Oxidation and Reduction. Reactions that involve electron transfer Batteries and chemistry

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1 Oxidation and Reduction Reactions that involve electron transfer Batteries and chemistry

2 Learning objectives Define oxidation and reduction process Identify oxidation/reduction and oxidizing/reducing agents Calculate oxidation numbers and use them to identify oxidation and reduction processes Describe basic principles of electrochemical and electrolytic cells Decribe features of common battery types Define units of electricity: volts and amps Use activity series to predict progress of reactions

3 What s in a battery A battery does work with electrical energy Chemical reactions provide the energy A circuit is required to conduct electricity

4 Energy and electricity All chemical reactions involve energy change Spontaneous reactions where energy is given out can provide the energy in form of electricity Volta made the first battery (Voltaic cell) All batteries involve electron transfer Electron transfer involves oxidation/reduction

5 What is oxidation? Associated with addition of oxygen atoms Corrosion of metal: Fe Fe 2 O 3 Burning of fuel: C CO 2 Loss of hydrogen atoms (in organic compounds Loss of electrons (Fe Fe e)

6 What is reduction? Associated with loss of oxygen atoms Preparation of metal: Fe 2 O 3 Fe Reaction with hydrogen Fe 2 O 3 + H 2 Fe + H 2 O Addition of hydrogen atoms (in organic compounds Gain of electrons (Fe e Fe)

7 Oxidation-Reduction (Redox) Oxidation is loss of electrons Na Na + + e - Reduction is gain of electrons Cl + e - = Cl - Formation of NaCl from elements is redox

8 Single displacement is redox Zn atoms Zn 2+ ions Cu 2+ ions Cu atoms (oxidized) (reduced)

9 Which is reduction? A. Mg = Mg e B. 2Cl - = Cl 2 + 2e C. Cl 2 + 2e = 2Cl - D. Fe 2+ = Fe 3+ + e

10 Agents of redox Oxidizing agent: causes oxidation of another substance Cu 2+ ions oxidize the Zn atoms Reducing agent: causes reduction of another substance Zn atoms reduce the Cu 2+ ions

11 Nuggets of redox processes Where there is oxidation there is always reduction Oxidizing agent Is itself reduced Gains electrons Causes oxidation Reducing agent Is itself oxidized Loses electrons Causes reduction

12 Identifying oxidation and reduction Follow the electrons Only reactants (things on left) need to be considered Metal elements: Generally form positive ions (lose electrons) Are reducing agents Are oxidized Nonmetal elements: Form negative ions (gain electrons) Are oxidizing agents Are reduced

13 Identifying oxidation and reduction

14 Identifying oxidation and reduction With elements forming ionic compounds identifying oxidation and reduction is usually straightforward Follow path of electrons from reactant to product Metals are oxidized, nonmetals are reduced What about covalent molecules and reactions involving only compounds? System of oxidation numbers is used Oxidation numbers keep track of electrons

15 A numbers game

16 Review of ionic charges Oxidation numbers are often, but not always the same as the ionic charge For groups 1 8A Cation charge = group number Anion charge = - (8 group number)

17 Calculating oxidation numbers

18 Using oxidation numbers in equations

19 Daniell cell An electrolytic cell that uses reduction of Cu 2+ by Zn to produce a voltage In right beaker is Cu and CuSO 4 In left beaker is Zn and ZnSO 4 External circuit for electrons Adding salt bridge completes internal circuit for ions and reaction occurs Zn + Cu 2+ = Zn 2+ + Cu

20 Galvanic cell: long distance Each metal in touch with a solution of its own ions External circuit carries electrons transferred during the redox process A salt bridge containing neutral ions completes the internal circuit. With no current flowing, a potential develops the potential for work Unlike reaction in beaker, energy released by reaction in cell performs useful work like lighting a bulb chemistry

21 Odes to a galvanic cell Cathode Where reduction occurs Where electrons are consumed Where positive ions migrate to Has positive sign Anode Where oxidation occurs Where electrons are generated Where negative ions migrate to Has negative sign

22 Cell notation Anode on left, cathode on right Electrons flow from left to right Oxidation on left, reduction on right Single vertical = electrode/electrolyte boundary Double vertical = salt bridge Anode: Zn Zn e Cathode: Cu e Cu

23 Diagramming a cell

24 Volts and amps Volt is the measure of potential the driving force to move electrons. Voltage depends on the type of chemical process and not on the size of the battery Other forms of potential: Pressure moves air or liquids Temperature moves heat Chemical potential moves reactions Amp is the flow of current. The size of the current flowing can be increased by making the electrodes larger (more reaction per second)

25 Predicting the behavior of electrochemical cells Voltaic cell involves spontaneous reactions An element higher in the activity series will reduce ions of metal lower Zn is higher than Cu Zn reduces Cu 2+ Cu is higher than Ag Cu reduces Ag + Ag will not reduce Zn 2+

26 Predicting chemical reactions

27 Electrolysis driving against the Voltaic cell Spontaneous process: electrons flow from left to right (cell performs work) Electrolysis Nonspontaneous process: apply voltage to electrodes: electrons flow from right to left (perform work on cell) Chemical reaction reversed restores potential energy to the bonds stream

28 Electrolysis of NaCl Formation of NaCl from Na and Cl 2 is spontaneous Electrolysis of molten NaCl to produce Na and Cl 2 requires work: Anode: 2Cl - Cl 2 + 2e Cathode: Na + + e = Na

29 A rusty nail: corrosion and electrochemistry The rusting of a nail is an electrochemical process Anode: Fe is oxidized to Fe 2+ Cathode: O 2 is reduced to H 2 O Why do things rust quicker in salt water?

30 Lithium batteries Lithium has a very large negative reduction potential Li = Li + + e.e = 3.04 V The basis for light-weight, high energy density batteries Low atomic mass of lithium High reduction potential Ability to make rechargeable batteries

31 Lead-acid batteries a unique system Lead battery technology is 100 years old Provides high current Rechargeable Inexpensive Rugged Oxidation: Pb + H 2 SO 4 = PbSO 4 + 2H + + 2e Reduction: PbO 2 + H 2 SO 4 + 2H + + 2e = PbSO 4 + 2H 2 O Discharge Overall: Pb + PbO 2 + 2H 2 SO 4 = 2PbSO 4 + 2H 2 O Recharge

32 Dry cell batteries Acid dry cell Zn anode Zn Zn e MnO 2 cathode 2MnO 2 + 2NH e Mn 2 O 3 + 2NH 3 + H 2 O Alkali cell Zn anode Zn + 2OH - ZnO + H 2 O + 2e MnO 2 cathode 2MnO 2 + H 2 O + 2e Mn 2 O 3 + 2OH -

33 Fuel cells and the hydrogen A battery with the electrolyte supplied from without Cathode: 2H 2 4H + + 4e Anode: O 2 + 4H + +4e 2H 2 O Overall 2H 2 + O 2 2H 2 O economy

34 Redox in life Corrosion Combustion Bleaching Fe + O 2 Fe 2 O 3 CH 4 + O 2 CO 2 + H 2 O Cl + e - = Cl -

35 Biological systems Respiration Cytochrome c (Fe 3+ ) + e - = cytochrome c (Fe 2+ ) Followed by: O 2 + 4e - + 4H + 2H 2 O Metabolism Ethanol acetaldehyde acetic acid CO 2 + H 2 O

36 Vitamin C and oxidation Vitamins are organic compounds important for maintaining health Vitamin C is also easily oxidized (it is a reducing agent) Body produces free radicals which oxidize aging, cancer, cardiovascular disease Antioxidants (like vitamin C) defend against radicals Question: should we take antioxidant supplements?