GALVANIC CELLSAHAR ADHAM

LECTUERE OF PHYSICAL CHEMISTRY

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.

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 electronsreduction: gain of electrons

LEO the lion says GER!

GER!

WHY STUDY ELECTROCHEMISTRY? Batteries Corrosion Industrial production

of chemicals such as Cl2, NaOH, F2 and Al

Biological redox reactions

The heme group

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

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–

Cell Potential

•Cell Potential or Electromotive Force (emf): The “pull” or driving force on the electrons.

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.

CELL POTENTIALS AND REDUCTION POTENTIALS

E°cell = E°reduced - E°oxidized

E°cell = E°cathode - E°anode

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!

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.

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).

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.

MERCURY BATTERYAnode:

Zn is reducing agent under basic conditions

Cathode:HgO + H2O + 2e- ---> Hg + 2 OH-

can not be recharged

NI-CAD BATTERYAnode (-)Cd + 2 OH- ---> Cd(OH)2 + 2e-Cathode (+) NiO(OH) + H2O + e- ---> Ni(OH)2 + OH-

rechargeable

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!

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

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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

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

V1.06

QUESTION 1For a galvanic cell, the electrode at which reduction occurs is called the:

A: Anode B: Cathode

ANSWERFor a galvanic cell, the electrode at which reduction occurs is called the:

B: Cathode

QUESTION 2For a galvanic cell, the electrode with negative polarity is called the:

A: Anode B: Cathode

ANSWERFor a galvanic cell, the electrode with negative polarity is called the:

A: Anode

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.

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)

QUESTION 5 What is the oxidation state of nitrogen in HNO3?

A: +3 B: +4

C: +5 D: -5

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

ANSWER

d. V + 2Cu2+ V2+ + 2Cu+

QUESTION 7What is the oxidative state of iodine in IO3

-?

A: +7 B: +6

C: +5 D: +4

ANSWERWhat is the oxidative state of iodine in IO3

-?

C: +5