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SAHAR ADHAM LECTUERE OF PHYSICAL CHEMISTRY. Galvanic cell. Comparison of Electrochemical Cells. galvanic. electrolytic. need power source. produces electrical current. two electrodes. anode (-) cathode (+). anode (+) cathode (-).
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SAHAR ADHAM LECTUERE OF PHYSICAL CHEMISTRY Galvanic cell
Comparison of Electrochemical Cells galvanic electrolytic need power source produces electrical current two electrodes anode (-) cathode (+) anode (+) cathode (-) conductive medium salt bridge vessel E°cell < 0. E°cell > 0.
Electron Transfer Reactions • Electron transfer reactions are oxidation-reduction or redox reactions. • Results in the generation of an electric current (electricity) or be caused by imposing an electric current. • Therefore, this field of chemistry is often called ELECTROCHEMISTRY.
You can’t have one… without the other! • oxidation: loss of electrons • reduction: gain of electrons LEO the lion says GER! GER!
WHY STUDY ELECTROCHEMISTRY? • Batteries • Corrosion • Industrial production of chemicalssuch as Cl2, NaOH, F2 and Al • Biological redox reactions The heme group
Cu Zn Galvanic Cells • One ½ cell rxn. occurs in each compartment. • Zn Zn2+ + 2e– in the anode. • Cu2+ + 2e– Cu in cathode. • But not without a connection. Cathode=Reduction Anode=Oxidation SO42– SO42– Zn2+ Cu2+ Zn + Cu2+ Zn2+ + Cu
2e– 2e– Ion (“salt”) Bridge • But even with a connection of the electrodes, no current flows. • We need to allow neutrality in the solutions with a salt bridge to shift counterions. Cu Zn SO42– SO42– Zn2+ Cu2+ Zn + Cu2+ Zn2+ + Cu
Cell Potential • Cell Potential or Electromotive Force (emf): The “pull” or driving force on the electrons.
The voltage generated by the Zn/Cu galvanic cell is +1.1V under standard conditions. • Standard conditions are: • T = 25°C and P = 1 bar for gases. • Solids and liquids are pure. • Solutions are 1 M in all species. • E°cell is sum of ½ cell E° values. Standard Reduction Potentials, E°
Cell potentials and Reduction potentials E°cell = E°reduced - E°oxidized E°cell = E°cathode - E°anode
All ½ cells are catalogued as reduction reactions & assigned reduction potentials, E°. • The lower reduction potential ½ rxn is reversed to become the oxidation. E°oxidation = –E°reduction • That makes spontaneous E°cell > 0. • But E°red can’t be found w/o E°ox! ½ cell Reduction Potentials
We need a standard electrode to make measurements against! • The Standard Hydrogen Electrode (SHE) • 2H+(aq) + 2e– H2(1 bar) E° 0 V • 1 bar H2 flows over a Pt electrode, and the full E°cell is assigned to the other electrode. E°SHE = 0 V. • E.g., standard calomel electrode: • Hg2Cl2(s) + 2e– 2 Hg(l) + Cl–E°SCE = +0.27V • a more physically convenient reference. Origin for Reduction Potentials
Shorthand for a complete redox cell is of the form: • Anode | anodic soln. || cathodic soln. | Cathode • So making a cell of Cu corrosion, • Cu | Cu2+ || NO3–, NO(g), H+ |Pt • where all ions should be suffixed (aq) and both metals should have (s). Galvanic Line Notation
Primary Battery: can not be recharged e.g. Mercury Battery Secondary Battery: rechargeable (storage batteries) e.g. Ni-Cad Battery Fuel Cell: reactants supplied from an external source e.g. H2/O2 fuel cells. Types ofgalvanic cells
Mercury Battery Anode: Zn is reducing agent under basic conditions Cathode: HgO + H2O + 2e- ---> Hg + 2 OH- can not be recharged
Ni-Cad Battery Anode (-) Cd + 2 OH- ---> Cd(OH)2 + 2e- Cathode (+) NiO(OH) + H2O + e- ---> Ni(OH)2 + OH- rechargeable
Why rechargeable?. • It isbecause the products of the reaction are solids that the Ni-Cd battery can be recharged • The solid hydroxides are sticky, and remain in place. • If current is applied, the reaction can be driven backwards!
HOW TO CHARGE? • 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.
Why not rechargeable ? But in mercury battery the ZnO is not sticky, and doesn’t remain attached to the electrode. This battery is not rechargeable
H2 as a Fuel Cars can use electricity generated by H2/O2 fuel cells. H2 carried in tanks or generated from hydrocarbons
17-44 Galvanic cells for which the reactants are continuously supplied.anode: 2H2 + 4OH 4H2O + 4ecathode :4e + O2 + 2H2O 4OH2H2(g) + O2(g) 2H2O(l) Fuel Cells
Mercury batteries take advantage of the high density of Hg to be quite small: used in watches, hearing aids, calculators, etc. Lithium-iodine batteries are particularly small and lightweight, but also very long-lived Often used in pacemakers, where they can last for 10 years
The End For more lessons, visit www.chalkbored.com
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Question 1 For a galvanic cell, the electrode at which reduction occurs is called the: A: Anode B: Cathode final Dr. Keck poll 5/50
ANSWER For a galvanic cell, the electrode at which reduction occurs is called the: B: Cathode
final Question 2 Dr. Keck For a galvanic cell, the electrode with negative polarity is called the: A: Anode B: Cathode poll 50/50
ANSWER For a galvanic cell, the electrode with negative polarity is called the: A: Anode
Which of the following statements is incorrect • In a galvanic cell, reduction occurs at the anode. • The cathode is labeled "+" in a voltaic cell. • Oxidation occurs at the anode in a voltaic cell. • Electrons flow from the anode to the cathode in all electrochemical cells. Question3
Consider the following notation for an electrochemical cell Zn|Zn2+ (1M)||Fe3+ (1M), Fe2+ (1M)|Pt What is the balanced equation for the cell reaction? • Zn(s) + 2Fe3+(aq) → 2Fe2+(aq) + Zn2+(aq) • Zn2+(aq) + 2Fe2+(aq) → Zn(s) + 2Fe3+(aq) • Zn(s) + 2Fe2+(aq) → 2Fe3+(aq) + Zn2+(aq) • Zn(s) + Fe3+(aq) → Fe2+(aq) + Zn2+(aq) • Zn(s) + Fe2+(aq) → Fe(s) + Zn2+(aq) Question 4
50/50 Dr. Keck poll final Question 5 What is the oxidation state of nitrogen in HNO3? A: +3 B: +4 C: +5 D: -5
ANSWER What is the oxidation state of nitrogen in HNO3? C: +5
Question 6 Consider the following electrode potentials: Mg2+ + 2e– Mg E° = –2.37 V V2+ + 2e– V E° = –1.18 V Cu2+ + e– Cu+ E° = 0.15 V Which one of the reactions below will proceed spontaneously from left to right? a. Mg2+ + V V2+ + Mg b. Mg2+ + 2Cu+ 2Cu2+ + Mg c. V2+ + 2Cu+ V +2+ Cu2+ d. V + 2Cu2+ V2+ + 2Cu+ e. none of these
d. V + 2Cu2+ V2+ + 2Cu+ ANSWER
Question 7 What is the oxidative state of iodine in IO3-? A: +7 B: +6 C: +5 D: +4 50/50 Dr. Keck poll final
ANSWER What is the oxidative state of iodine in IO3-? C: +5