“ Free Energy” <ul><li>All spontaneous (product favored) reactions can be harnessed in some way to do work! </li></ul>OR Burn fuel to create steam To turn the turbines How can you store “ free energy?” Power Generation
ELECTROCHEMISTRY Unit 4 Study of chemical reactions that produce electrical currents or voltages and of the chemical reactions that are caused by the action of currents or voltages.
A 3.41 x 10 -6 g sample of a compound is known to contain 4.67 x 10 16 molecules. This compound is <ul><li>CO 2 </li></ul><ul><li>CH 4 </li></ul><ul><li>NH 3 </li></ul><ul><li>H 2 O </li></ul>
What is different in the second reaction? Ag + + Cl - AgCl(s) Ag + + Cu(s) Cu 2+ + Ag(s) Cu Ag Ag + The charge on Ag and Cu change. That’s an Oxidation- reduction reaction (REDOX) Cu 2+
Ag + + Cu(s) Cu 2+ + Ag(s) Copper loses two electrons Cu was OXIDIZED Ag was REDUCED Ag + was the OXIDIZING AGENT Cu was the REDUCING AGENT Charge balance vs. mass balance 2 2 Ag + gains electrons
Identifying REDOX OXIDATION NUMBERS <ul><li>Any element in any neutral form-- standing alone </li></ul><ul><li>OXIDATION NUMBER = 0 </li></ul>Bookkeeping method only---not real CHARGES S 8 O.N. = 0 O 2 , O 3 , O O.N. = 0 I 2 (s) , I 2 (g) , I O.N. = 0 2) Any Element as a single ion – standing alone OXIDATION NUMBER = Charge on the ION S 2- O.N. = -2 O 2- O.N. = -2 I - Ox no = -1 Cu 2+ O.N. = +2 Al 3+ O.N. = +3
In a neutral COMPOUND some elements often have a fixed OXIDATION NUMBER H almost always is +1 O almost always is -2 F always is -1 3) In a NEUTRAL COMPOUND the SUM of all the atomic oxidation numbers is ZERO CH 4 CH 3 COOH
4) In a COMPLEX ION the SUM of all the atomic oxidation numbers is equal to the TOTAL CHARGE on the ion. ClO 3 − MnO 4 − PO 3 3-
What is the oxidation number of P in H 3 PO 4 ? What is the oxidation number of Cr in Cr 2 O 7 2- ? What is the oxidation number of C in (NH 4 ) 2 CO 3 ? What is the oxidation number of C in C 3 H 8 (propane)?
Another Way Use the structure – the oxidation number of an atom is equal to # of valence electrons − # of lone pair e - − # of bond e - IF the atom is the most electronegative in the bond C = 4 – 0 – 0 = +4 (oxygen is more electronegative) This method treats atoms like ions – it assumes all the bonded electrons go to the more electronegative atom in the bond
Propane again… Using the structure method, what is the oxidation number of each carbon in propane (C 3 H 8 )?
What is the oxidation number of the oxygen-bonded C in CH 3 COOH (acetic acid)?
OXIDATION IS THE LOSS OF ELECTRONS REDUCTION IS THE GAIN OF ELECTRONS (Oxidation Number is reduced) Oil Rig O I L R I G Oxidation and Reduction Reactions LEO the LION goes GER L E O G E R
2Al + Fe 2 O 3 2Fe + Al 2 O 3 THERMITE REACTION
Identifying REDOX S 8 + 12O 2 8SO 3 Ionic or covalent? O.N.? REDOX? Ox. Agent; Red. Agent? 2AgNO 3 + Na 2 S Ag 2 S + 2NaNO 3 REDOX? 5As 4 O 6 + 8MnO 4 - + 18H 2 O 20 AsO 4 3- + 8 Mn 2+ + 36H + REDOX?
S 8 + 12O 2 8SO 3 Is it redox? If so, What is oxidized? What is the reducing agent?
2 AgNO 3 + Na 2 S Ag 2 S + 2 NaNO 3 Is it redox? 5 As 4 O 6 + 8 MnO 4 - + 18 H 2 O 20 AsO 4 3- + 8 Mn 2+ + 36 H + What is oxidized and reduced? What is the oxidizing agent (OA) and reducing agent (RA)?
. Uncontrolled spontaneous reaction How can this be harnessed to do work?
. Controlled redox reaction Zn Zn 2+ + 2e − Cu 2+ + 2e − Cu Zn + Cu 2+ Cu + Zn 2+ + overall reaction but electrons are forced outside of the cells
Electrochemical Cells Devices to harness a chemical reaction which produces an electric current Daniel's Cell Zn is oxidized and is the reducing agent Zn(s) Zn 2+ (aq) + 2e- ANODE (oxidation) Cu 2+ is reduced and is the oxidizing agent Cu 2+ (aq) + 2e- Cu(s) CATHODE (reduction) Zinc anode Copper cathode Cu +2 Cu +2 Cu +2 Cu +2 1.10 v Zn +2 Zn +2 Zn +2 Zn +2
Simple Electrochemical Cell Also called voltaic or galvanic cells
Electrochemical Cell Explain what happens in this system. Follow the circulation of one negative charge in the cell.
CELL POTENTIAL, E <ul><li>Electrons are “driven” from anode to cathode by an electromotive force or emf . </li></ul><ul><li>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 . </li></ul>Zn and Zn 2+ , anode Cu and Cu 2+ , cathode 1.10 V 1.0 M 1.0 M
CELL POTENTIAL, E <ul><li>For Zn/Cu cell, potential is +1.10 V at 25 ˚C and when [Zn 2+ ] and [Cu 2+ ] = 1.0 M. </li></ul><ul><li>is the STANDARD CELL POTENTIAL, E o </li></ul><ul><li>a quantitative measure of the tendency of reactants to proceed to products when all are in their standard states at 25 ˚C. </li></ul>
Calculating Cell Voltage <ul><li>Balanced half-reactions can be added together to get overall, balanced equation. </li></ul>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.
CELL POTENTIALS, E o <ul><li>We can not measure 1/2 reaction E o directly. Therefore, measure it relative to a STANDARD HYDROGEN CELL. </li></ul>2 H + (aq, 1 M) + 2e- H 2 (g, 1 atm) E o = 0.00 V
Overall reaction is reduction of H + by Zn metal Zn(s) + 2H + (aq) Zn 2+ + H 2 (g) E o =+0.76 V E o for Zn Zn 2+ (aq) + 2e - is +0.76 V E o for Zn 2+ + 2e - Zn is –0.76 V Zn is a better reducing agent than H 2 .
Cu/Cu 2+ and H 2 /H + Cell <ul><li>E o = +0.34 V </li></ul>Acceptor of electrons Supplier of electrons Cu 2+ + 2e- Cu Reduction Cathode H 2 2 H + + 2e- Oxidation Anode Positive Negative What is happening here?
TABLE OF STANDARD REDUCTION POTENTIALS 2 E o (V) Cu 2+ + 2e- Cu +0.34 2 H + + 2e- H 0.00 Zn 2+ + 2e- Zn -0.76 oxidizing ability of ion reducing ability of element
Standard Redox Potentials, E o <ul><li>Any substance on the right will reduce any substance higher than it on the left. </li></ul><ul><li>Zn can reduce H + and Cu 2+ . </li></ul><ul><li>H 2 can reduce Cu 2+ but not Zn 2+ </li></ul><ul><li>Cu cannot reduce H + or Zn 2+ . </li></ul>
Predict the outcome of the following reaction. Classify each of the reactions as processes as product-favored or reactant-favored. Fe(s) + Sn 2+ (aq) O 2 (g) + Au(s) Na(s) + H 2 O(l) Al 3+ (aq) + Hg(l)
Summary Activity A lead acid battery uses the following two reactions to provide a voltage: Pb + SO 4 2- PbSO 4 + 2e - +0.36V PbSO 4 + 2H 2 O PbO 2 + 4H + + SO 4 2- + 2e - -1.70V What is the overall reaction? What is the overall standard cell potential? Which reaction is the oxidation and which reaction is the reduction?