1 Focus 4: Oxidation-reduction reactions as source of energy SAVE PAPER AND INK!!! When you print out the notes on PowerPoint, print "Handouts" instead of "Slides" in the print setup. Also, turn off the backgrounds (Tools>Options>Print>UNch eck "Background Printing")!

2 Oxidation-reduction Reactions Oxidation-reduction or redox reactions are reactions where electrons are transferred.Oxidation-reduction or redox reactions are reactions where electrons are transferred. This results in the generation of an electric current (electricity)This results in the generation of an electric current (electricity) Therefore, this field of chemistry is often called ELECTROCHEMISTRY.Therefore, this field of chemistry is often called ELECTROCHEMISTRY.

3 Terminology for Redox Reactions OXIDATION—loss of electron(s) by a species; increase in oxidation number; increase in oxygen.OXIDATION—loss of electron(s) by a species; increase in oxidation number; increase in oxygen. REDUCTION—gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in hydrogen.REDUCTION—gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in hydrogen. OXIDIZING AGENT or oxidant—the substance that accepts the electron or the species is reduced.OXIDIZING AGENT or oxidant—the substance that accepts the electron or the species is reduced. REDUCING AGENT or reductant—the substance that donates the electron or the species that is oxidized.REDUCING AGENT or reductant—the substance that donates the electron or the species that is oxidized. OXIDATION—loss of electron(s) by a species; increase in oxidation number; increase in oxygen.OXIDATION—loss of electron(s) by a species; increase in oxidation number; increase in oxygen. REDUCTION—gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in hydrogen.REDUCTION—gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in hydrogen. OXIDIZING AGENT or oxidant—the substance that accepts the electron or the species is reduced.OXIDIZING AGENT or oxidant—the substance that accepts the electron or the species is reduced. REDUCING AGENT or reductant—the substance that donates the electron or the species that is oxidized.REDUCING AGENT or reductant—the substance that donates the electron or the species that is oxidized.

4 Activity Series of Metals Recall from Prelim Chem the activity series of metals where metals can be arranged from the most active to the least active

5 OXIDATION-REDUCTION REACTIONS in metal displacement reactions What happens when a copper wire is placed in a is placed in a solution of silver nitrate? Observations: What is the explanation? Equation: Cu(s) + 2 Ag + (aq) ---> Cu 2+ (aq) + 2 Ag(s)

6 What is happening in this redox? Which is the oxidation reaction? Which is the Which is the reduction reaction? reduction reaction?

7 Worksheet on Redox See sheet: “HSC Chemistry- Revision sheet on oxidation and reduction” Textbook, pg 45 Exercise Q1 - 8 Worksheet 10 – Oxidation and reduction

8 You can’t have one… without the other! Reduction (gaining electrons) can’t happen without an oxidation to provide the electrons. You can’t have 2 oxidations or 2 reductions in the same equation. Reduction has to occur at the cost of oxidation LEO the lion says GER! GER!

9 Another way to remember OIL RIG

10 Where do other redox reactions occur? BatteriesBatteries CorrosionCorrosion Industrial production of chemicals such as Cl 2, NaOH, F 2 and AlIndustrial production of chemicals such as Cl 2, NaOH, F 2 and Al Biological redox reactionsBiological redox reactions The heme group

11 Electrochemical Cells A device that uses a chemical reaction (redox) to create an electric current.A device that uses a chemical reaction (redox) to create an electric current. Product favored reaction ---> voltaic or galvanic cell ----> electric currentProduct favored reaction ---> voltaic or galvanic cell ----> electric current Reactant favored reaction ---> electrolytic cellReactant favored reaction ---> electrolytic cell (It uses an electric current is used to create a chemical change) These are voltaic cells A battery is actually several cells joined together in a common outer casing. Eg a car battery.

12 AnodeCathode Basic Concepts of Electrochemical Cells

13 What are the components of a galvanic cell? Electrode – the metallic conducting plates of a galvanic cell Anode – This is negative in a galvanic cell. It is the metallic plate where ox….. Cathode – This is positive. It is the metallic plate where red…… Electrolyte – a substance that releases ions when in solution (or when melted it carries a current – meaning in a electrolytic cell) Half-cell – refers to either the oxidation or reduction half of an electrochemical cell Salt bridge-a bridge containing an electrolyte that joins the half-cells so as to allow a movement of ions to maintain a balance of charges

14 What happens in a galvanic cell or voltaic cell?.

15 Electrons travel thru external wire and the voltmeter moves from left to right. Electrons travel thru external wire and the voltmeter moves from left to right. Salt bridge allows anions and cations to move between electrode compartments so as to ensure the whole solution is neutral.Salt bridge allows anions and cations to move between electrode compartments so as to ensure the whole solution is neutral. Electrons travel thru external wire and the voltmeter moves from left to right. Electrons travel thru external wire and the voltmeter moves from left to right. Salt bridge allows anions and cations to move between electrode compartments so as to ensure the whole solution is neutral.Salt bridge allows anions and cations to move between electrode compartments so as to ensure the whole solution is neutral. <-- Anions Cations-- > Zn --> Zn e- Cu e- --> Cu Oxidation Anode Negative Oxidation Anode Negative Reduction Cathode Positive Reduction Cathode Positive RED CAT

16 Terms Used for Voltaic Cells

17 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.

18 TABLE OF STANDARD REDUCTION POTENTIALS 2 E o (V) Cu e- Cu H + + 2e- H0.00 Zn e- Zn-0.76 oxidizing ability of ion reducing ability of element To determine an oxidation from a reduction table, just take the opposite sign of the reduction!

19 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 +

20 This is found on your HSC data sheet. (Also see your wiki)

21 Measuring standard electrode potentials All electrode potentials are measured relative to the standard hydrogen electrode. The standard electrode potential, E° of an electrode is the potential of that electrode in its standard state relative to the standard hydrogen electrode.

22 CELL POTENTIAL, E For Zn/Cu cell, potential is V at 25 ˚C and when [Zn 2+ ] and [Cu 2+ ] = 1.0 M.For Zn/Cu cell, potential is V at 25 ˚C and when [Zn 2+ ] and [Cu 2+ ] = 1.0 M. This is the STANDARD CELL POTENTIAL, E oThis is the STANDARD CELL POTENTIAL, E o —a quantitative measure of the tendency of reactants to proceed to products when all are in their standard states at 25 ˚C and 100 kPa pressure and electrolyte conc of 1.0 mol/L and compared to the standard hydrogen electrode.—a quantitative measure of the tendency of reactants to proceed to products when all are in their standard states at 25 ˚C and 100 kPa pressure and electrolyte conc of 1.0 mol/L and compared to the standard hydrogen electrode.

23 Cd --> Cd e- or Cd e- --> Cd Fe --> Fe e- or Fe e- --> Fe E o for a Voltaic Cell All ingredients are present. Which way does reaction proceed?

24 From the table, you see Fe is a better reducing agent than CdFe is a better reducing agent than Cd Cd 2+ is a better oxidizing agent than Fe 2+Cd 2+ is a better oxidizing agent than Fe 2+ E o for a Voltaic Cell

25 More About Calculating Cell Voltage Assume I - ion can reduce water. 2 H 2 O + 2e- ---> H OH - Cathode 2 I - ---> I 2 + 2e- Anode I H 2 O --> I OH - + H 2 2 H 2 O + 2e- ---> H OH - Cathode 2 I - ---> I 2 + 2e- Anode I H 2 O --> I OH - + H 2 Assuming reaction occurs as written, E˚ = E˚ cat + E˚ an = ( V) - ( V) = V Minus E˚ means rxn. occurs in opposite direction (the connection is backwards or you are recharging the battery)

26 Charging a Battery When you charge a battery, you are forcing the electrons backwards (from the + to the -). To do this, you will need a higher voltage backwards than forwards. This is why the ammeter in your car often goes slightly higher while your battery is charging, and then returns to normal. In your car, the battery charger is called an alternator. If you have a dead battery, it could be the battery needs to be replaced OR the alternator is not charging the battery properly.

27 Dry Cell Battery Anode (-) Zn ---> Zn e- Cathode (+) 2 NH e- ---> 2 NH 3 + H 2

28 Alkaline Battery Nearly same reactions as in common dry cell, but under basic conditions. Anode (-): Zn + 2 OH - ---> ZnO + H 2 O + 2e- Cathode (+): 2 MnO 2 + H 2 O + 2e- ---> Mn 2 O OH -

29 Mercury Battery Anode: Zn is reducing agent under basic conditions Cathode: HgO + H 2 O + 2e- ---> Hg + 2 OH -

30 Lead Storage Battery Anode (-) E o = V Pb + HSO > PbSO 4 + H + + 2e- Cathode (+) E o = V PbO 2 + HSO H + + 2e- ---> PbSO H 2 O

31 Ni-Cad Battery Anode (-) Cd + 2 OH - ---> Cd(OH) 2 + 2e- Cathode (+) NiO(OH) + H 2 O + e- ---> Ni(OH) 2 + OH -

32 H 2 as a Fuel Cars can use electricity generated by H 2 /O 2 fuel cells. H 2 carried in tanks or generated from hydrocarbons

33 Balancing Equations for Redox Reactions Some redox reactions have equations that must be balanced by special techniques. MnO Fe H + ---> Mn Fe H 2 O Mn = +7 Fe = +2 Fe = +3 Mn = +2

34 Balancing Equations Consider the reduction of Ag + ions with copper metal. Cu + Ag + --give--> Cu 2+ + Ag

35 Balancing Equations Step 1:Divide the reaction into half-reactions, one for oxidation and the other for reduction. OxCu ---> Cu 2+ Red Ag + ---> Ag Step 2:Balance each element 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

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

37 Balancing Equations Balance the following in acid solution— VO Zn ---> VO 2+ + Zn 2+ VO Zn ---> VO 2+ + Zn 2+ Step 1:Write the half-reactions OxZn ---> Zn 2+ RedVO > VO 2+ Step 2:Balance each half-reaction for mass. OxZn ---> Zn 2+ Red VO > VO 2+ + H 2 O 2 H + + Add H 2 O on O-deficient side and add H + on other side for H-balance.

38 Balancing Equations Step 3:Balance half-reactions for charge. OxZn ---> Zn e- Rede- + 2 H + + VO > VO 2+ + H 2 O Step 4:Multiply by an appropriate factor. OxZn ---> Zn e- Red 2e- + 4 H VO > 2 VO H 2 O Step 5:Add balanced half-reactions Zn + 4 H VO > Zn VO H 2 O

39 Tips on Balancing Equations Never add O 2, O atoms, or O 2- to balance oxygen.Never add O 2, O atoms, or O 2- to balance oxygen. Never add H 2 or H atoms to balance hydrogen.Never add H 2 or H atoms to balance hydrogen. Be sure to write the correct charges on all the ions.Be sure to write the correct charges on all the ions. Check your work at the end to make sure mass and charge are balanced.Check your work at the end to make sure mass and charge are balanced. PRACTICE!PRACTICE!

40 Exercises Textbook, pg 71 Q