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DETERMINING GIBBS FREE ENERGY AND ENTROPY CHANGES Introduction In this experiment you will determine the enthalpy change, entropy change and Gibbs free energy change for dissolving borax (Na2B4O5(OH)4.10H2O) in water: Na2B4O5(OH)4. 10H2O(s) 2Na+(aq) + B4O5(OH)42-(aq) + 10H2O(l) [equation 1] This will be done by determining the solubility product (Ksp) of borax in water at two different temperatures. Solubility product (which you may meet in the equilibrium unit depending on your choice of IA) is a measure of the soubulity of a compound, based on the concentrations of ions in a saturated solution. For borax, the solubility product is given by: Ksp = [Na+]2[B4O5(OH)42-] [equation 2] The square brackets in the equation mean ‘concentration’. The concentration of Na+ is squared because equation 1 shows two Na+ ions for every borate ion. The concentration of the borate ion in equilibrium with solid borax will be determined by titration with HCl according to the equation: B4O5(OH)42-(aq) + 2HCl(aq) + 3H2O(l) 4H3BO3(aq) + 2Cl-(aq) [equation 3] Once we have calculated the concentration of borate ions, equation 1 tells us that the concentration of Na + ions is simply double this. The Ksp for borax will be determined at room temperature and at about 60oC. Knowing Ksp values at two temperatures allows you to calculate the enthalpy change for the reaction using Equation 4. Don’t worry about the maths behind this equation, it is something you might meet at degree level but is well beyond IB level. ln Ksp 1 Ksp 2 = ΔH 1 ( R T2 − 1 ) T1 [equation 4] Note: temperature in all these equations must be in Kelvins. We can calculate Go at each of our two temperatures by using the following equation: ΔGo = -RT lnKsp [equation 5] Finally we are able to use the change in Gibb’s free energy, the temperature and the enthalpy change to calculate the entropy change. Note that whilst ΔH and ΔS do not change with temperature, ΔG does so make sure the temperature you use corresponds to the value of ΔG you use. ΔGo = ΔHo – TΔSo [equation 7] Source: Kalamazoo Valley Community College, http://classes.kvcc.edu/chm130/Thermodynamic%20Study%20of%20a%20Reaction%20-%20Borax.pdf Materials and Equipment Water baths set to 60OC Graduate pipette Pipette fillers Burette (plus stand and clamp) Thermometer Standard laboratory glassware 60 cm3 saturated borax solution stored at room temperature 60 cm3 saturated borax solution stored at 60OC 0.10 mol dm-3 hydrochloric acid Bromothymol blue Procedure 1. Record the temperature of the room temperature borax solution. 2. Without disturbing the solid at the bottom, carefully transfer about 60 cm3 of the room temperature borax solution into a clean and dry beaker – perhaps use a bulb-pipette to do these transfers as that way you will not need to lift/disturb the container. 3. Use a graduated pipette to transfer three 10.00 cm3 samples into three separate clean but not necessarily dry conical flasks. 4. Add 20 cm3 of distilled water and a splash of bromothymol blue indicator to each flask. The solutions should turn blue. 5. Titrate each sample with HCl solution until the solution changes from blue to yellow-green. 6. Use your two most closely agreeing titrations to perform your calculations – you should be able to get two that agree to within 0.1 cm3. 7. Repeat steps 1 to 6 with your 60OC borax solution. Analysis Note: success in these calculations will depend largely on how clearly you label each answer, so you’re always clear exactly what you are calculating Calculations: a) Use your average titre for the room temperature solution to determine the number of moles of borate in a 10 cm3 sample of the solution. b) Use the answer to ‘a’ to determine the concentration of borate in the samples, and then the concentration of Na+. c) Use equation 2 and your answers to ‘b’ to calculate Ksp at room temperature. d) Repeat steps a-c for the 60OC solution. e) Use your answers to c and d, the temperatures you recorded and equation 4 to calculate ΔHO. You will need to rearrange the equation to get your answer. f) Use equation 5 to calculate ΔGO at each of the two temperatures. g) Use equation 6, your answer to e and a corresponding ΔGO/temperature pair to calculate ΔSO. Repeat the calculation with the other ΔGO/temperature pair. Questions: 1. How do your two values of ΔSO compare to each other? 2. Can you find a literature value of ΔSO to compare with? If so, how does it compare? 3. Try to determine the relative uncertainty in your calculations. Is this an acceptable value? How does this uncertainty compare to the % difference between yours and the literature value? 4. What are the main souces of error and uncertainty and how could these be overcome? Source: Kalamazoo Valley Community College, http://classes.kvcc.edu/chm130/Thermodynamic%20Study%20of%20a%20Reaction%20-%20Borax.pdf