Thermodynamics of Borax
C3C Justin Nevins
Chemistry 200, Section M1A
The purpose of the lab was to determine how the solubility of Borax (Na2B4(OH)4) and other thermodynamic quantities such as enthalpy, entropy, and Gibbs free energy depend on temperature. When Sodium borate octahydrate (Borax) dissociates in water it forms two sodium ions, one borate ion and eight water molecules. The chemical reaction is shown as:
A simple acid-base titration can be used to determine the concentration of the borate ion base. By dissolving Borax into distilled (DI) water at two different temperatures, the amount of borate that went into the solution at each temperature can be measured. The balanced equation:
represents the titration of the borax where the endpoint of the reaction is signaled by the change of bromocresol purple indicator, from purple to yellow.
To understand how temperature affects thermodynamic quantities equation 1 – equation 4 shown in Appendix A were used to calculate the solubility product constant, enthalpy, entropy, and Gibbs free energy respectively. Using these equations, the aforementioned thermodynamic quantity’s dependence on temperature is more understood by the lab’s completion.
To start the experiment two separate titrations were set up, one at room temperature and the other in an ice bath. For the room temperature Borax titration, a saturated solution was created by adding 1.5 grams of solid Borax to 50mL of DI water and a stir bar to a beaker that was stirred for at least ten minutes. To assure that equilibrium was sustained throughout the stirring, it was stopped periodically to assure that there was solid Borax present in the beaker keeping a saturated solution. Next, a burette was filled with approximately 50mL of the .103M Hydrochloric Acid solution (HCl). For the room temperature Borax titrations the temperature of the saturated solution was measured first. Then, DI water and bromocresol purple indicator were added to two separate flasks of the saturated solution. Each HCl solution was then titrated to its yellow endpoint and the HCl volume was recorded.
For the ice bath temperature Borax, the titration was completed with the same procedure as the room temperature Borax.
Results and Discussion
For both room temperature titrations at the start of the lab, the initial temperature was found to be 18˚C, while the two titrations set in an ice bath were found to be 8˚C. After each titration was complete, the volumes of the .103M HCl solution needed to titrate the saturated solution were recorded in Table 1.
Table 1. Titration Results
vol HCl (mL)
Room Temperature #1
Room Temperature #2
Ice Bath #1
Ice Bath #2
After the Borax dissociated in the water it was important to calculate the concentrations of both the Na+ and the , because these are needed to calculate the solubility product constant (Ksp) of the solution. By using the titration endpoint, the equivalence point was approximated and the latter was calculated. The equation:
was used to find the appropriate values of and are shown in Appendix B. To find the concentration of Na+, the concentration of the borate ion was multiplied by two because the ratio presented in reaction 1 shows that for every mole of borate produced there are two moles of Na+ produced. The values were then averaged for both room temperature titrations as well as the two ice bath titrations. The values found were shown in Table 2.
Table 2. Concentration of Ions
After these values were calculated, the average concentrations of both the borate ion and N+ ion were used in equation 1 to find the solubility product constant at room temperature and ice bath temperature. The Ksp values were found to be...
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