Determination of K’ and Molar Gibbs Free Energy Via I3 Formation

Sara Speak
Partners: Don S., Natalie R. and Abby V.
Experiment performed:

Abstract: Iodine is the heaviest of the halogens and is very weakly soluble in water. Solid iodine can be made more soluble in water if it is added to potassium iodine to create a triiodide salt. This mixture proceeds to the direction of dynamic equilibrium and is dependent of the masses of the reactants (which proves it follows the las of mass action). Using a spectrophotometer, the formation of I3 can be monitored while gradually saturating the I2 solution with KI aliquots. The experimental Gibbs Free Energy was calculated at -2.24*104 J/mol for 0.03 mM I2 and -2.30*104 J/mol for 0.06mM I2 with a 95% confidence interval of 190.0 and 211.3 respectively. The value of K’ was also calculated as 8,624 for 0.03 mM I2 and 11,400 for 0.06mM I2 with a 95% confidence interval of 246.9 and 332.5 respectively. These values were compared to the literature of Gibbs Free Energy at -1.63*104 J/mol and 748 K’ 1. The proposed source for the errors is inadequate pipetting technique during the addition of KI aliquots.
Methods: First, a solution of 0.030mM and 0.060mM of I2 was prepared from a stock solution of 0.4mM I2. Although an exact concentration was not important, it was essential to know and record the accurate concentration. Then two more solutions were prepared where the same concentration of I2 as before but with an excess of KI. To make these solutions, the I2 stock solution was diluted with the ~0.10M KI stock until the final I2 concentration is 0.03mM. This step was repeated to prepare 0.06mM I2 solution. Exact concentrations and dilutions can be seen in Appendix A3.
With a volumetric pipette, 3mL of 0.03mM of I2 was transferred into a quartz or silica cuvette. Using the Photodiode Array Spectrophotometer, the absorption spectrum between 300-650nm was measured and recorded. This absorption of 0.03mM I2 was considered the background and was subtracted as KI solutions were added. This was repeated with the 0.06mM I2 solution to obtain the background. Then, 2µL of the ~0.10M KI was added to the cuvette containing the 3mL of I2. After each addition of KI, the solution was capped and gently shaken to ensure a thorough mix. Then the absorption was collected over the same range and recorded.
This method was repeated until a total of 20µL of KI was added. The absorbance at 353nm proceeding the KI addition was labeled as Ax for the 0.03mM I2. Then the absorbance for saturated I2/KI was measured which provided Aa for the solution of 0.030mM of I2. These steps were repeated for 0.06mM of I2 and successive KI aliquots to obtain the Ax and Aa values. A list of all reagents and grades can be seen in Table 1.

Table 1.
Reagents Chemical Formula Manufacturer Concentration/Grade Chemical/Physical Hazards
Iodine I2 Sigma-Aldrich