Electrochemistry

I. Galvanic Cells
A. A galvanic cell is a device in which chemical energy is changed to electrical energy. This process involves the transfer of electrons in redox reactions. Electrons flow through the wire from reducing agent to oxidizing agent, and ions flow from one compartment to the other to keep the net charge at zero. The electrode compartment in which oxidation occurs is called the anode; the electrode compartment in which reduction occurs is called the cathode.
B. The driving force in the movement of electrons is known as the electromotive force (emf) or the cell potential. The unit of cell potential is the volt. The voltage of a galvanic cell is measured by a voltmeter.

II. Standard Reduction Potentials
A. The scientific community has universally accepted the half-reaction potentials based on the assignment of zero volts to the process 2H+ + 2e- à H2. The E values corresponding to reduction half-reactions with all solutes at 1 M and all gases at 1 atm are called standard reduction potentials.
B. A cell will always run spontaneously in the direction that produces a positive cell potential.

III. Cell Potential, Electrical Work, and Free Energy
A. emf = potential difference (V) = work (J) / charge (C). Electric potential equals negative work divided by charge (E = -w/q). Free energy is related to electric potential in the equation DG = -nFE.

IV. Dependence of Cell Potential on Concentration
A. A cell in which both compartments have the same components but at different concentrations is called a concentration cell. An increase in concentration of one product or reactant will follow Le Chatelier's principle and drive the reaction to the left or right, respectively.
B. The Nernst equation relates the Free Energy at a given temeperature and Q: -nFE = -nFE' + RTln(Q) or E = E - RT/nF ln(Q). This equation is also commonly used at 25C: E = E - 0.0592/n log(Q).
C. A pH meter is a familiar example of an instrument that measures concentration using an observed potential. By measuring the electrical potential difference between a known solution and the unknown solution the pH of the unknown solution can be determined. Electrodes that are sensitive to the concentration of a particular ion are called ion-selective electrodes.
D. The equilibrium constant can be determined from the electrical potential of a redox reaction by the equation: log(K) = N*E / 0.0592 at 25C.

V. Batteries
A. A battery is a galvanic cell or, more commonly, a group of galvanic cells connected in series, where the potentials of the individual cells add to give the total battery potential.
B. Lead storage batteries came into existence around 1915. In this battery, lead serves as the anode, and lead coated with lead dioxide serves as the cathode. Both electrodes dip into an electrolyte solution of sulfuric acid.
C. The common dry cell battery was invented more than 100 years ago by George Leclanche (1839-1882), a French chemist. There are many different types of dry batteries including the silver cell and the mercury cell.
D. A fuel cell is a galvanic cell for which the reactants are continuously supplied. The Apollo missions used a fuel cell based on the reaction of hydrogen and oxygen to form water.

VI. Corrosion
A. Corrosion can be viewed as the process of returning metals to their natural state-the ores from which they were originally obtained. Most metals develop a thin oxide coating, which tends to protect their internal atoms against further oxidation.
B. The corrosion of iron is an electrochemical reaction where one part of the metal is an anode and an adjacent part is the cathode. The iron in anode decomposes into Fe2+ and electrons. These two products then flow to the cathodic area where they react to from rust and hydroxide ions.
C. There are several methods of preventing rust to develop on iron. 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. Chromium and tin form a durable protective layer while tin actually sacrifices itself by being oxidized before the iron. Alloying involves creating a mixture of iron and a noble metal so that the alloy has properties of the noble metal. Cathodic protection involves attaching an active metal to the iron so that the active metal is oxidized instead of the iron.

VII. Electrolysis
A. An electrolytic cell uses electrical energy to produce chemical change. The process of electrolysis involves forcing a current through a cell to produce a chemical change for which the cell potential is negative; that is, electrical work causes and otherwise non-spontaneous chemical reaction to occur. Electrolysis can be used to charge a battery, produce aluminum metal, or chrome plate an object.
B. The properties of electrolysis can be used to predict which species of ion in a solution will be plated first. For example, if Ag+, Cu2+, and Zn2+ are in solution it can be determined while will be reduced first according to the reduction potentials of the species. A greater reduction potential for a reaction requires less charge to happen.