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Exp. 31 The Thermodynamics of the Dissolution of Borax p. 347

Exp. 31

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Exp. 31 The Thermodynamics of the Dissolution of Borax p. 347

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    1. Exp. 31 The Thermodynamics of the Dissolution of Borax p. 347 To determine the solubility product (Ksp) of borax as a function of temperature To determine the standard free energy (?G), standard enthalpy (?H), and standard entropy (?S) changes for the dissolution of borax in an aqueous solution.

    2. Exp. 31 Introduction Large deposits of borax are found in Death Valley in the Mojave Desert in California. Borax is mined as either: tincal, Na2B4O5(OH)48H2O, or kernite, Na2B4O74H2O.

    3. Exp. 31 Introduction Uses for borax: Cleansing agent Manufacture of glazing paper and varnishes Flux in soldering and brazing Manufacture of borosilicate glass.

    4. Borosilicate Glass www.solsticeglass.com Borosilicate glass is stronger than "soft" glass and has been used for everything from stovetop cookware to nuclear waste containment. One of its most frequent uses is to make scientific glassware such as beakers and test tubes. Chemically, borosilicate glass substitutes boron oxide particles in place of the soda and lime particles found in soft glass. The boron oxide serves as a flux or glue to hold the silicate particles together with aluminum oxide and sodium oxide. Because the boron oxide particles are so small, the silicate is held together more closely resulting in a much stronger glass.

    5. Exp. 31 Introduction The free energy change of a chemical process is proportional to it equilibrium constant: ?Go = -RTlnK (eq. 31.1) Where R, the gas constant, is 8.314 x 10-3 kJ/molK and T is the temperature in kelvins and K is the equilibrium constant.

    6. Exp. 31 Introduction K is expressed for the chemical system in equilibrium when the reactant and products are in their standard states. For a slightly soluble salt in an aqueous system, the precipitate corresponds to the reactants the ions in solution correspond the products

    7. Exp. 31 Introduction Therefore, for Ag2CrO4(s) ? 2Ag+(aq) + CrO42-(aq) Ksp= [Ag+]2[CrO42-] and ?Go = -RTlnKsp = -RTln[Ag+]2[CrO42-] Since the free energy change is a function of the enthalpy change and the entropy change for the process: ?Go = ?Ho - T?So

    8. Exp. 31 Introduction Combining these two expressions and rearranging to solve for lnKsp: lnKsp = - ?Ho/R(T-1) + ?So/R (y = mx + b) If you measure lnKsp at varying temps and plot lnKsp (y) vs. 1/T (x): Slope = - ?Ho/R y-intercept = ?So/R Since R is constant, ?Ho and ?So can be determined.

    9. Exp. 31 Introduction Since the values of Ksp may be (+) or (-) for slightly soluble salts and T-1 is always positive, the plot of lnKsp vs. 1/T appears in the 1st and 4th quadrants as shown in the unlabeled figure on page 348.

    10. Exp. 31 Introduction In this experiment, you will determine ?Go, ?Ho, & ?So for the aqueous solubility of tincal, Na2B4O5(OH)48H2O: Na2B4O5(OH)48H2O(s) ? 2Na+(aq) + B4O5(OH)42-(aq) + 8H2O(l) The solubility product at equilibrium for the solubility of borax is Ksp=[Na+]2[B4O5(OH)42-]

    11. Exp. 31 Introduction B4O5(OH)42- is the conjugate base of the weak boric acid, and it is capable of accepting two protons from a strong acid: B4O5(OH)42- + 2H+ + 3H2O ? 4H3BO3. Therefore, the [B4O5(OH)42-] can be measured with an HCl titration.

    12. Exp. 31 Introduction This analysis is also a measure of the molar solubility of borax in water at a given temp according to the stoichiometry, one mole of B4O5(OH)42- forms for every mole of borax that dissolves: [B4O5(OH)42-] = molar solubility of borax

    13. Exp. 31 Introduction Temperature affects the molar solubility of most salts: Solubility of borax @ 0oC = 2.01g/100mL Solubility of borax @ r.t. 6.3g/100mL Solubility of borax @ 100oC = 170g/100mL

    14. Exp. 31 Introduction According to the stoichiometry of the dissolution of borax, [Na+] = 2 [B4O5(OH)42-] so, the solubility product for borax at a given temp is: Ksp = [Na+]2 [B4O5(OH)42-] = [2 [B4O5(OH)42-]]2 [B4O5(OH)42-] = 4[B4O5(OH)42-]3 = 4[molar solubility of borax]3

    15. Exp. 31 Introduction The objectives of this experiment are: Determine the [B4O5(OH)42-] and the molar solubility of borax at 5 different temperatures using a standardized HCl titration. Calculate the solubility product (Ksp) of borax at each temp. Plot the lnKsp vs. T-1 of the measurements. Determine ?Ho & ?So from the plot and calculate ?Go.

    16. Exp. 31 Procedural Notes Part A. Omit, will be provided by stockroom Part B. each team will maintain a different bath temperature. Part B.4. Add borax until it no longer dissolves.

    17. Exp. 31 Report Sheet Questions 1, 3 Your formal report is due next week.

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