William Francis Giauque, 1895-1982, won the 1949 Nobel Peace Prize in Chemistry for proving the third law of thermodynamics by developing methods for studying matter at extremely low temperatures, those below 1 Kelvin. Due to his observations, the behavior of chemicals at higher temperatures can be predicted. Not only did Giauque carry out the experiments, but he designed much of the apparatus used. He developed a two step process for reaching temperatures below 1 Kelvin and thermometers that could read those temperatures. His work not only made possible the calculation of change in entropy for a reaction, but also the calculation of entropy of certain elements and compounds. Giauque also is credited with the discovery of isotopes of oxygen. This opened the floodgates for the research and discovery of isotopes of other elements. His work in discovering the isotopes of oxygen also caused the change in in moving from O16 scale to C12 in determination of atomic weights.Before Giauque, the lowest temperature reached was 1 K. The method for reaching low temperatures before Giauque's method was introduced involved a series of gas expansions. Nitrogen was the first gas to be liquefied. Once that was done, a gas with a lower boiling point could be condensed. Helium was the last gas to be condensed, which occurs around 4 K. If this was done very accurately and carefully, then this liquid helium could be cooled to 1 K. It was then determined that absolute zero could not be reached using current methods. Giauque devised a plan to go significantly beyond 1 K by using a method connected to the thirds law of thermodynamics, which was first mention by Nernst. Nernst's Law stated that for perfect crystalline structures, the suggested entropy approaches zero as the temperature approaches absolute zero. Giauque concluded that by magnetizing a compound, all of the electron spins will line up creating virtually no randomness, entropy equal to zero. For Giauque's method involved bringing a compound to the temperature of liquid helium and then putting it through a two step process. The first step would be isothermal magnetization followed by the second step of adiabatic demagnetization. Using this technique a temperature as low as 0.003 K was reached.
William Giauque also needed to develop a thermometer that could actually show that these extremely low temperatures were being reached. He knew that even gas thermometers would fail at these low temperatures. He began to look upon physical properties that he could use to determine the temperature and focused upon the magnetic susceptibility.
Many papers were produced leading to his presentation of the Nobel prize in 1949. They were almost all based in the research of compounds and elements at extremely low temperatures and their properties that could be determined by that research. One of those articles, "Chlorine. The Heat Capacity, Vapor Pressure, Heats of Fusion and Vaporization, and Entropy", is discussed further in this article. This is an excellent example of how Giauque compared spectroscopic to calorimetric data to solidify his findings.
The purpose of this paper was to compare calorimetric experimentally determined entropy of chlorine with the previously calculated spectroscopic value. Additional findings in this paper are the vapor pressure and heats of fusion and vaporization for chlorine. Both, third law related and spectroscopic, methods for determining entropy were concluded as being excellent techniques and are in agreement with each other because of the structure chlorine and relation to other like elements. The entire paper shows how Giauque was tireless in his search of concrete truth in the experimental determination of every little detail. Every possible defect or reason for experimental failure was examined and proven to yield reasonable results. An example of this is within the first three paragraphs he writes about checking the Gold Calorimeter II for reactivity with the chlorine and how that would interfere with the reaction. Through experimentation he concluded that since the chlorine that he was using was going to be dried, pure, and would be around its boiling temperature at an extremely short time, the interaction with gold would be close to none and therefore would not interfere with the reaction significantly. He checked his thermometer also, to make sure that the thermocouples were correct.
To prepare the chlorine, 3 M HCl was dropped on manganese dioxide. The hydrogen chloride gas produced was removed by water and the chlorine was dried by concentrated sulfuric acid and then phosphorus pentoxide. The chlorine was placed in a calorimeter and it was found to have 5:10000 impurity on a molal basis. This was refined by fractionated the chlorine in a vacuum jacketed column. The chlorine was then tested and it was found that the impurities were reduced to 3:100000.
In determining the vapor pressure, a mercury manometer was used to measure the pressure. Apparatus was set up to prevent slow diffusion of the gases. Two series of observations were concluded to find the vapor pressure at the triple point of chlorine to be 1.044 cm Hg at the triple point of 172.12 K. The equation that represents the vapor pressure from the triple point to 240.05 K is written:
log10 P(inter.cm. Hg) = -(1414.8/T) - 0.01206*T + 1.34E-005 * T2 + 9.91635 Using the equation, the boiling point of chlorine was found to be 239.05 K. Giauque showed many tables comparing his experimental observations with previously observed data and calculated data of his own. Each table shows the accuracy of his detailed work.The heat capacity was calculated by observing the temperature change when heat was added and presented in both graph and table format. The data was determined to be within 0.1-0.2% accuracy above 35 K. As the temperature approaches the boiling point of hydrogen, the percentage accuracy decreases due to the decreasing heat capacity and temperature coefficient of the resistance thermometer. The values that Giauque calculated were compared to previously determined values by Eucken and Karwat. The new data showed close relation to the previously determined data.
The heat of fusion was calculated by inputting heat several degrees below the melting point and ending several degrees above it and then calculating using the known heat cavity of each phase. The average of the four runs executed during this experiment is 1531 + 1 cal. mol-1, which is compared to the value calculated by Eucken and Karwat, 1615.
The heat of vaporization was determined by attaching an absorption bulb filled with 1.7 M potassium iodide and 2 M sodium hydroxide. Careful consideration was used so that the solution was not sucked back into the entrance. The heat was gradually introduced so that the vaporization would occur slowly. Again, four runs of forty minute time intervals were executed at the boiling point and the average of these runs was found to be 4874 + 4 cal. mol-1 at 760 mm Hg. Once again, Giauque made sure that his observation concurred with his deviation calculation and previously recorded data.
Finally the entropy of chlorine was determined by adding up the pieces of information as stated in the following table:
0-15 K., Debye Function hcv/ k = 115 0.331 The Entropy of Chlorine
15-172.12 K., graphical 16.573
Fusion 1531/172.12 8.895
172.12-239.05 K., graphical 5.231
Vaporization 4878/239.05 20.406Entropy of actual gas at boiling point 51.44
Correction for gas imperfection 0.12Entropy of ideal gas at boiling point 51.56
The correction for gas imperfection was found by finding the difference in the the entropies of ideal and actual. Once more, Giauque compared the calorimetric data that he determined experimentally with the spectroscopic values that were previously determined and showed that the accuracy of his findings to be impeccable. The paper is summed up by restating all of the properties of chlorine that were determined during the execution of this paper.
Chlorine
Melting point 172.12 K
Boiling point 239.15 K
Heat of fusion 1531 cal. mol-1
Heat of vaporization 4878 cal. mol-1
Entropy 51.56 cal. K-1 mol-1 @ 239.05 K
53.32 cal. K-1 mol-1 @ 298.10 KVapor Pressure equation
log10 P(inter.cm. Hg) = -(1414.8/T) - 0.01206*T + 1.34E-005 * T2 + 9.91635 William Giauque had an incredible passion for the pursuit of the truth. He has showed that in the precision of every piece of work that he compiled. He has been one of the most prolific people in the development of physical chemistry and his work has made it possible for further advances in technology and society.
Bibliography:
Frank N. Magill, The Nobel Prize Winners: Chemistry, Volume II: 1938-1968, Salem Press, Pasadena, CA, 1990, pg.
533-540.Laylin K. James, Nobel Laureates in Chemistry 1901-1992, American Chemical Society and the Chemical Heritage Foundation, U.S.A., 1993, pg. 321-327.
Giauque, W. F. and Powell, T. M. Chlorine. The Heat Capacity, Vapor Pressure, Heats of Fusion and Vaporization, and Entropy. Journal of American Chemical Society 61, 1970-4 (1939).