Work of Dr. W.F. Giauque

William Frances Giauque received the Nobel Prize in Chemistry in 1949 (2). The award was present to him because of his work with thermodynamics, specifically the third law of thermodynamics (2). His work proved the third law as a fundamental Natural law moreover made it possible for the calculation of entropy of many elements and compounds (3). Giauque's work expanded the Nernst heat theorem. The theorem states as an entropy change accompanying any physical or chemical transformation approaches zero as the temperature approaches zero.

This proof shows that as a substance is cooled, the amount of disorder present decreases. In theory and now proven, thanks to Giauque, a crystal's motion (vibration, rotational) was shown to decrease as the crystal's temperature approaches zero (3). How Giauque accomplished this was not by cooling the substance down to absolute zero because that is not feasible. Since liquid helium is the coldest substance available it only brings the substance down to a certain point. How he accomplish this feat was to cool the crystal down with liquid helium while holding it in a magnetic field that brought about increased order. The liquid helium was removed along with any heat and also the magnetic field was removed. The crystal solid then falls to just a few thousandths of a degree above zero. The substance that was used successfully was gadolinium sulfate which is a paramagnetic substance. Paramagnetic is important in the fact that without it the substance can not be held in the magnetic field which is needed to increase the order of the substance. The discover of the use of a magnetic field by Giauque to increase order in a substance was important in the fact proving the Third Law of Thermodynamics. As a foot note, recently scientists have used lasers to increase the order of a substance and they were awarded the Nobel prize also for being able to achieve temps just millionths of a degree above absolute zero.

The paper of Giauque that will be dealt with involves the experimental measured range of hydrogen sulfide. The title is "Hydrogen Sulfide. The Heat Capacity and Vapor Pressure of Solid and Liquid. The Heat of Vaporization. A Comparison of Thermodynamic and Spectroscopic Values of the Entropy" (1). It was published in May, 1936 in the Journal of American Chemical Society. This paper gives a detailed analysis of H₂S as it moves through its transitions of state. From the data collected the resulting entropy's of the H₂S were calculated between the transitions of state.

The beginning of the paper discusses the processes of producing H₂S from the combination of elements. One observation to note, is that the preparation of almost pure H₂S is extremely critical for the test performed to have optimum accuracy. As Giauque discusses, impurities in the H₂S will throw off the results. One problem that arises is premelting caused by the liquid-soluble solid-insoluble impurities. The depending on the impurities the melting point and boiling point of any substance can be askew by as much as a few degrees.

The next section of this paper discusses the process of measuring vapor pressure and transitions of state of hydrogen sulfide. The experiment involved taking the pressure using a monometer measured at specific temperatures to determine the transition points of hydrogen sulfide, specifically the triple point. The triple point was observed at 187.61 K and 17.389 cm Hg. This is shown on the phase diagram as the point where solid, liquid and gas are all present. The transitions of state were observed 103.52 K for solid states. At 187.61 K the melting point was observed and at 212.77 K the boiling point was measured.

At 126.22 K Giauque observed an increase in heat capacity between the solid-liquid phase boundaries. He noted that at this temperature the molecules lose some molecular rotation but gained it back below or above this temperature. The possible reason for this is abrupt gain in heat capacity is the association of the molecules of like phase causing something similar to cohesion of hydrogen bonds with an increase of energy needed to break the bonds present. Since no transition was observed Giauque could not give a reliable explanation for this observation and the presence of a first order phase transition.

In the attached diagram the transitions are noted by first order phase transitions. A first-order phase transition is one where on a graph of temperature vs. heat capacity the line is discontinuous to represent a phase transition. This shows that the heat supplied drives the transition not to raise the temperature which is why there is increase in heat capacity. This can be seen on the graph at points where the line jumps up and down.

Giauque calculated the entropy of hydrogen sulfide was done using several methods. The equation used was S= ^T C_p d ln T (1). To determine entropy from almost absolute zero to 16 K the Debye function was used. The entropy values from 16 K to 212.77 K were obtained graphically. The values obtained during transition coincide with what is consistent with the Third Law of Thermodynamics, stated earlier. Example of this is from the boiling point down to the lowest recorded temperature, the entropy decreases as an increase in order occurs. As the temperature decreases, an increase in order of the system is present as it proceeds from gas to liquid and then to solid form. This is shown at the transitions of state by that as the compound goes from the solid to the liquid form there is a decrease in order correlated with an increase in entropy and vice versa.

Hydrogen sulfide is not a perfect gas but Giauque proved that the Third Law of Thermodynamics still applies to it. This is important in the fact that the Third Law is a Natural law and is not limited to only theoretically perfect compounds. Giauque's work is an example of taking a theoretical law, Nernst equation, and experimentally proving it.

Sources:

1) W.F. Giauque, R.W. Blue , "Hydrogen Sulfide. The Heat Capacity and Vapor Pressure of Solid and Liquid. The Heat of Vaporization. A Comparison of Thermodynamic and Spectroscopic Values of the Entropy." May 1936, Journal of American Chemical, p 831-837.

2) "Nobel Prize Winners - Chemistry", Vol. II, Edited by F.N. Magill, p 533-540.

3) Nobel Prize in Chemistry 1949 found on the internet at http://www.nobel.se/laureates/chemisrtry-1949-press.html