Exp31–Objectives.ppt

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.,Exp. 31 Introduction,Large deposits of borax are found in Death Valley in the Moj

2、ave Desert in California. Borax is mined as either: tincal, Na2B4O5(OH)48H2O, or kernite, Na2B4O74H2O.,Exp. 31 Introduction,Uses for borax: Cleansing agent Manufacture of glazing paper and varnishes Flux in soldering and brazing Manufacture of borosilicate glass.,Exp. 31 Introduction,The free energy

3、 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.,Exp. 31 Introduction,K is expressed for the chemical system in equilibrium when the reactant and products are in their standa

4、rd states. For a slightly soluble salt in an aqueous system, the precipitate corresponds to the reactants the ions in solution correspond the products,Exp. 31 Introduction,Therefore, for Ag2CrO4(s) 2Ag+(aq) + CrO42-(aq) Go = -RTlnKsp = -RTlnAg+2CrO42- Since the free energy change is a function of th

5、e enthalpy change and the entropy change for the process: Go = Ho - TSo,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 Sin

6、ce R is constant, Ho and So can be determined.,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.,Exp. 31 Introduction,In this experimen

7、t, 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 Ksp=Na+2B4O5(OH)42-,Exp. 31 Introduction,Because B4O5(OH)42- is the conjugate base

8、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.,Exp. 31 Introduction,This analysis is also a measure of the molar stability of borax in water at a given temp according

9、 to the stoichiometry, one mole of B4O5(OH)42- forms for every mole of borax that dissolves: B4O5(OH)42- = molar solubility of borax,Exp. 31 Introduction,Temperature affects the molar solubility of most salts: Solubility of borax 0oC = 2.01g/100mL Solubility of borax r.t. 6.3g/100mL Solubility of bo

10、rax 100oC = 170g/100mL,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 = Na+2 B4O5(OH)42- = 2 B4O5(OH)42-2 B4O5(OH)42- = 4B4O5(OH)42-3 = 4molar solubility of borax3,Exp. 31

11、 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.,Exp. 31 Procedural Not