ChemBytes - Chapter 12-13

Redox Reactions, Electrochemistry, Cell Voltage

CHEMByte 26: Walther Nernst-The Man and His Equations. A towering genius of physical chemistry in the late 19th and early 20th centuries, Walther Nernst made his mark at age 25 when he applied the principles of thermodynamics to the electrochemical cell and was able to give a reasonable explanation of the potential it produced. His simple equation related the cell potential to the concentrations of reactants and products and got him a highly prized professorship at the 700 year old University of Gottingen which was at the time challenging Berlin and the Kaiser Wilhelm Institute as the central science faculty on the continent of Europe. The textbook of physical chemistry, which Nernst wrote while he was there as a young professor, was without equal for forty years and served to introduce generations of students to the field.
Nernst's greatest contributions, however, were to come after he advanced to Berlin in 1905. There, he proposed what has become known as the third law of thermodynamics, that DS approaches zero as temperature approaches absolute zero, a concept that won him the Nobel Prize in 1920, a year before Einstein and two years before Neils Bohr. Nernst also did important work in photochemistry and he advanced a theory for the ready dissociation of ionic compounds in water.
His life story was a peculiar counterpoint to his huge log of scientific successes. Nernst was a practical fellow, very much given to invention and technology. He succeeded in developing a ceramic lamp about the time Edison was looking for a proper combination of filament and globe to serve as an electric light bulb, patented his invention, and sold it to European industrialists for a million dollars (at a time when there were no income taxes). Nernst's invention turned out to be hardly worth the paper this million dollars was printed on and word of this costly failure set Edison back significantly in his quest for universal support for his successful light bulb invention.
During WW I, Nernst served Germany as a science and technology advisor, lost two sons who died in the trenches in France, and was decorated for all these sacrifices by his grateful nation. But the marriage of his two daughters to men of Jewish descent changed all of that as the National Socialist (Nazi) party took control of Germany in the 1930s, leaving a broken and disappointed Walther Nernst to resign all his offices in protest and to live out the last decade of his life in disgust of the country he believed he had sacrificed so much for.

CHEMByte 27: "Animal Electricity" and the Voltaic Pile
One of the most important discoveries in electrical science was made by the Italian physician and professor of anatomy and surgery, Luigi Galvani. Galvani had been interested in electrical effects on living tissue, and so he obtained an electrostatic generator and a Leyden jar, a simple device for storing up static charge. His famous observation was that contacting the muscle of a dead frog with a metal probe in the presence of his electrostatic generator caused the muscles of the frog to twitch. Galvani made the further important discovery that glass or bone probes would not produce the same effect, thus demonstrating that metal could conduct electricity and that glass or bone could not. Unfortunately, Galvani belonged to a time when people believed in innate forces and principles contained within substances and organisms. One example of this was phlogiston, the principle in a substance that was believed to cause combustion. Therefore he ascribed his results with the frog as being something contained within the frog, which he called "animal electricity."

The interpretation of these results was questioned by the Italian professor of physics, Alessandro Volta. He repeated some of Galvani's experiments on the frog and observed that the muscle twitch could occur without the electrostatic generator if the frog's tissue was probed by two dissimilar metals. Volta concluded that the twitch was the result of the electrical stimulus applied to the frog, not the result of some vital force already present in the animal. Volta extended his investigations to a variety of combinations of dissimilar metals and compiled a list telling which metal was positive and which was negative in each combination. Further experimentation by Volta showed that an electric potential could have resulted from these metal combinations in the presence of salt water. To maximize the effect he constructed a "voltaic pile" consisting of pairs of silver and zinc plates in contact but separated from each neighboring pair by a pad moistened with salt water. Strong shocks could be felt by touching the plates at opposite ends of this pile. Each of these pairs of plates is what we now call a galvanic (or voltaic) cell, and Volta's arrangement placed these cells in series so that their potentials would add together. Thus a source of dependable electric current became available for the experiments by others like Humphry Davy and Michael Faraday, which would move society into the electric age.

CHEMByte 28: The Invention of the Fuel Cell Was Earlier than You Might Think. We usually associate the fuel cell, particularly the hydrogen-oxygen fuel cell, with the space age since it is so useful in spacecraft and holds so much of the promise for an era of pollution-free energy. It comes as a surprise to most people that the fuel cell, specifically the hydrogen-oxygen fuel cell, was discovered in December 1838 by Sir William Grove in England. Grove was trained as a lawyer and received a knighthood for his services as a judge, but during a period of ill health he reduced his activities in the courtroom and turned his attention to science. His accomplishments in science clearly outweigh those in law although his fame as a scientist should be much greater than it is.

The electrodes in Grove's first fuel cells consisted of glass tubes that were sealed at the top with their open bottoms immersed in an electrolyte of dilute sulfuric acid. One of the tubes in each cell pair was filled with hydrogen gas and the other was filled with oxygen gas. Each tube contained an electrode, which was a tube of platinum foil. The foil had been specially treated to increase its surface area and was connected to a wire that passed through the glass at the sealed end of each tube. In order to intensify the effect of his fuel cell he connected fifty of them in series thus getting fifty times the potential of a single cell. Grove reported that such a battery of his fuel cells could produce "a shock which could be felt by five persons joining hands, and which when taken by a single person was painful," and the battery could produce "a brilliant spark visible in broad daylight." Now the extraordinary promise of the fuel cell is being realized, some 160 years after Advocate Grove's invention.

CHEMByte 29: Chlorine and Industrial Electrochemistry. Chlorine is one of the chemical elements. It is only slightly soluble in water and neither the gas nor the liquid is explosive or flammable. Chlorine gas has a greenish-yellow color and a characteristic, penetrating odor. It is about 2.5 times heavier than air. Liquid chlorine is clear amber in color and is about 1.5 times as heavy as water. At atmospheric pressure, it boils at about -100°C. One volume of liquid chlorine, when vaporized, will yield about 460 volumes of chlorine gas, which is an important piece of information since chlorine is generally transported throughout the world in the liquid state in specially constructed tank trucks and railroad cars. It is a very reactive material, entering into a wide variety of chemical reactions with organic and inorganic substances and materials. Dissolved in water, it is an oxidizing agent of moderate strength which can be purchased under the trade name Clorox.

Listed as a "hazardous substance," chlorine is subject to regulation under the Clean Water Act and more generally under regulations governing Best Management Practices and the Occupational Health and Safety Act, and the Toxic Substances Control Act. It is an article of commerce that is manufactured and used on a scale of millions of pounds per year for everything from plant and animal drugs to cleaning fluids and the polymers of all manner of plastics materials used for protection, construction, and containing. Chlorine is manufactured today as it has been for a century by variations of the Hooker Chloralkali Process.
The electrochemical cells used for manufacturing chlorine by the Hooker Chlor-alkali Process are special cells known as diaphragm cells, consisting of three essential, identifiable parts:
(1) Anodes in an electrolyte chamber.
(2) Cathodes in an electrolyte chamber.
(3) A diaphragm separating the chambers.
A purified brine solution is fed into the anode compartment and an electrical current is applied. The principal products are chlorine, hydrogen, and a cell liquor (solution) which is a mixture of NaCl and NaOH. For the passage of two Faradays of electricity, the following chemical equation can be written:
2 NaCl(aq) + 2 H2O(liq) Cl2(g) + H2(g) + 2 NaOH(aq)
The brine solution that feeds into the operating diaphragm cell is essentially a saturated sodium chloride solution, about 23% by weight NaCl, though it may contain some sodium sulfate, sodium carbonate , and sodium hydroxide. The electrochemical reactions that occur are principally the discharge of chloride ions and the electrolysis of water to produce hydrogen gas and hydroxide ions:
2 Cl-(aq) + 2 e- Cl2(g) Oxidation reaction discharges chloride ions
2 H2O(liq) H2(g) + 2 OH-(aq) Reduction reaction from electrolysis of water
At first, the chlorine molecules formed dissolve in the electrolyte solution in the anode compartment but as it becomes saturated, bubbles of chlorine gas emerge and are carried away. Hydrogen and caustic (NaOH) solution are removed separately.
The voltage to drive the reaction is the thermodynamic potential, often referred to as the decomposition potential, plus the voltage required to overcome cell resistance and electrode overvoltage -- overpotential -- which is the additional voltage needed to drive the reaction. Overvoltage arises because of the nature of the electrode surface and is especially a factor in the electrolysis of water. Oxygen has a higher overvoltage than chlorine.
CHEMBytes: Additional Problems on electrochemistry
  1. Complete and balance the following reactions which occur in (acid/basic) solution by the ion-electron half-reaction method:
    acidic solution
    (a) CuS + NO3- Cu2+ + NO + SO42-
    (b) ClO3- + As2S3 Cl- + H2AsO4- + SO42-
    (c) MnO42- MnO2 + MnO4-
    basic solution
    (d) Al + NO3- + OH- Al(OH)4- + NH3
    (e) ClO- + Fe(OH)3 Cl- + FeO42-
    (f) HOO- + Cr(OH)3- CrO42- + OH-
  2. Consult a table of standard electrode potentials and select an (oxidizing/reducing) agent capable of the following transformations:
    oxidizing agent:
    (a) Cl- Cl2
    (b) Pb Pb2+
    (c) Fe2+ Fe3+
    reducing agent:
    (d) Fe2+ Fe
    (e) Ag+ Ag
    (f) Mn2+ Mn
  3. Which of the following oxidizing agents become stronger as the hydronium ion concentration increases? Which are unchanged? Which become weaker?
    (a) Cl2 (b) Cr2O72- (c) Fe3+ (d) MnO4-
  4. By using the appropriate half-cell potentials, calculate the cell potential and the equilibrium constant for the following reaction:
    Fe3+ + I- Fe2+ + 1/2 I2
    State what you expect to happen when equal volumes of 2 M Fe3+ and 2M I- solutions are mixed.
  5. A cell consists of a standard Ag, Ag+ half cell (1M Ag+) combined with another half-cell in which a silver wire dips into a solution of 1M Br- which is saturated and in contact with solid AgBr. The electrode of this latter cell is negative, and the cell generates 0.77 volt. What is the concentration of Ag+ in equilibrium with 1M Br- and solid AgBr? What is the apparent solubility product (Ksp) of AgBr?
  6. From the following standard electrode potentials, calculate the equilibrium constant for the indicated reaction:
    Cu2+ + 2e- Cu      E° = 0.34V
    Cu2+ + e- Cu+      E° = 0.15V
    Cu + Cu2+ 2Cu+      Keq = ?