Thermodynamics of the Dissolution Borax

Thermodynamics of the Dissolution of Borax Lina Jawadi Objectives: Study a system of salt and water solution. Determining a variety of important thermodynamics quantities from the solubility information at various temperatures.

Background: The salt and water solution in this experiment has a relatively simple solubility equilibrium of borax in water.

Na2B4O7 . 10 H2O 2Na + + B4O5(OH)42- + 8H2O.

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This reaction is an equilibrium process and 8 water molecules from the hydrated salt are lost to the reaction medium.

The equilibrium constant expression for this reaction is

K= [Na+]2 [B4O5(OH)42-] [H2O]8 [Na2B4O7 . 10 H2O

In this experiment we will always make sure there is some solid borax remaining in the sample mixture before removing some of it to analyze it. Therefore, we can assume that the concentration of solid borax is constant. In addition, the water molecules which were originally part of the borax’s crystalline matrix are lost to the sample mixture; however, it doesn’t significantly affect the concentration of the water. The equilibrium constant expression can now be simplified to become:

K = [Na+]2 [B4O5(OH)42-]

The first equilibrium expression and the balanced solubility equilibrium reaction allow us to express either borate ion or sodium ion in terms of the other. So, it is possible to determine the constant in terms of either ion. After substituting borax ion in place of the sodium ion,

([Na+]=2[B4O5(OH)42-], K = (2 [B4O5(OH)42-] )2*[B4O5(OH)42-] )we get:K=4 [B4O5(OH)42-]3.

Finding the concentration of borate ion in any sample at any given temperature gives us the solubility product at that temperature.

Equipment: 5mL pipet , 250 mL beaker.

Procedure:

1. Using the pipet add 5mL of distilled water to 10 test tubes.
2. Mark the level with the marker and pour the water out. Mark the test tube with the assigned temperature.
3. Weigh 30-32g of solid sodium borate decahydrate (borax) in a 250 mL beaker and then add 150 mL of water.
4. Place the mixture on a hot plate, but don’t allow its temperature to exceed 50oC.
5. After all the borax dissolves, add more and let the temperature reach 45oC.
6. Once it starts slightly exceeding 45oC, remove it from the hot plate and place instead a beaker filled with 150 mL of distilled water.  Continue stirring the mixture until it reaches the desired temperature.
7. Quickly pour 5 ml of the mixture into two test tubes without transferring any solid borax. (record the temperature before and after the transfer.
8. When the water bath reaches 45oC, place the test tube in it until precipitation of borax has dissolved.
9. In a flask with 50-75 mL of water and 10 drops of bromocresol green indicator, add 125 mL of the borax solution.
10. Before titrating the borax with acid make sure that it has a blue color. Using the buret filled with 50 mL oh HCl, add the acid to the borax until it reaches the endpoint, which is when the solution turns from a blue color to a yellow. Record the initial and final volumes. (For calculations: find the moles of borax using the volume of HCl and its molarity. Then find the solubility product constant and plot the ksp vs. (1/t) graph and from it obtain the values of? H and? S. Make sure the temperature is in kelvin when doing the calculations. Finally, find the value of? G using both equations and plot the?

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