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The purpose of this laboratory is to measure the enthalpy (AH) and entropy (AS) of the dissolution of borax (sodium tetraborate decahydrate) in water. Enthalpy and entropy will be measured using the temperature dependence of the solubility product constant (Ksp) for the dissolution of borax in water. A graph will be made to determine the enthalpy of the dissolution process. Once the enthalpy of the dissolution of borax is found it will be possible to solve for the entropy of dissolution of borax in water. Introduction The solubility of a salt is dependent on the temperature of the solution. When equilibrium is established in a saturated solution at a specific temperature, the rate of formation of ions in solution is equal to the rate of deposition of solid. The equilibrium constant for the dissolution of a solid in a solvent is called the solubility product constant (Ksp). It is equal to the product of the concentration of ions in solution. Since the concentration of ions can change with temperature, the equilibrium constant for the reaction is temperature dependent. Sodium tetraborate decahydrate ("borax") dissociates in water to form sodium ions, borate ions and water: Na2B407 10 H20 (s) 2 Na+ (aq) + B4O5(OH)42- (aq) + 8 H20 (1) The equilibrium constant expression is: Ksp = [Na+)?[B4O5(OH)42-] (11.1) Note that there are two sodium ions produced for each borate ion (B4O5(OH)42-) in t reaction. (You need to remember the stoichiometry for your calculations!) Thus, t equilibrium constant for the reaction can be determined if the total borate concentration is known.
The changes in free energy (AG), enthalpy (AH), and entropy (AS), are related to the equilibrium constant (K or Kup) for a chemical reaction at temperature, T, by the following equation: AG' = -RxTx In K = AH - TAS where R is the universal gas constant and is equal to 8.314 J/mol K. Equation 11.2 can be rearranged to eliminate agº and solve for the natural logarithm of K (In K) in terms of AHO, AS and T: (11.2) AS + R In K ΔΗ". (11.3) R Note that equation 11.3 resembles the equation for a line, y = mx + b, so that a plot of In K versus 1/T should result in a straight line with a slope equal to -AH/R and a y-intercept equal to 45%R. In this experiment, you will need to first determine the concentration of borate ion. (B4Os(OH)22'). To do this, you need to recall that since borate ions are bases, they can react with acids. Thus, their concentration can be determined by a simple acid- base titration requiring two moles of H+ ions for each mole of borate ion: B4Os(OH)42 + 2 H+ + 3 H2O → 4 B(OH)3 (Again, you need to remember your stoichiometry for your calculations.) If a set of saturated borax solutions are formed at various temperatures and titrated to determine borate ion concentration, the data can be used to compare Ksp values at each temperature. A plot of in Ksp verses 1/T will have a slope -AH/R. Therefore, the slope can ha lead to onlarlate AU and the internant non ha riced to determine AS
Pre Lab Questions 1) Explain, clearly and concisely, why it is important to not transfer solids when transferring the borax solution. 2) Explain, clearly and concisely, how your experimental results will be affected if the solution solidifies in the transfer pipet when transferring the borax solution