Borax

Lab: Thermodynamics of Saturated Borax Solutions

Background: Borax is commonly called sodium tetraborate decahydrate and has the formula Na2B4O7•10H2O. However, based upon its chemical behavior as the sodium salt with the conjugate base of weak boric acid, a more descriptive formula for borax is Na2B4O5(OH)4•8H2O, also called tincal. Borax dissolves and dissociates in water according to the equation below:

Na2B4O5(OH)4•8H2O(s) ( 2Na+(aq) + B4O5(OH)42-(aq) + 8H2O(l)

At equilibrium, the solubility-product constant for borax is determined by: Ksp = [Na+(aq)]2 [B4O5(OH)42-(aq)]

The borate ion, B4O5(OH)42-(aq), can accept two protons from a strong acid in an aqueous solution. Therefore, the molar concentration of the borate anion in a saturated solution can be determined during a neutralization titration with standardized hydrochloric acid. The concentration of the sodium ions in the saturated solution will be twice the molar concentration of the borate ions.

The free energy of the dissolution of borax is related to the equilibrium constant by the equation:

(Gº = -RT ln Ksp , where R = 8.314 x 10-3 kJ/mol•K and T is the Kelvin temperature

The free energy is also related to the changes in enthalpy ((Hº ) and entropy ((Sº) by the equation:

(Gº = (Hº - T(Sº

Setting these two free energy equations equal to each other gives the expression: -RT ln Ksp = (Hº - T(Sº

Re-arranging and solving for the ln Ksp results in an equation in the form of a linear relationship, y = mx + b. By plotting a graph of ln Ksp versus 1/T, the slope of the line equals -(Hº/R and the y-intercept equals (Sº/R.

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Problem: Can you determine the Ksp values for borax at various temperatures, then use this information in order to determine the free energy, enthalpy and entropy changes for the dissolution of solid borax?

Standardization of HCl Procedure:

1. Dry 2-3 g of the primary standard, anhydrous Na2CO3(s), for several hours in a drying oven...