H+(aq) | H2(g) Cathode Positive Anode Negative Electrons <---------- Cu2+ + 2e- --> Cu H2 --> 2 H+ + 2 e- The sign of the electrode in Table 20.1 is the polarity when hooked to the H+/H2 half-cell.

Using Standard Potentials, Eo In which direction do the following reactions go? Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s) Goes right as written 2 Fe2+(aq) + Sn2+(aq) ---> 2 Fe3+(aq) + Sn(s) Goes LEFT opposite to direction written What is Eonet for the overall reaction?

Eo for a Voltaic Cell Cd --> Cd2+ + 2e- or Cd2+ + 2e- --> Cd Fe --> Fe2+ + 2e- or Fe2+ + 2e- --> Fe All ingredients are present. Which way does reaction proceed? Calculate Eo for this cell.

E at Nonstandard Conditions The NERNST EQUATION E = potential under nonstandard conditions n = no. of electrons exchanged F = Faraday’s constant R = gas constant T = temp in Kelvins ln = “natural log” Q = reaction quotient

Eo and Thermodynamics ∆Go = -nFEo Eo is related to ∆Go, the free energy change for the reaction. ∆G˚ is proportional to –nE˚ ∆Go = -nFEo where F = Faraday constant = 9.6485 x 104 J/V•mol of e- (or 9.6485 x 104 coulombs/mol) and n is the number of moles of electrons transferred

Eo and ∆Go ∆Go = - n F Eo For a product-favored reaction Reactants ----> Products ∆Go < 0 and so Eo > 0 Eo is positive For a reactant-favored reaction Reactants <---- Products ∆Go > 0 and so Eo < 0 Eo is negative

Eo and Equilibrium Constant DGo = -RT ln K DGo = -nFEo

Dry Cell Battery Anode (-) Zn ---> Zn2+ + 2e- Cathode (+) Primary battery — uses redox reactions that cannot be restored by recharge. Anode (-) Zn ---> Zn2+ + 2e- Cathode (+) 2 NH4+ + 2e- ---> 2 NH3 + H2

Alkaline Battery Nearly same reactions as in common dry cell, but under basic conditions. Anode (-): Zn + 2 OH- ---> ZnO + H2O + 2e- Cathode (+): 2 MnO2 + H2O + 2e- ---> Mn2O3 + 2 OH-

Lead Storage Battery Secondary battery Uses redox reactions that can be reversed. Can be restored by recharging

Ni-Cad Battery Anode (-) Cd + 2 OH- ---> Cd(OH)2 + 2e- Cathode (+) NiO(OH) + H2O + e- ---> Ni(OH)2 + OH-

Fuel Cells: H2 as a Fuel Fuel 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.

Hydrogen—Air Fuel Cell Figure 20.12

H2 as a Fuel Comparison of the volumes of substances required to store 4 kg of hydrogen relative to car size. (Energy, p. 290)

Storing H2 as a Fuel One way to store H2 is to adsorb the gas onto a metal or metal alloy. (Energy, p. 290)