Data Analysis and Conclusions
The Joule-Thomson coefficient was easily calculated from the experimental data obtained. The Joule-Thomson coefficient is represented by the equation:
Therefore, it can be obtained from creating a graph by plotting change in temperature against the change in pressure observed over time in the experiment. The Joule-Thomson coefficient is equal to the slope of that graph. For the four gases used, Argon, Nitrogen, Helium and Carbon dioxide, the Joule-Thomson coefficients were calculated to be 0.4053, 0.2387, -0.0756, and 1.2167 respectively. These values are only slightly off from the known values that are shown in the graph below.
This experiment was performed at room temperature. Argon was expected to show a Joule-Thomson coefficient of approximately 4.0 K/bar, while the values for Helium, Nitrogen and Carbon dioxide were expected to be 0.06, 0.25 and 1.10 K/bar respectively. The Joule-Thomson coefficient describes the change in temperature that gases experience when they are forced through an obstacle. Therefore the more a gas cools down during this process the higher the value of the Joule-Thomson coefficient. Inversely, the more a gas warms up during this process the more negative the value of the Joule-Thomson coefficient.
Helium has a negative value because it warms up once it passes through the frit (expanding). The other three gases have positive values because they cool down when expanded. Helium warms up because in gas form it has many molecular collisions that occur. During each collision kinetic energy is converted to potential energy. As the gas expands molecules are further apart and there are fewer collisions. Because there are fewer collisions there is a decrease in potential energy causing an increase in kinetic energy which is an increase in temperature. The three gases, Argon, Nitrogen and Carbon dioxide cool down because as the gas passes through the frit, it expands causing the distance between its molecules to be greater. The potential energy of the molecules is increased because of intermolecular forces. This increase in potential energy causes a decrease in kinetic energy which is a decrease in temperature.
The experimental values of the Joule-Thomson coefficient could be slightly off due to a few sources of error. The series of tubing could have been unsuccessfully flushed out. This would cause the apparatus to have two types of gases in it, which would reslut in the data being slightly off. Another source of error could be that the gases did not get to room temperature before they were passed through the frit of the column. Although there was a significant distance of tubing for the gas to pass through, during the experiment all of the tubing remained cold to the touch. If the gas was not at room temperature it could be the source of some error. Overall, this laboratory was successful in obtaining accurate values for the Joule-Thomson coefficient of Argon, Nitrogen, Helium and Carbon dioxide.
