Dissolution of Borax Reem Alhussain Lab Partners: Dana Fayyad Date of Submission: April 21, 2009 This experiment was conducted to study sparingly soluble salt, Borate in our case. After finding the Ksp from different temperature, one can determine the rest of the thermodynamic quantities such as the enthalpy change, the entropy change, the entropy change and the gibbs free energy change. The reaction that was dealt with is Na2B4O7(10H2O)(s) 2Na+(aq)+8H2O(l) at different temperatures to obtain the thermodynamic quantities. The average Ksp value was 0.000233. The free energy change was calculated to be 20352.82J. Introduction: Borax, Na2B4O5(OH)4·8H2O, dissolves slightly in water to give sodium ions, a borate ion, and water according to the equation: Na2B4O5(OH)4·8H2O(s)? 2Na1+(aq) + B4O5(OH)42-(aq) + 8H2O(l) The K expression is: K = [Na1+]2[B4O5(OH)42-] This can of course be simplified to : Borax(s) ? 2Na1+(aq) + Borate2-(aq) K = [Na1+]2[Borate2-] The concentration of the borate ion can be determined by titration with a standardized HCl solution according to the equation: B4O5(OH)2-(aq) + 2HCl(aq) +3H2O(l) ? 4H3BO3(aq) + 2Cl1-(aq) The sodium ion concentration will be twice that of the borate ion. The HCl solution is standardized by titration with sodium carbonates: Na2CO3+ 2HCl? 2NaCl+ CO2H2O By evaluating the K at different temperatures (the enthalpy (?H0) for the dissolution process can be found from the equation: ln(k1/k2) = ?H /R (1/T-1/T) The Gibb?s free energy change (?G) for the dissolution process can be evaluated at each temperature using the equation: ?G = -RTln(K) Once ?G and ?H are known to us by the equation above, the entropy change (?S) can be evaluated from the Gibb?s equation: ?G0 = ?H0 - T?S0 The enthalpy change and the entropy change should not change with temperature while the Gibb?s free energy change will vary with temperature. Purpose: The purpose of this lab is to determine a variety of thermodynamic quantities from the solubility information at various temperatures for a system of a barely soluble salt in water. The three thermodynamic quantities focused on in this lab are change in standard enthalpy, standard entropy, and standard free energy. Procedure: Take out three test tubes and place 5 mL water into them. Mark the level of the water with marker, then pour out water. You will need these later. Each lab bench will be responsible for one of the four temperatures (25, 30, 35, or 40 deg. C). At the end of class, everyone will be entering their data for their temperature into the spreadsheet for the class to compute. Add 2 scoops of borax + 50 mL dH2O to a beaker. Heat your borax solution to ~45 deg. C on hotplate. If the borax should completely dissolve at 45 deg. C, add more to the beaker. You want the solution saturated Once solution is ~45 deg. C, take off hotplate and place beaker with plain dH2O on hotplate. Again, make sure the water temp is < 50 deg. C. Let your borax solution cool to assigned temperature, then pour 5 mL into 3 test tubes (three trials!). Add 5 mL solution to Erlenmeyer flask w/ 50-75 mL dH2O + 10 drops of bromocresol green indicator. Titrate with HCl solution (clean buret filled with ~20 mL HCl solution) until the solution color goes from green to yellow (endpoint). Results Class Data Graph of lnKsp vs. 1/T Calculations Graphically Determined Values: By combining the ?G = -RT ln Ksp equation with ?G = ?H ? T ? S, we got the new equation ln Ksp = (-?H/R)x(1/T) + (?S/R), in which ln Ksp = Y and (1/T) =X. The data for both ln Ksp set 1 and 2 was graphed including their best fit line and the equation for the best fit line. By graphing the data, we can use the equation for the best fit line (y = -19627x + 59.57) to find the (-?H/R) because it equals the slope and (?S/R) because it equals the Y-intersept. Change in Enthalpy, ?H: -19627 (slope) = (-?H/R) -19627 = (-?H/8.314) ?H = 158190.48 Change in Enthalpy, ?S: 59.57 (y-intercept) = (?S/R) ?S = 495.26 Using the graphically determined values for the ?H and ?S, we can plug these values into the ?G = ?H ? T ? S equation to find the ?G (free energy change) for each Temperatures and compare them to the ?G we calculated from the solubility product using the equation ?G = -RT ln Ksp. In the ?G = ?H ? T ? S equation we are to assume that ?H and ? S are constant over the temperature change because though they are affected by temperature, the effect is very minute. Handwritten Calculations Discussion/Conclusion During this lab we tried to determine the concentration of borax solution in our solution. Each group were assigned a different temperature. Borax concentration was determined by titrating hydrogen chloride to the borax solution until the color changed from blue to yellow. It is necessary to precisely measure the amount of solid borax used because the chemist can ultimately take the count of sodium ion in solution or the count of borate ion in the sample mixture. The chemist can determine the solubility product constant by using the concentration of either sodium ion or borate ion in terms of the other and solve for the Ksp value conveniently that way. Since all the borate ions formed from borax dissociation reacted with the HCl titrated from the buret, we used the stoicheometry to find the concentration of borate ions and plugged that into the Ksp = 4 [B4O5(OH)42-]3 equation to find the Ksp. For all values of ksp the ?G (free energy change) was calculated using the formula ?G = -RT ln Ksp equation. Meanwhile, we graphed the 1/T vs. ln Ksp (for both sets of data) to make use of the ln Ksp = (-?H/R)x(1/T) + (?S/R) equation, in which ln Ksp = Y and (1/T) =X. We used the best fit line of Ksp to find that ?H = 158190.48and ?S = 495.26. Using these figures, we once again figured out the ?G but using the ?G = ?H ? T ? S. Then the ?G vs. Temperature data was graphed two sets of ?Gs. Ksp should vary with temperature. According to the Le Chatelier principle, a change in pressure, volume, and or temperature can distort the equilibrium constant. When temperature increases the Ksp value decreases and vice versa. However, in the lab experiement, it is seen that group and 4 have the same temperatures, however their ksp values are different because of the volume differences both groups have in terms of HCL and borax. The graph of ln Ksp vs 1/T does have deviating values. This is due to the fact that some of the Ksp and temperature values are calculated incorrectly. For instance group 1?s Ksp values are very extreme and far apart. Their average does not give a close to accurate Ksp value because their Ksp value fluctuates too much from trial to trial. The determination of the change in enthalpy and the change in entropy is seen through ta relationship. Delta H relates to the slope of the graph and the y aziz related to the change in entropy. Delta of free nergy is obtained by subtracting the enthalpy change from the temperature multiplied to the entropy change. As temperature increases, entropy increases. There are many possible errors for this lab. Not recording precise temperatures in this lab is a major cause for errors because the bulk of thermodynamics lab is obtaining the data at specific temperatures for accurate information leading to the calculation of the free energy change, the enthalpy change, and the entropy change. Boiling the water to a greater temperature than it needs to be will cause the solid in the solution to dissolve completely, thus not leading behind any solid.
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