Galvanic cell
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Transcript of Galvanic cell
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GALVANIC CELLSAHAR ADHAM
LECTUERE OF PHYSICAL CHEMISTRY
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galvanic electrolytic
needpowersourcetwo
electrodes
produces electrical current
anode (-)cathode (+)
anode (+)cathode (-)
salt bridge vessel
conductive medium
Comparison of Electrochemical Cells
E°cell > 0.E°cell < 0.
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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.
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YOU CAN’T HAVE ONE… WITHOUT THE OTHER!
oxidation: loss of electronsreduction: gain of electrons
LEO the lion says GER!
GER!
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WHY STUDY ELECTROCHEMISTRY? Batteries Corrosion Industrial production
of chemicals such as Cl2, NaOH, F2 and Al
Biological redox reactions
The heme group
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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.
Zn
Zn2+ Cu2+
Zn + Cu2+ Zn2+ + Cu
SO42– SO4
2–
CuAnode=Oxidation
Cathode=Reduction
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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.
Zn
Zn2+ Cu2+
Zn + Cu2+ Zn2+ + Cu
SO42– SO4
2–
Cu
2e– 2e–
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Cell Potential
•Cell Potential or Electromotive Force (emf): The “pull” or driving force on the electrons.
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STANDARD REDUCTION POTENTIALS, E°
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.
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CELL POTENTIALS AND REDUCTION POTENTIALS
E°cell = E°reduced - E°oxidized
E°cell = E°cathode - E°anode
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½ CELL REDUCTION POTENTIALS 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!
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ORIGIN FOR 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.
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GALVANIC LINE NOTATION
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).
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TYPES OF GALVANIC CELLSPrimary 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.
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MERCURY BATTERYAnode:
Zn is reducing agent under basic conditions
Cathode:HgO + H2O + 2e- ---> Hg + 2 OH-
can not be recharged
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NI-CAD BATTERYAnode (-)Cd + 2 OH- ---> Cd(OH)2 + 2e-Cathode (+) NiO(OH) + H2O + e- ---> Ni(OH)2 + OH-
rechargeable
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WHY RECHARGEABLE?.It is because 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!
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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.
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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
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H2 AS A FUEL
Cars can use electricity generated by H2/O2 fuel cells.H2 carried in tanks or generated from hydrocarbons
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Galvanic cells for which the reactants are continuously supplied.anode: 2H2 + 4OH 4H2O + 4e
cathode :4e + O2 + 2H2O 4OH
2H2(g) + O2(g) 2H2O(l)
Fuel Cells
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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
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THE END
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V1.06
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QUESTION 1For a galvanic cell, the electrode at which reduction occurs is called the:
A: Anode B: Cathode
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ANSWERFor a galvanic cell, the electrode at which reduction occurs is called the:
B: Cathode
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QUESTION 2For a galvanic cell, the electrode with negative polarity is called the:
A: Anode B: Cathode
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ANSWERFor a galvanic cell, the electrode with negative polarity is called the:
A: Anode
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QUESTION 3Which of the following statements is incorrect
a. In a galvanic cell, reduction occurs at the anode.
b. The cathode is labeled "+" in a voltaic cell.
c. Oxidation occurs at the anode in a voltaic cell.
d. Electrons flow from the anode to the cathode in all electrochemical cells.
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ANSWERa. In a galvanic cell, reduction occurs at the
anode.
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QUESTION 4Consider the following notation for an electrochemical cell
Zn|Zn2+ (1M)||Fe3+ (1M), Fe2+ (1M)|Pt
What is the balanced equation for the cell reaction?
a.Zn(s) + 2Fe3+(aq) → 2Fe2+(aq) + Zn2+(aq)b.Zn2+(aq) + 2Fe2+(aq) → Zn(s) + 2Fe3+(aq)c.Zn(s) + 2Fe2+(aq) → 2Fe3+(aq) + Zn2+(aq)d.Zn(s) + Fe3+(aq) → Fe2+(aq) + Zn2+(aq)e.Zn(s) + Fe2+(aq) → Fe(s) + Zn2+(aq)
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ANSWER
Zn(s) + 2Fe3+(aq) → 2Fe2+(aq) + Zn2+(aq)
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QUESTION 5 What is the oxidation state of nitrogen in HNO3?
A: +3 B: +4
C: +5 D: -5
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ANSWERWhat is the oxidation state of nitrogen in HNO3?
C: +5
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QUESTION 6Consider the following electrode potentials:Mg2+ + 2e– Mg E° = –2.37 VV2+ + 2e– V E° = –1.18 VCu2+ + e– Cu+ E° = 0.15 VWhich one of the reactions below will proceed
spontaneously from left to right?
a. Mg2+ + V V2+ + Mgb. Mg2+ + 2Cu+ 2Cu2+ + Mgc. V2+ + 2Cu+ V +2 + Cu2+
d. V + 2Cu2+ V2+ + 2Cu+
e. none of these
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ANSWER
d. V + 2Cu2+ V2+ + 2Cu+
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QUESTION 7What is the oxidative state of iodine in IO3
-?
A: +7 B: +6
C: +5 D: +4
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ANSWERWhat is the oxidative state of iodine in IO3
-?
C: +5