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Chemistry 1011

Chemistry 1011. Introductory Chemistry II http://www.mi.mun.ca/~pfisher/chemistry.html Password for final exams Midgley. Chemistry 1011. TOPIC Electrochemistry TEXT REFERENCE Masterton and Hurley Chapter 18. 18.2 Standard Voltages. YOU ARE EXPECTED TO BE ABLE TO:

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Chemistry 1011

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  1. Chemistry 1011 Introductory Chemistry II http://www.mi.mun.ca/~pfisher/chemistry.html Password for final exams Midgley Chemistry 1011 Slot 5

  2. Chemistry 1011 TOPIC Electrochemistry TEXT REFERENCE Masterton and Hurley Chapter 18 Chemistry 1011 Slot 5

  3. 18.2 Standard Voltages YOU ARE EXPECTED TO BE ABLE TO: • Define the standard electrode potential of a half cell • Order species according to their ease of oxidation or reduction based on a table of standard reduction potentials • Calculate the net cell voltage, Eo, of a combination of half cells from standard electrode potential data • Determine whether a given redox reaction will be spontaneous or non-spontaneous Chemistry 1011 Slot 5

  4. Cell Voltage • The force that pushes the electrons through the external circuit of a cell is known as the • Potential difference, or • Electromotive force (emf), or • Voltage • It is measured in volts • The magnitude of the voltage depends on • The nature of the redox reaction • The concentrations of the ions in solution, (or pressures of any gases) Chemistry 1011 Slot 5

  5. Standard Voltage • In order to compare the voltages of different cells, or to calculate the expected voltage of a given cell, measurements are taken under standard conditions: • Current flow is almost zero • All ions and molecules in solution are at a concentration of 1.0 mol/L • All gases are at a pressure of 1.0 atm Chemistry 1011 Slot 5

  6. The Zinc – Hydrogen Voltaic Cell Chemistry 1011 Slot 5

  7. The Standard Voltage of the Zinc – Hydrogen Voltaic Cell Zn(s) + 2H+(aq) Zn2+(aq) + H2(g) 1.0 mol/L 1.0 mol/L 1.0 atm Zn | Zn2+|| H+| H2 | Pt • Cell voltage with no current flowing is +0.762V • This is the standard voltage for this cell Zn(s) + 2H+(aq, 1.0M) Zn2+(aq, 1.0M) + H2(g, 1.0atm) Eo = +0.762V Chemistry 1011 Slot 5

  8. Standard Half Reaction Voltages • Each half reaction has a standard voltage • Eoox (standard oxidation voltage) • Eored (standard reduction voltage) Eo = Eoox + Eored • Only Eo can be measured - the standard voltage of a half reaction cannot be measured directly Chemistry 1011 Slot 5

  9. Obtaining Values for Standard Half Reaction Voltages • Standard half reaction voltages are determined by arbitrarily assigning the value of zero to the standard reduction half reaction for hydrogen ions to give hydrogen gas 2H+(aq,1.0M) + 2e- H2(g,1.0atm) Eored (H+ H2) = 0.000V Since Eo = Eoox + Eored Eoox (Zn  Zn2+) = +0.762V Chemistry 1011 Slot 5

  10. Obtaining Values for Standard Half Reaction Voltages • Once one half reaction standard voltage is established, others can be deduced: • For: Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s) the standard cell voltage is +1.101V Zn(s) Zn2+(aq,1.0M) + 2e- Eoox (Zn  Zn2+) = +0.762V Cu2+(aq,1.0M) + 2e- Cu(s) Eored (Cu2+Cu) = ??V Since Eo = Eoox + Eored +1.101V = +0.762V + Eored Eored = +0.339V Chemistry 1011 Slot 5

  11. Standard Reduction Potentials • Standard half cell voltages are found in tables of standard potentials • These are the values for reduction half reactions based upon the convention that 2H+(aq,1.0M) + 2e- H2(g,1.0atm) Eored (H+ H2) = 0.000V • Standard reduction potential = Eored Chemistry 1011 Slot 5

  12. Standard Reduction Potentials Oxidizing Agent Reducing Agent Eored (V) Li+(aq) + e- Li(s) -3.040 Na+(aq) + e- Na(s) -2.714 Zn2+(aq) + 2e- Zn(s) -0.762 Ni2+(aq) + 2e- Ni(s) -0.236 2H+(aq) + 2e- H2(g) 0.000 Cu2+(aq) + 2e- Cu(s) +0.339 Ag+(aq) + e- Ag(s) +0.799 NO3-(aq) + 4H+(aq) + 3e- NO(g) + 2H2O+0.964 MnO4-(aq) + 8H+(aq) + 5e- Mn2+(aq) + 4H2O+1.512 F2(g) + 2e- 2F-(aq)+2.889 Chemistry 1011 Slot 5

  13. Standard Reduction Potentials • Elements above hydrogen in the table of standard reduction potentials will react with a solution of hydrogen ions to produce hydrogen gas M(s) + 2H+(aq) M2+(aq) + H2(g) M2+(aq) + 2e- M(s) Eored =negative M(s) M2+(aq) + 2e- Eoox =positive 2H+(aq) + 2e- H2(g) Eored = 0.000V M | M2+|| H+| H2 | Pt Eocell= positive • Elements below hydrogen in the table of standard reduction potentials will NOT react with a solution of hydrogen ions to produce hydrogen gas Chemistry 1011 Slot 5

  14. Standard Voltages for Voltaic Cells • The table of standard reduction potentials gives standard voltages for reduction half reactions • Standard voltages for oxidation half reactions are obtained by reversing these reactions and changing the sign of the Eored value • If: Zn2+(aq) + 2e- Zn(s) Eored=-0.762 • Then: Zn(s) Zn2+(aq) + 2e-Eoox=+0.762 Chemistry 1011 Slot 5

  15. Computing Standard Cell Potential • The standard voltage of a cell is the sum of the standard potentials for the two half reactions • For the cell: Zn | Zn2+|| Cu2+| Cu Zn(s) Zn2+(aq) + 2e-Eoox =+0.762V Cu2+(aq) + 2e- Cu(s) Eored =+0.339V Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s) • Eocell= Eoox + Eored =+ 0.762 + 0.339 = 1.101V Chemistry 1011 Slot 5

  16. Oxidizing Agents • An oxidizing agent is a species that can gain electrons • The strongest oxidizing agents are the species that gain electrons most readily • They have the largest positive Eored values • Oxidizing strength increases moving down the left column of the table of standard reduction potentials • Oxidizing agents in the table of standard reduction potentials can oxidize any species above Chemistry 1011 Slot 5

  17. Reducing Agents • A reducing agent is a species that readily loses electrons • The strongest reducing agents are the species that lose electrons most readily • They have the largest negativeEored values (The largest positiveEoox values) • Reducing strength increases moving up the right column of the table of standard reduction potentials • Reducing agents in the table of standard reduction potentials can reduce any species below Chemistry 1011 Slot 5

  18. Strong Reducing and Oxidizing Agents Reducing agent causes another species to be reduced - it is oxidized Li(s)  Li+(aq) + e-Eoox =+3.040V Oxidizing agent causes another species to be oxidized - it is reduced F2(g) + 2e- 2F-(aq)Eored = +2.889V Table of Standard Reduction Potentials R = strongest reducing agent O = strongest oxidizing agent R O Chemistry 1011 Slot 5

  19. Spontaneity of Redox Reactions • In order for a redox reaction to occur spontaneously, the calculated cell potential MUST BE POSITIVE • Questions: • Will copper metal be oxidized to Cu2+ ions by dilute hydrochloric acid? • Will copper metal be oxidized to Cu2+ ions by dilute nitric acid? Chemistry 1011 Slot 5

  20. Reaction of Copper with Dilute Hydrochloric Acid?? • Possible oxidation half reaction: Cu(s) Cu2+(aq) + 2e-Eoox =-0.339V • Possible reduction half reaction (H+ and Cl- ions are present - Cl- ions cannot be reduced): 2H+(aq) + 2e- H2(g) Eored = 0.000 • Net possible reaction: Cu(s) + 2H+(aq) Cu2+(aq) + H2(g) • Net calculated cell voltage Eocell= Eoox + Eored =- 0.339 + 0.000 = -0.339 V • Reaction will not be spontaneous i.e no reaction Chemistry 1011 Slot 5

  21. Reaction of Copper with Dilute Nitric Acid?? • Possible oxidation half reaction: Cu(s) Cu2+(aq) + 2e-Eoox =-0.339V • Possible reduction half reactions(H+ and NO3- ions are present): 2H+(aq) + 2e- H2(g) Eored = 0.000V NO3-(aq) + 4H+(aq) + 3e- NO(g) + 2H2OEored = +0.964V • Net spontaneous reaction (Add multiples of the two half reactions so that same #electrons (6) in each half): 3Cu(s) + 2NO3-(aq) + 8H+(aq) 3Cu2+(aq) + 2NO(g) + 4H2O • Net calculated cell voltage: Eocell= Eoox + Eored =- 0.339 + 0.964 = +0.629 V • Reaction will be spontaneous i.e reaction takes place Chemistry 1011 Slot 5

  22. Voltaic Cells with Inert Electrodes • Half cells will frequently be constructed with inert electrodes (often carbon or platinum) • The Hydrogen half cell is one example: H+| H2 | Pt • A cell with two inert electrodes might be: Pt | Fe2+(aq) | Fe3+(aq) || Cl -(aq) | Cl2(g) | Pt Chemistry 1011 Slot 5

  23. The Leclanché Cell • The Leclanché cell is the ordinary commercial flashlight battery Zn | Zn2+||MnO2 | Mn2O3 | C • Anode half reaction: Zn(s) Zn2+(aq) + 2e-Eoox =+0.762V • Cathode half reaction (complex): 2MnO2(s) + 2NH4+(aq) + 2e- Mn2O3(s) + 2NH3(aq) + H2O Eored =+0.7 V • Net cell voltage Eocell = 1.5V Chemistry 1011 Slot 5

  24. Chemistry 1011 Slot 5

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