Chemistry 163 Name: Exercise 7: Borax solvation calculations Lab 4: The thermodynamics of borax solvation Purpose: To determine the thermodynamic quantities ΔH° and ΔS° for the solvation reaction of borax in water by measuring the solubility product constant over a temperature range. ΔH o $ 1 ' ΔSo + Equation: ln K = − & ) R %T ( R Let y = ln K and x = (1/T) and the equation has the form of the linear relationship y = mx + b, where m is the slope of the line, given by – (ΔH°/R), and b is the y-intercept, € by (ΔS°/R). Thus, if the quantities ln K and 1/T could be plotted, you could given measure the slope and intercept of a best-fit line you could draw through those data points and determine ΔH° and ΔS° for this reaction. So what is K for this reaction? Note that the borax solvation reaction equilibrium constant is the solubility product Ksp for borax or Ksp = [Na+]2 [B4O5(OH)42–]. By the stoichiometry of the reaction [Na+] = 2 [B4O5(OH)42–], so Ksp = 4 [B4O5(OH)42–]3. The amount of B4O5(OH)42– in solution is determined by titration with HCl solution using an indicator to mark the equivalence point. Skills: • Use of a graph to determine a best-fit line • Use of a best-fit line equation to determine thermodynamic quantities Chemical equation: Na2B4O7•10H2O (s) ↔ 2 Na+ (aq) + B4O5(OH)42– (aq) + 8 H2O (l) Data: Concentration of the HCl stock solution = 0.2000 M temperature of 5 mL aliquot (°C) 55.0 45.0 35.0 25.5 15.5 volume of HCl at equivalence point (mL) 65 50 40 30 22 Analysis 1. Determine the concentration of the dissolved B4O5(OH)42– in each sample titrated; write out a complete calculation for beaker #1 below. Then enter formulae in the spreadsheet that will allow the calculation of [B4O5(OH)42–] for all beakers. 2. Calculate Ksp for beaker #1; write out the complete calculation below. Then enter formulae in the spreadsheet to calculate Ksp for each borax solution. 3. Should all the Ksp values be the same for the different beakers? Explain why or why not. 4. Prepare a graph of your results, plotting the two quantities x and y (as defined in the introduction); be very careful to assign the correct labels for each axis, as well as their proper units. Using either your calculator’s functions or the graphing program’s functions, determine the equation of the best-fit line through your data points. Determine the correlation coefficient (r2). You may either write the equation and the correlation coefficient below, or include them on the graph. Comment on how good a line you have, and attach a copy of the graph to this sheet. 5. From the parameters of the best-fit line equation, calculate ΔH° and ΔS° for this reaction. Make sure to keep track of units, especially for R, so that the units of ΔH° and ΔS° work out sensibly. Write out the calculations below. 6. Interpret the values of ΔH° and ΔS° you calculated; that is, is the reaction endo or exothermic? Does the reaction generate entropy or destroy it? What observations can you make during the lab that will support these values?