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Data Analysis Part 1: Determination of KHT Saturation Temperatures Be sure to show all calculations required, and don't forget proper units! 1. Refer back to the introduction of this lab and write down the two equations provided to solve for AG. 2. Set the two equations equal to each other and isolate In Ksp on one side of the equals sign by dividing both sides by (-RT). 3. Create a graph using Excel plotting In Kp on the y-axis and 1/T (make sure the units for temperature are Kelvin) on the x-axis. a. Use the value obtained for the slope of the graph to solve for AH* using the equation derived in Question 2. b. Use the value obtained for the y-intercept of the graph to solve for AS using the equation derived in Question 2. 4. Calculate AG* at 298K using the equation provided in the introduction - this represents the free energy associated with dissolving KHT in water at 25°C. 5. To calculate the standard enthalpy change of a reaction (AHran) for the process KHT (s) KHT (aq) the following equation can be used: AHrxn=AH; (products) - AH; (reactants) Given the following information, calculate AHan and compare to the experimentally obtained value (Question 3a). Calculate the percent error between the theoretical and experimental values. AH; of KHT (s) = -1545 kJ/mol AH, of KHT (aq) = -1497 kJ/mol
Data Table 1: Determination of KHT saturation Temperature I Mass of flask Mass of flask + KHT Mass of KHT Volume of H2O (ML) 25.00 30.00 35.00 40.00 45.00 50.00 Saturation Saturation 1/T Temperature Temperature (1/K) °C 69°C 77°C 68°C 74°C 62°C 36°C (K) 342 K 350 K 341 K 347 K 335 K 309 K 129.5 g 130.5 g 1.000 g Parameter Concentration of KHT in 50.00 mL (from table 1) Volume of solution Mass KCI Moles KCI Concentration KCI Total [K+] in solution = [KHT] Total [HT-] in solution = [KHT] Ksp = [K+]=[HT-] Saturation Temperature (°C) Saturation Temperature (K) 0.00292 1/K 0.00286 1/K 0.00293 1/K 0.00288 Data Table 2: Observation of the common ion effect 1/K 0.00299 1/K 0.00324 1/K Value 0.106 [KHT]= [K+]-[HT-] 0.213 0.177 0.152 0.133 0.118 0.106 50.00 mL 1.000 g 0.0134 mole 0.268 M 0.374 0.106 0.0396 58 °C 331 K Ksp = [K+]=[HT-] 0.0454 0.0313 0.0231 0.0177 0.0139 0.0112 Ln Ksp -3.09 -3.46 -3.77 -4.03 -4.28 -4.49
Solubility Product and Thermodynamic Values Purpose The purpose of this experiment is to determine the solubility product (K) for potassium hydrogen tartrate, or KHT, at several different temperatures in order to calculate various thermodynamic values of the solubility reaction. Introduction The concept of solubility-whether a substance dissolves in water or not-is commonly encountered in chemistry. Solubility rules are often used to determine whether or not a compound is expected to be dissolved in water. However, the solubility of a compound is actually an equilibrium reaction -- compounds can therefore be classified by the degree in which the compound is soluble using equilibrium concepts. For example, solubility rules can be used to determine that silver chloride (AgCl) is insoluble. In fact, AgCl can dissolve in water-it's just a very minute amount. The degree at which a compound can dissolve in water can be noted using the solubility product constant, or Kip- For example, the reaction for the solubility of AgCl in water is: AgCl (s) = Ag+ (aq) + Cl- (aq) Therefore, the expression for the solubility constant is: Ksp = [Ag*][CI] As with any other equilibrium expressions, solids are omitted from the solubility constant expression. All reactions can be characterized by their thermodynamic properties, and dissolving compounds in water is no exception. The change in enthalpy (AH) of a reaction describes the change in internal energy of the reaction, and the sign of this value can help determine whether a reaction is exothermic or endothermic. A negative AH value signifies that heat was given off during a reaction, or that the reaction is exothermic. On the other hand, a positive AH value signifies that heat was required in order for the reaction to occur - therefore, the reaction is endothermic. The change in entropy (AS) of a reaction is a measurement of the disorder in a system. A chemical system will always proceed in a direction in which the entropy of the universe increases. This means that, in lab, a AS value should be positive (or zero, in some cases). Finally, the change in free energy (AG) of a reaction can be calculated using the values for AH and AS at a given temperature. The equation relating AG to the other thermodynamic values is: AG = AH-TAS The sign of AG relates to whether a reaction will be spontaneous or not. When AG is a negative value, the reaction is considered spontaneous. However, if the value of AG is positive, the reaction is considered non-spontaneous. A second equation can be used to relate AG to an equilibrium constant (such as K):
AG = -RT In Ksp Therefore, the value for Keq can be determined at various temperatures in order to obtain values for AG at these same temperatures. By combining the two equations for AG known, the values for AH and AS for the reaction can also be determined. The reaction studied in this experiment is dissolving KHT in water: KHT (s) K+ (aq) + HT- (aq) This compound does not readily dissolve in water; however, can be forced into solution by applying heat to the reaction. As the solution cools, it will reach a temperature at which the KHT will begin to precipitate back out of solution - this temperature is known as the saturation temperature, and is the temperature that should be recording during the experiment.