Thermodynamics of The Dissolution of Borax [PDF]

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Thermodynamics of the Dissolution of Borax Experiment 26

Hypothesis: In performing a dissolution of borax in an aqueous solution, the solubility product, standard free energy, standard enthalpy, and standard entropy change can be calculated.

Results:

Part B: Preparation of Borax Test Solutions Sample

1

2

3

Volume of sample (mL)

5

5

5

Temperature of water bath (ºC)

5.5

23

61

Part C: Analysis of Borax Test Solutions Buret reading, initial (mL)

0

6.2

11.9

Buret reading, final (mL)

5.5

11.9

18

Volume of HCl titrant added (mL)

5.5

5.7

6.1

Temperature (K)

278.5K

296K

334K

1/T (K-1)

0.003590664

0.003378378

0.002994012

0.0014

0.0014

0.0015

0.0007

0.0007

0.0008

0.02

0.02

0.02

Part D: Calculations

Amount of HCl used (mol) 2-

Amount of B4O5(OH)4 (mol) 2-

[B4O5(OH)4 (mol/L)

Molar solubilty of borax (mol/L)

0.02

0.02

0.02

Solubility product, Ksp

3.20E-05

3.20E-05

3.20E-05

ln Ksp

-10.34977466

-10.34977466

10.34977466 - ΔHº /R

1

ΔSº /R

-

ΔHº (kJ/mol) ΔSº (J/mol)

10.34977466 8.314 -

ΔGº (kJ/mol), at 298 K

86.04802648 25650.62589

Calculations:

Discussion: In this experiment, the purpose was to find the standard free energy, standard enthalpy, and standard entropy changes. Borax (B4O5(OH)42 ) was titrated with the strong acid titrant hydrochloric acid (HCl), in this acid-base titration. There were three temperatures tested, the cold water bath had a temperature of 278.5K, warm bath had a temperature of 296K, and the hot baath had a temperature of 334 K. Graph 1 below shows the ln of Ksp vs. I/T- this was analyzed through the slope intercept form, y=mx+b. The slope was -Δ/R, the y-intercept was ΔSº /R. Looking at the trendline of the Graph, it is negative- showing that as the temperature increases/decreases, solubility also increases/decreases.

The values from this Graph allowed to find these values: ΔHº (enthalpy), ΔSº (entropy), and ΔGº (free energy). The enthalpy value was 8.314, which is also the value of the R-constant. Since this value is positive, it shows the reaction to be endothermic. The enthalpy value was -10.349. The ΔGº was calculated using these values, it turned out to be 25.65 J/mol. Since this free energy value is greater than 0, it is nonspontaneous.

Graph 1 -8 -8.5 -9

ln Ksp

-9.5 -10 -10.5 -11 -11.5 -12 0

0

0

0

0

0

0

0

0

0

1/T (K-1)

Conclusion:

In this experiment to determine the solubility product of borax as a function of temperature, the standard free energy, standard enthalpy, and standard entropy changes for the dissolution of Borax in an aqueous solution were also determined. The solubility product for all three

temperatures were 3.2 *10^5. The standard enthalpy was 8.314 kJ/mol. The standard entropy was -86.04802648 kJ/mol. The standard free energy was 25.65 kJ/mol.

Laboratory Questions:

3. Part B.2. The solid borax reagent is contaminated with a water-soluble substance that does not react with hydrochloric acid. As a result of this contamination, will the Ksp of the borax be reported as too high, too low, or unaffected. Explain.

The Ksp of the borax would be too high since the molar solubility is higher- this is because adding too much of water-soluble substance would allow for a diluted solution and more HCl would have to be titrated. 4. Part B.4. For the borax solution at 48°C, no solid borax is present in the test tube. Five milliliters is transferred to the corresponding calibrated test tube and subsequently titrated with the standardized hydrochloric acid solution. How will this oversight in technique affect the reported Ksp of borax at 48°C—too high, too low, or unaffected? Explain. The Ksp of the borax would be low, since the molar solubility is lowered. Since the solid borax has dissociated more, a lower amount of HCL to titrate is required. 5. Part B.5. A “little more” than 5 mL of a saturated solution is transferred to the corresponding calibrated test tube and subsequently titrated with the standardized hydrochloric acid solution. How will this “generosity” affect the reported molar solubility of borax for that sample—too high, too low, or unaffected? Explain.

It would be too high, since there is more HCL required, there is more borax present. It would take a longer time for the equivalent point to reached. 6. Part C.2. The saturated solution of borax is diluted with “more than” 25 mL of deionized water. How does this dilution affect the reported number of moles of B4O5(OH)42– in the saturated solution—too high, too low, or unaffected? Explain.

If the amount of borax is diluted, it would require more HCL to be titrated. The moles of borax would be too high.

7. Judy’s data plot has a lesser slope than Nancy’s. Which student will have the more positive deltaH? for the dissolution of borax? Explain. Hint: for interpretation remember that a slope of -100 is less than a slope of -150. If Judy’s plot has a lesser slope than Nancy’s, Judy’s plot will have a more positive delta H. With a small slope, the delta H will be less negative or more positive.

Reference: Beran, J.A., (2010), Pages 221 - 230 Laboratory Manual for Principles of General Chemistry. 9e. Print