General Chemistry II Laboratory Manual, 2019 Revision 133 Determination of Some Thermodynamic Data for the Dissolution of Borax Pre-I ab 1) Why is methyl orange used as the indicator for the endpoint of the titration in this lab? belau x we are usmg strung acrd 2) What is the Ksp for lithium tetraborate if 50.0 mL of a saturated solution of lithium tetraborate requires 12.11 mL of 0.500 M HCl to titrate the tetraborate ion? Resde HO 3) Based on the Ksp you calculated in the previous question, what is AG° for the dissolution of lithium tetraborate at 25°C? Ke Determination of Some Thermodynamic Data for the Dissolution of Borax Background: Borax, or sodium tetraborate (Na2B4Os(OH)4), is used in many laundry products to aid in the cleaning of clothes. There are even some commercial products that are exclusively borax on the market which are sold as laundry aids. Borax is water soluble, but its solubility varies markedly with temperature. At room temperature, borax is somewhat soluble. We can write the solubility equilibrium and the solubility product constant for borax as shown below. NazB4Os(OH)4(s) = 2Na*(aq) + B4Os(OH)4²-(aq) Ksp = [Na*]°[B4Os(OH)4²] If we have a solution that is saturated with borax, the concentration of the tetraborate anion can be determined by titration with a solution of hydrochloric acid resulting in the formation of boric acid. B4Os(OH)4²- + 2H* + 3H2O → 4H3BO3 Each mole of the tetraborate ion requires two moles of hydrochloric acid to react completely with it. If the concentration of the tetraborate ion is known, the concentration of sodium ion is equal to two times the concentration of the tetraborate; assuming that both ions in solution come only from the dissolution of borax. In this lab you will use the titration with HCl to determine the Ksp Values for borax over a series of different temperatures. This data will then be used to determine different thermodynamic values for the dissolution reaction of borax. We have learned several equations that can be used to calculate the Gibbs' Free Energy (AG) for a reaction. ulos 0ten AG= AH – TAS AG-ΔH- TAS sr lo AG = -RT(InK) ort o1w ow basinoleb igrons bbA If we combine these two equations, we get: AH– TAS = -RT(lnK). ol noteotbai ounsto Iyrrom to anob E bbA This equation can be rearranged to give: w bodasoai mioq eoolaviupo or (8 gnibe HC ΔΗ-ΤAS %3D InK = %3D -RT oginot Ea noimtoa xmod boinunca which rearranges further to give: 20n LecOLq ( 1 R T AS -ΔΗ ΔS InK = ΔΗ or InK = %3D 6. RT R R The boxed form of the equation suggests that if we determine the values of Ksp for borax we will get a straight over a series of temperatures and then make a plot of InKsp Vs. AS -ΔΗ and whose intercept is R line whose slope is