1 © 2006 Brooks/Cole - Thomson OXIDATION-REDUCTION REACTIONS Indirect Redox Reaction A battery functions by transferring electrons through an external wire from the reducer to the oxidizer. The spontaneous flow of electrons from one substance to another is the driving force for this reaction.

2 © 2006 Brooks/Cole - Thomson Oxidation: Zn(s) ---> Zn 2+ (aq) + 2e- Reduction: Cu 2+ (aq) + 2e- ---> Cu(s) Cu 2+ (aq) + Zn(s) ---> Zn 2+ (aq) + Cu(s) Electrons are transferred from Zn to Cu 2+, but there is no useful electric current. Direct Redox Reaction With time, Cu plates out onto Zn metal strip, and Zn strip “disappears” as it becomes ions.

3 © 2006 Brooks/Cole - Thomson This is accomplished in a GALVANIC or VOLTAIC cell. A group of such cells is called a battery.

4 © 2006 Brooks/Cole - Thomson Electrons travel through external wire. Salt bridge allows anions and cations to flow into anode and cathode to keep charges balanced.Salt bridge allows anions and cations to flow into anode and cathode to keep charges balanced. Electrons travel through external wire. Salt bridge allows anions and cations to flow into anode and cathode to keep charges balanced.Salt bridge allows anions and cations to flow into anode and cathode to keep charges balanced. Fe --> Fe e- Cu e- --> Cu <--AnionsCations--> OxidationAnode: Fe gets oxidized OxidationAnode: ReductionCathode: Cu 2+ gets reducedReductionCathode: reduced FeFe Fe ANATOMY OF A BATTERY

5 © 2006 Brooks/Cole - Thomson The Cu|Cu 2+ and Ag|Ag + Cell Write the 2 half reactions for this battery. What species is getting oxidized? What is getting reduced? Which is the oxidizer? Which is the reducer? Show how we can find the voltage of this cell.

6 © 2006 Brooks/Cole - Thomson Electrochemical Cell: Describe what particles are doing… Electrons move from anode to cathode in the wire. Anions & cations move thru the salt bridge.

7 © 2006 Brooks/Cole - Thomson CELL POTENTIAL, E (volts) Electrons are “driven” from anode to cathode by an electromotive force.Electrons are “driven” from anode to cathode by an electromotive force. For Zn/Cu cell, this is indicated by a voltage of 1.10 V at 25 ˚C and when [Zn 2+ ] and [Cu 2+ ] = 1.0 M.For Zn/Cu cell, this is indicated by a voltage of 1.10 V at 25 ˚C and when [Zn 2+ ] and [Cu 2+ ] = 1.0 M. Write the 2 half reactions. Show how the voltage is calculated.Write the 2 half reactions. Show how the voltage is calculated. Standard reduction potentials are measured at standard conditions (1 M, 25 o C)Standard reduction potentials are measured at standard conditions (1 M, 25 o C) Positive voltage indicates which way the cell runs spontaneously!Positive voltage indicates which way the cell runs spontaneously! Zn and Zn 2+, anode Cu and Cu 2+, cathode 1.10 V 1.0 M

8 © 2006 Brooks/Cole - Thomson Calculating Cell Voltage Balanced half-reactions can be added together to get overall, balanced equation.Balanced half-reactions can be added together to get overall, balanced equation. Zn(s) ---> Zn 2+ (aq) + 2e- Cu 2+ (aq) + 2e- ---> Cu(s) Cu 2+ (aq) + Zn(s) ---> Zn 2+ (aq) + Cu(s) Zn(s) ---> Zn 2+ (aq) + 2e- Cu 2+ (aq) + 2e- ---> Cu(s) Cu 2+ (aq) + Zn(s) ---> Zn 2+ (aq) + Cu(s) If we know E o for each half-reaction, we could get E o for net reaction.

9 © 2006 Brooks/Cole - Thomson Zn/Cu Electrochemical Cell Zn(s) ---> Zn 2+ (aq) + 2e-E o = V Cu 2+ (aq) + 2e- ---> Cu(s)E o = V Cu 2+ (aq) + Zn(s) ---> Zn 2+ (aq) + Cu(s) E o = V E o = V Cathode, positive, sink for electrons Anode, negative, source of electrons +

10 © 2006 Brooks/Cole - Thomson Cd --> Cd e- or Cd e- --> Cd Fe --> Fe e- or Fe e- --> Fe E o for a Voltaic Cell Use standard reduction potential table. Which two half reactions occur? Calculate E o for this cell. Label the oxidation reaction and reduction reaction. Label anode and cathode. Show direction of electron flow. Indicate movement of anions and cations in salt bridge.