The Thermodynamics of the Dissolution of Borax
Objective:
1. Determine ∆H, enthalpy change; ∆S, enthropy change ; ∆G, free energy change and
Ksp, dissolution constant of Borax.
Experiment
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Prepare a saturated solution of borax.
Samples of the saturated solution is dissolved in 6 different temperatures.
A 5mL sample of the supernate from each of the 6 test tube is pipeted.
The superanate is titrated with standardized HCl.
B4O5(OH)42- + 2 H+ + 3 H2O ' 4 H3BO3(aq)
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Determine the molar solubility of borate B4O5(OH)42- at each temperature.
Mole B4O5(OH)42- = L HCl used x Conc of HCl (mol/L) x 1mol B4O5(OH)42-/2 mol HCl
[B4O5(OH)42- ] = Mole B4O5(OH)42-/0.005 L
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The Ksp = 4 x [B4O5(OH)42- ]
To determine the ∆H, ∆S, and ∆G, the Ksp is graphed versus 1/T.
From the equation:
ln Ksp = -∆Ho/R(1/T) + ∆So/R;
where -∆Ho/R is the slope of the line and ∆So/R is the y intercept.
Procedure: Use the procedure below for the preparation of borax
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Check-out a buret.
Each student will titrate one trial of all the temperatures.
Part A.
• The HCl has been standardized for you.
• We always make three times more than needed of standardized solutions in the
past and always have run out. Some students are taking more than needed. Only
take 100 mL of standardized HCl. NO MORE!
Part B.
#2. Sample preparation
• Transfer aliquots of saturated borate solution into 6 test tubes. It is important that
the aliquots include solid sodium borate.
#3. Bath preparation.
• Water baths are set at 57o, 48o, 40o and 35o.
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Label your test tubes with your names. Use wax pencil or permanent
markers. Labeling tape will peel off in the water bath. Show courtesy by not
picking up someone else test tube.
After the sample has settled, transfer 5 mL of the supernatant with a disposable
Pasteur pipet to the calibrated test tube. Use the bulb from your medicine
dropper. Dispose used Pasteur pipet in the broken glass box.
Part C
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Should the 5 mL borate solution solidify before titration, redissolve by placing the
test tube in a warm water bath.
Quantitatively transfer the sample into an Erlenmeyer flask.
Analyze borax sample by titration with standardized HCL as written. Bromcresol
green changes from blue to green(end point)to yellow.