Answer Happy

Thermodynamics from Equilibrium: Determination of AG, AH°, and AS Purpose Determine a variety of thermodynamic quantities from the solubility information of a sparingly soluble salt at various temperatures. Introduction The system you will be studying involves a relatively simple solubility equilibrium of borax (Na2B407-10H20) in water: Na2B407 10H2O(s) 2Na+ (aq) + B405(OH)42 (aq) + 8H₂O(l) Which has an solubility product equilibrium expression as follows, once you remove the solid and liquid terms: K[Na]2 [B405(OH)4²] Since the concentration of sodium ion is two times the concentration of borate, we can plug the borate concentration value in for both the sodium and borate terms. This simplifies what we have to measure so that we can solve the problem by finding only the concentration of borate. K [(2 [B405(OH)421)12 [B405(OH)42 Ksp 4 [B405(OH)4²-7³ We find the borate concentration by titrating with HCl. The titration reaction is: B405(OH)42 (ag) + 2HCl(ag) + 3H2O()-> 4 B(OH)3(g) + 2 CI¯ (ag) Remember that K is dependent upon temperature, so if we record the amount of HCI needed to complete the titration at different temperatures, we can find Ksp at different temperatures. This will allow us to calculate enthalpy and entropy for the reaction. Procedures: The experiment was carried out as follows: Make a saturated solution of borax at 60 °C. You will know it is saturated if there is solid that won't dissolve. A saturated solution is at equilibrium, so any concentrations we measure will be the equilibrium concentrations. Take 5 mL of the saturated solution and titrate with HCl. Cool the solution 10 degrees then take another portion and titrate it. Repeat until you are at room temperature. The following data was collected:
Data Collection Record the concentration of HCI used in the experiment (M) Table 1. Data Collection Actual temp Vol. borate solution (ml) Initial buret reading (ml) Final buret reading (ml) -60°C trial 59.0 5.05 4.32 19.58 -50°C trial 50.2 4.97 19.58 29.90 -40°C trial 40.7 5.01 29.90 36.43 AG=AH - TAS Which means that you can combine the two equations for AG into: -RT In Ksp-AH-TAS -30°C trial 31.7 4.97 36.43 40.56 0.500 -20°C trial AG---RT In Ksp The free energy change at a given temperature is itself related to both the change in enthalpy, and the change in entropy, by the following equation: 21.8 4.98 40.56 42.94 Calculations for titration: Multiple the concentration of HCl times the volume used in the titration (found by subtracting the initial buret reading from the final reading). This is the moles of HCI used. Using the titration equation shown above, calculate the moles of borate from the moles of HCI. Divide by the 5 mL portion of borate solution to get concentration of borate in molarity. Use the borate concentration is the bold Ksp equation above to solve for Kap. You will do this for each temperature. How to make a graph: Now that you have equilibrium constant values at different temperatures, we can solve for free energy.
In order to solve for AH and AS, we need to make this equation into something we can easily graph as a straight line. You divide both sides of the equation by-RT, simplify, and define the x axis to be 1/T and the y axis to be In K: In Ksp In Ksp In K y A T Inverse temperature -AH RT -AH R m + TAS RT AS R That will make -AH/R and AS/R the slope and Y-intercept, respectively. Thus, by generating one graph, two values can be arrived at simultaneously, AH and AS. x + b Convert your temperatures into Kelvin and calculate the natural log of your Ksp values. Calculate the inverse of each temperature value. Collect those numbers in the following table: Copy this table into Excel (or a similar program). Create a scatterplot and add a trendline. Calculations of thermodynamic values: Use the slope of the trendline to determine AH and the y-intercept to determine AS. Calculate the value for AG-AH-TAS using the values for AH and AS from your graph,
What is the slope from your graph? J/mol
What is the value of AH? kJ
What is the y-intercept?
What is the AS?
Do you expect the solubility of Borax to increase or decrease as temperature increases? Select the option that best explains why. Solubility will increase, because as T' increases the -AH-/RT term becomes smaller therefore K will get smaller. Solubility will decrease, because as T' increases the -AH-/RT term becomes smaller therefore K will get larger. Solubility will decrease, because as T' increases the -AH-/RT term becomes smaller therefore K will get smaller. Solubility will increase, because as 7 increases the -AH-/RT term becomes smaller therefore K will get larger.
Why was it necessary to make sure that some solid was present in the main solution before taking the samples to measure Ksp? Select all options that are correct. To make sure the solution was saturated with sodium and borate ions. To make sure no more sodium borate would dissolve in solution. To ensure the dissolution process was at equilibrium.