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Exp' 31 Objectives

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To determine the standard free energy, standard enthalpy, and ... Large deposits of borax are found in Death Valley in the Mojave Desert in California. ... – PowerPoint PPT presentation

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Title: Exp' 31 Objectives


1
Exp. 31 Objectives
  • To determine the solubility product of borax as a
    function of temperature.
  • To determine the standard free energy, standard
    enthalpy, and standard entropy changes for the
    aqueous dissolution of borax.

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
Exp. 31 Introduction
  • The free energy change of a chemical process is
    proportional to its equilibrium constant
  • ?Go -RTlnK
  • Where R, the gas constant, is 8.314 x 10-3
    kJ/molK and T is the temperature in kelvins.

5
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

6
Exp. 31 Introduction
  • Therefore, for
  • Ag2CrO4(s) ? 2Ag(aq) CrO42-(aq)
  • ?Go -RTlnKsp -RTlnAg2CrO42-
  • Since the free energy change is a function of the
    enthalpy change and the entropy change for the
    process
  • ?Go ?Ho - T?So

7
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 vs. 1/T
  • Slope - ?Ho/R
  • y-intercept ?So/R
  • Since R is constant, ?Ho and ?So can be
    determined.

8
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.

9
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 KspNa2B4O5(OH)42-

10
Exp. 31 Introduction
  • Because B4O5(OH)42- is the conjugate base of the
    weak boric acid, 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
    a HCl titration.

11
Exp. 31 Introduction
  • This analysis is also a measure of the molar
    stability 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

12
Exp. 31 Introduction
  • Temperature affects the molar solubility of most
    salts
  • Solubility of borax _at_ 0oC 2.01g/100mL
  • Solubility of borax _at_ r.t. 6.3g/100mL
  • Solubility of borax _at_ 100oC 170g/100mL

13
Exp. 31 Introduction
  • According to the stoichiometry of the dissolution
    of borax,
  • Na 2 B4O5(OH)42-
  • Therefore, the solubility product for borax at a
    given temperature is
  • Ksp Na2 B4O5(OH)42- 2 B4O5(OH)42-2
    B4O5(OH)42- 4B4O5(OH)42-3 4molar
    solubility of borax3

14
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.

15
Exp. 31 Procedural Notes
  • Part A. Omit
  • Part B. setup assign one team member to watch
    the bath temperatures.
  • Part B.3. Add borax until it no longer
    dissolves.

16
Exp. 31 Report Sheet
  • Questions 1, 3, 4
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