Chapter 18 - Electrochemistry

18.1 Galvanic Cells

• **** An oxidation–reduction (redox) reaction involves a transfer of electrons from the reducing agent to the oxidizing agent
• It involves a loss of electrons and that reduction involves a gain of electrons
• It is useful to break a redox reaction into half-reactions, one involving oxidation and one involving reduction
• A galvanic cell uses a spontaneous redox reaction to produce a current that can be used to do work.
• Oxidation occurs at the anode. Reduction occurs at the cathode
• A galvanic cell consists of an oxidizing agent in one compartment that pulls electrons through a wire from a reducing agent in the other compartment
  • The “pull,” or driving force, on the electrons, is called the cell potential
  • The unit of electrical potential is the volt (abbreviated V), which is defined as 1 joule of work per coulomb of charge transferred.
• How can we measure the cell potential?
  • One possible instrument is a crude voltmeter, which works by drawing current through a known resistance.
  • The key to determining the maximum potential is to do the measurement under conditions of zero current so that no energy is wasted
  • Under such conditions, the cell potential is equal in magnitude and opposite in sign to the voltage setting of the potentiometer.
• Since these instruments are more convenient to use, they have replaced potentiometers in the modern laboratory

18.2 Standard Reduction Potentials

• There is no way to measure the potentials of the individual electrode process
• If we want potentials for the half-reactions (half-cells), we must arbitrarily divide the total cell potential
• The standard hydrogen potential is the reference potential against which all half-reaction potentials are assigned
• The accepted convention is to give the potentials of half-reactions as reduction processes
• When a half-reaction is reversed, the sign of e° is reversed
• When a half-reaction is multiplied by an integer, e° remains the same
• A galvanic cell runs spontaneously in the direction that gives a positive value for cell
  • A complete description of a galvanic cell usually includes four items: The cell potential and the balanced cell reaction
  • The direction of electron flow is obtained by inspecting the half-reactions and using the direction that gives a positive ecell
  • Designation of the anode and cathode.
  • The nature of each electrode and the ions present in each compartment

18.3 Cell Potential, Electrical Work, and Free Energy

• Work is viewed from the point of view of the system
• It flows out of the system is indicated by a minus sign
• Work is never the maximum possible if any current is flowing
• In any real, spontaneous process some energy is always wasted—the actual work realized is always less than the calculated maximum.
• Faraday: Coulombs of charge per mole of electrons
• The actual work obtained from a cell is always less than the maximum because energy is lost through frictional heating of the wire when current flows

18.4 Dependence on Cell Potential on Concentration

• A galvanic cell in which both compartments have the same components but at different concentrations
• The electrons flow in the direction that tends to equalize the concentrations
• The difference in concentration is the only factor that produces a cell potential in this case, and the voltages are typically small
• Nernst was one of the pioneers in the development of electrochemical theory and is generally given credit for first stating the third law of thermodynamics. He won the Nobel Prize in chemistry in 1920
• At equilibrium, the components in the two cell compartments have the same free energy
• A pH meter is a familiar example of an instrument that measures concentration using an observed potential
  • The pH meter has three main components: a standard electrode of known potential, a special glass electrode that changes potential depending on the concentration of ions in the solution into which it is dipped, and a potentiometer that measures the potential between the electrodes
  • Electrodes that are sensitive to the concentration of a particular ion are called ion-selective electrodes, of which the glass electrode for pH measurement is just one example

18.5 Batteries

• A battery consists of a galvanic cell or group of cells connected in series that serve as a source of direct current
• The lead storage battery has been a major factor in making the automobile a practical means of transportation.
• Lead storage battery
  • Anode: lead
  • Cathode: lead coated with PbO2
  • Electrolyte: H2SO4(aq)
• Dry cell battery
  • Contains a moist paste instead of a liquid electrolyte
  • Anode: usually Zn
  • Cathode: carbon rod in contact with an oxidizing agent
• The calculators, electronic games, digital watches, and portable CD players that are so familiar to us are all powered by small, efficient batteries
• A fuel cell is a galvanic cell for which the reactants are continuously supplied
  • Galvanic cells in which the reactants are continuously supplied
  • New fuel cells are under development that can use fuels such as methane and diesel directly without having to produce hydrogen first.
  • The H2/O2 fuel cell is based on the reaction between H2 and O2 to form water

18.6 Corrosion

• It involves the oxidation of metals to form mainly oxides and sulfides
• Some metals, such as aluminum and chromium, form a thin protective oxide coating that prevents further corrosion
• Metals, such as copper, gold, silver, and platinum, are relatively difficult to oxidize. These are often called noble metals
• The corrosion of iron to form rust is an electrochemical process
  • Iron can be protected from corrosion by coating it with paint or with a thin layer of metal such as chromium, tin, or zinc; by alloying; and by cathodic protection
  • This coating is not an infallible shield against corrosion, however; when steel is exposed to oxygen in moist air, the oxide that forms tends to scale off and expose new metal surfaces to corrosion.
  • Moisture must be present to act as a kind of salt bridge between anodic and cathodic regions
• Prevention of corrosion is an important way of conserving our natural resources of energy and metals
• The primary means of protection is the application of a coating, most commonly paint or metal plating, to protect the metal from oxygen and moisture.
• Alloying is also used to prevent corrosion
• Cathodic protection: A method most often employed to protect steel in buried fuel tanks and pipelines

18.7 Electrolysis

• ****An electrolytic cell uses electrical energy to produce a chemical change that would otherwise not occur spontaneously
  • Electrolysis is used to place a thin coating of metal onto steel
  • It is used to produce pure metals such as aluminum and copper
  • Electrolysis has great practical importance; for example, charging a battery, producing aluminum metal, and chrome plating an object are all done electrolytically
• The causes of overvoltage are very complex
  • The phenomenon is caused by difficulties in transferring electrons from the species in the solution to the atoms on the electrode across the electrode–solution interface.
  • Because of this situation, e° values must be used cautiously in predicting the actual order of oxidation or reduction of species in an electrolytic cell

18.8 Commercial Electrolytic Process

• Because metals are typically such good reducing agents, most are found in nature in ores, mixtures of ionic compounds often containing oxide, sulfide, and silicate anions
• The noble metals, such as gold, silver, and platinum, are more difficult to oxidize and are often found as pure metals
• Aluminum is one of the most abundant elements on earth, ranking third behind oxygen and silicon.
• Production of aluminum metal from its ore proved to be more difficult than the production of most other metals.
• Noble metal impurities in the anode are not oxidized at the voltage used; they fall to the bottom of the cell to form a sludge, which is processed to remove the valuable silver, gold, and platinum.
  • Metals that readily corrode can often be protected by the application of a thin coating of a metal that resists corrosion
  • Addition of a nonvolatile solute lowers the melting point of the solvent, molten NaCl in this case.
• In the past 30 years, a new process has been developed in the chlorine alkali industry that employs a membrane to separate the anode and cathode compartments in brine electrolysis cells
  • The membrane is superior to a diaphragm because the membrane is impermeable to anions
  • Although membrane technology is now just becoming prominent in the United States, it is the dominant method for chlorine–alkali production in Japan.

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