Solubility and Spontaneity0

Solubility and Spontaneity0 - Solubility and Spontaneity...

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Solubility and Spontaneity OBJECTIVE: You will be using your understanding of acids, bases, and titration to determine the solubility product (K sp ) of a compound commonly called Borax. The free energy, G, of a reaction and the equilibrium constant (like K sp ) depend on temperature. By varying the temperature of Borax solutions and measuring K sp , you will be able to calculate the thermodynamic parameters of the Borax dissolution reaction. BACKGROUND: Borax, or tetraborate decahydrate, has the empirical formula Na 2 B 4 O 7 . 10H 2 O. The water is not covalently bound, but forms a complex with the molecule (hydrate). The structure of borax is actually: The dissolution reaction of borax in water is: Eqn 1: Na 2 [B 4 O 5 (OH) 4 ] . 8H 2 O(s)   2Na + (aq) + B 4 O 5 (OH) 4 2- (aq) + 8H 2 O(l) The tetraborate anion is a weak base. It reacts with water (hydrolyzes) to form the hydroxide ion (OH - ) , a strong base (and boric acid, H 3 BO 3 , an extremely weak acid whose effect on pH is negligible): Eqn 2: B 4 O 5 (OH) 4 2- (aq) + 5H 2 O(l) 4H 3 BO 3 (aq) + 2OH - (aq) The amount of tetraborate in solution is precisely related to the amount of OH - . We can determine the amount of OH - ions in a solution of borax by titration with a strong acid (HCl). We will use bromcresol green as an indicator to tell us when we’ve reached the endpoint of the titration. In the titration, the hydroxide ions (OH - ) react with H + . According the LeChatelier’s principle, as the hydroxide (OH - ) is removed from solution by reaction with H + , the tetraborate [B 4 O 5 (OH) 4 2- ] anion will continue to hydrolyze (react with water) to replace
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hydroxide ions. Eventually, all of the tetraborate will have hydrolyzed and the H + ion will have reacted with a stoichiometric amount of hydroxide. We can calculate the amount of OH - in the borax solution from the amount of H + used in the titration. The amount of OH - can then be related to the tetraborate concentration in a saturated solution. Once the tetraborate concentration is known, we can calculate the sodium ion (Na + ) concentration, and then calculate K sp for borax using the equilibrium expression for the dissolution reaction. Na 2 [B 4 O 5 (OH) 4 ] . 8H 2 O(s)   2Na + (aq) + B 4 O 5 (OH) 4 2- (aq) + 8H 2 O(l) Eqn.3 K sp = [Na + ] 2 [B 4 O 5 (OH) 4 2- ] By performing titrations of saturated borax solutions at different temperatures, we can measure the temperature dependence of K sp , and hence the determine the thermodynamic parameters G, H, and S: Eqn.5 G o = H o – T S o and G o = -RTlnK (T in Kelvins, R = 8.3145 J/K . mol)
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Solubility and Spontaneity0 - Solubility and Spontaneity...

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