APPLICATIONS OF ELECTROCHEMISTRY

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Michael Howard
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1 APPLICATIONS OF ELECTROCHEMISTRY

2 SPONTANEOUS REDOX REACTIONS APPLICATIONS OF ELECTROCHEMICAL CELLS

3 BATTERIES A galvanic cell, or series of combined galvanic cells, that can be used as a source of direct electric current at a constant voltage. Advantage: portable, completely selfcontained*; no auxiliary components such as salt bridge * possess all features & facilities required to function independently

4 BATTERIES Greater voltage can be achieved by using multiple voltaic cells in a single battery. Ex. 12-V automobile battery Higher emfs can also be achieved by using multiple batteries in series Battery produces a voltage that is the sum of the emfs of the individual cell

5 BATTERIES The emf of a battery is determined by the substance that are oxidized at the anode & reduced at the cathode & the usable life of the battery depends on the the quantities of these substances packaged in the battery.

6 BATTERIES Different applications require batteries with different properties Ex: battery required to start a car must be capable of delivering a large electrical current for a short period of time Battery that powers a heart pacemaker must be very small & capable of delivering a small but steady current over an extended time period.

7 CHARACTERISTICS OF COMMON COMMERCIAL BATTERY 1) Materials used to construct must be stable under the conditions in which it is to be used 2) Minimize health & environmental concerns upon use & disposal

8 TYPES OF BATTERIES 1) Primary cells Cant be recharged; must be recycled or discarded after its emf drops to zero 2) Secondary cells Can be recharged from an external power source after its emf has dropped 3) Fuels cell/flow batteries Voltaic cells that uses conventional fuels such as H 2 & CH 4

9 PRACTICAL CELLS/BATTERIES ELECTROCHEMISTRY

10 PRIMARY CELLS Cannot be recharged; must be recycled or discarded after its emf drops to zero Example: Dry cell / Leclanché Alkaline cell

11 DRY CELL (LECLANCHÉ / FLASHLIGHT BATTERY) It consist of a zinc container which acts as the ANODE and a carbon CATHODE. The electrolyte is a moist paste of NH 4 Cl, ZnCl 2, and MnO 2. Dry cell because no free liquid (moist paste) 2

12 DRY CELL Drawbacks: When current is drawn rapidly from the cell, products such as ammonia (NH 3 ) build up on the electrodes causing voltage to drop. Also, because the electrolyte medium is acidic, Zn metal slowly dissolves.

13 DRY CELL (ALKALINE)

The mercury battery employs HgO as oxidizing agent, the silver uses Ag 2

14 MERCURY & SILVER (BUTTON) BATTERIES Both use a zinc container as ANODE (reducing agent) in the basic medium. The mercury battery employs HgO as oxidizing agent, the silver uses Ag 2 O Both use a steel can around the cathode. The solid reactants are compacted with KOH & separated with moist paper

15 SECONDARY CELLS Can be recharged when it runs down by supplying electrical energy to reverse the cell reaction and re-form the reactant. Example: LEAD STORAGE BATTERY Common car battery * Demand for lightweight and readily recharged batteries.

16 NICKEL CADMIUM (NICAD) BATTERY It is an expensive type of battery. Being small and light, it is a metal containing NiO 2. The electrolyte is KOH.

17 NICKEL-CADMIUM

18 NICKEL-CADMIUM Drawbacks: Cadmium is a toxic heavy metal. Its use increases the weight of batteries & produces an environmental hazard. Eventually recycled as they lose their ability to recharge.

19 NICKEL METAL HYDRIDE (NIMH)

20 NICKEL METAL HYDRIDE

21 LITHIUM (LI-ION) BATTERIES

22 LITHIUM BATTERIES

23 LEAD STORAGE BATTERY Is the main source of current for starting lighting and ignition systems in motor vehicles. The ANODE consists of porous lead and the CATHODE consists of groups of plates of lead covered with lead dioxide (PbO 2 ). The electrolyte between the electrolysis is H 2 SO 4.

24 LEAD STORAGE BATTERY

25 FUEL CELLS

26 H 2 -O 2 FUEL CELL Cathode: O 2(g) + 2H 2 O (l) + 4e - 4 OH - (aq) Anode: 2 H 2(g) + 4 OH - (aq) 4 H 2(l) + 4e - Net REDOX: 2 H 2(g) + O 2(g) 2 H 2 O (l) E =1.23 V Electrolytes: semipermeable membranes Basis for pollution-free fuel cell powered vehicles

27 PROTON EXCHANGE MEMBRANE (PEM)

28 FUEL CELL STACK & CAR

29 OTHER FUEL CELLS

CORROSION Corrosion is a familiar phenomenon that occurs in the environment. For sure, you have seen iron nails and jewelry tarnishing, and copper, bronze and brass turning green.

30 CORROSION Corrosion is a familiar phenomenon that occurs in the environment. For sure, you have seen iron nails and jewelry tarnishing, and copper, bronze and brass turning green. Corrosion is a serious problem. It damages buildings and monuments. A considerable amount of money is spent on the repair and reconstruction of these structures on the implementation of measures to prevent corrosion.

31 CORROSION Undesirable redox reactions of metals The deterioration of metals by an electrochemical process. Spontaneous redox reaction in which metal is attacked by some substances in its environment & converted to an unwanted compound. Example: Rust on iron Tarnish on silver Green patina formed on copper and brass

32 CORROSION Viewed as the process of returning metals to their natural state the ores from which they were originally obtained. Corroded metals often loses its structural integrity & attractiveness. Nearly all metals, oxidation is a thermodynamically favorable process in air at room temperature. When oxidation is not inhibited in some way, it can be very destructive. However, oxidation can result in the formation of an insulating protective oxide layer that prevents further reaction of the underlying metal.

33 RUSTING OF IRON Most familiar corrosion process; requires both O 2 & H 2 O Other factors such as ph of solution, presence of salt, contact with metals more difficult to oxidize than iron & stress on the iron can accelerate rusting Electrochemical in nature because SRP of reduction of Fe2+ is less positive (+) than reduction of O 2. Thus Fe can be oxidized by O 2 (g). Cathode: O 2(g) + 4 H + (aq) + 4e - 2 H 2 O (l) E =1.23 V Anode: Fe (s) Fe 2+ (aq) + 2e - E = V

34 RUSTING A region on the surface of iron metal can serve as an anode at which oxidation of Fe to Fe 2+ occurs. The electrons produce migrate through the metal to another portion of the surface that serves as the cathode at which O 2 is reduced.

Iron in contact with a solution whose ph is above 9 does not corrode. Corrosion is enhanced by the presence of salt.

35 RUSTING Because cathode is generally the area having the largest supply of O 2, rust (hydrated iron (III) oxide) often deposits there. Reduction of O 2 requires H + so as concentration of H + is lowered (ph inc) reduction of O 2 becomes less favorable. Iron in contact with a solution whose ph is above 9 does not corrode. Corrosion is enhanced by the presence of salt. Evident in areas where heavy salt are present like seashores. The effects of salts is readily explained by the voltaic mechanism: the ions of a salt provide the electrolyte necessary for completion of the electrical circuit.

36 PREVENTION/PROTECTION Painting Covering with a coat of paint to protect its surface against corrosion Electroplating Covering with a coat of metal such as Sb & Zn to protect its surface against corrosion Effective compared to painting Example: Galvanized iron Allboying is the processing of mixing 2 or more kinds of metals together to get better characteristics than sole metal. Example: bronze (Cu & Sb); brass (Cu & Zn) Cathodic protection Making the corroding metal as the cathode to prevent from oxidation

37 PAINTING A means of preventing O 2 & H 2 O from reaching the iron surface However, if the paint is scratched, pitted or dented to expose even the smallest area of bare metal to O 2 & H 2 O, corrosion will form under the paint layer.

38 ELECTROPLATING/METAL PLATING Uses electrolysis to deposit a thin layer of one metal on another metal to improve beauty or resistance to corrosion. Uses the principle of electrolysis with active electrodes.

39 CATHODIC PROCTECTION Process in which metal that is to be protected from corrosion is made the cathode in the galvanic cell. An iron nail can be protected from rusting by connecting the nail to a piece of zinc. Without such protection, an iron nail quickly rusts in water. The metal that is oxidized while protecting the cathode is called the sacrificial anode

CATHODIC PROCTECTION A method often employed to protect steel in buried fuel tanks and pipelines. An active metal, such as Mg, is connected to a wire to the pipeline or tank to be protected.

40 CATHODIC PROCTECTION A method often employed to protect steel in buried fuel tanks and pipelines. An active metal, such as Mg, is connected to a wire to the pipeline or tank to be protected. Because Mg is a better reducing agent than Fe, electrons are furnished by the Mg rather than by the Fe, keeping the Fe from being oxidized. As oxidation occurs, Mg anode dissolves, and so it must be replaced periodically.

41 CATHODIC PROTECTION

42 CATHODIC PROCTECTION Ships hulls are protected in a similar way by attaching bars of Ti metal to the steel hull. In salt water the Ti acts as the anode and is oxidized instead of steel hull.

43 NONSPONTANEOUS REDOX REACTIONS APPLICATIONS OF ELECTROLYTIC CELLS

44 ELECTROPLATING/METAL PLATING Uses electrolysis to deposit a thin layer of one metal on another metal. Example: electroplating of Ni on a piece of steel Looking at the overall rxn, it appears as if nothing has been accomplished. During the electrolysis, however, Ni atoms from the Ni anode are transferred to the steel cathode, plating the steel electrodes with a thin layer of Ni atoms. Since E cell is zero, only a small emf is needed to provide the push to transfer the Ni atoms from one electrode to another.

45 ELECTROPLATING/METAL PLATING Application of a thin coating of metal that resists corrosion. Ex: tin cans which are actually steel cans with a thin coating of tin An object can be plated by making it the cathode in a tank containing ions of the plating metal.

46 ELECTROPLATING OF SILVERWARE (a) The silverware is being withdrawn from electroplating bath. (b) The polished final product.

47 ELECTROMETALLURGY Processes which uses electrolysis methods for obtaining a pure metal from its ores or for refining (purifying) the metal. Example: Production of aluminum metal Production of copper metal

48 PRODUCTION OF ALUMINUM METAL

49 PRODUCTION OF COPPER METAL

50 CHEMICAL IMPACT

58 END OF ELECTROCHEMISTRY

UNIT-I ELECTROCHEMISTRY PART-A

UNIT-I ELECTROCHEMISTRY PART-A 1. What is electrochemistry? 2. What do you understand by electrode potential? 3. Define E.M.F of an electrochemical cell? 4. Define (a) Single electrode potential (b) Standard