Essay on Thermodynamic Investigation of the Joule-Thompson Effect

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Thermodynamic Investigation of the Joule-Thompson Effect and Coefficient Determination for Helium and Carbon Dioxide

Niki Spadaro, Megan Cheney, and Jake Lambeth
University of North Florida, CHM4410C Fall 2010

The Joule-Thomson coefficient explains the behavior of any real gas when changes in intensive properties, such as temperature and pressure, occur. The coefficients for helium and carbon dioxide were determined using a Joule-Thomson apparatus that created constant enthalpy within the system. Using literature values for the coefficients at room temperature, the experimental results allow examination of each gas’s unique nature.

Introduction Enthalpy is a critical study in thermodynamics. It is a measurement of a system’s
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A pressure versus temperature graph allows comparisons between the two, in which isenthalpic curves are present. Figure 1 illustrates the comparison.

Figure 1. Isenthalpic curves on a temperature versus pressure diagram (Image provided by http://www.chem.queensu.ca/courses/09/chem221/ )
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The tangent of the maximum point on an isenthalpic curve is a horizontal line, indicating that no temperature change occurs and µJ-T= 0. This constant temperature is known as the inversion temperature. It is apparent that this point coincides with the boundary of the shaded area in Figure 1. For a certain pressure, temperatures below the inversion temperature, or within the shaded region, signify cooling. The coefficient is a positive value due to a positive tangent line at any point along the isenthalpic curve. At higher temperatures, a negative coefficient exists due to a negative slope. (Gould & Tobochnik, pp.34).

Methods and Materials The Joule-Thomson apparatus (Leybold Didactic, Huerth, Germany) consisted of a glass cylinder with five outlets and a glass filter subdivision. One side of the division was connected to the helium or carbon dioxide gas pressure cylinder that was supplied in the laboratory, a pressure sensor, and a NiCr-Ni thermocouple, which measures the temperature inside that chamber. The other chamber contained outlets for another thermocouple and for the transferred gas. A temperature controlled water bath was used to set the system to the

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