formal Report

Topics: Chemistry, Chemical equilibrium, Solubility Pages: 12 (2194 words) Published: September 16, 2014


Simultaneous Determination of Several Thermodynamic Quantities: K, ∆G°, ∆H°, and ∆S°
Submitted by: Feldan P. Villarta
Submitted to: Mrs. Gloria Jesusa D. Baltazar
Chemistry 73 (Laboratory)
August 11, 2014

Simultaneous Determination of Several Thermodynamic Quantities: K, ∆G°, ∆H°, and ∆S°
Feldan P. Villarta

I. INTRODUCTION
The solubility product constant, Ksp​, is the equilibrium constant for a solid substance dissolving in an aqueous solution. It represents the level at which a solute dissolves in solution. A more a substance dissolves, the higher the Ksp value it has.

In this experiment, a system of a sparingly soluble salt in water is studied. From the solubility information at various temperatures, the changes in standard enthalpy, standard entropy, and standard free energy were established. II. THEORETICAL BACKGROUND

The reaction that is studied in this experiment is the dissolution of borax in water. “Borax” is a naturally occurring compound; it is in fact the most important source of the element boron, and it has been used for many years as a water softening agent. Borax is a rather complicated ionic salt which has the chemical formula Na2B4O7•10H2O (Petrucci, 2007). When it dissolves, it dissociates as follows:

Na2B4O7 • 10H2O(s) 2Na(aq) + B4O5(OH)42(aq) + 8H2O(l) rnx (1)

The solubility product expression for this system is written below. Ksp = [Na]2 [B4O5(OH)42] (2) To determine a value for the solubility product, a method must be found to assay either the amount of sodium ion, or borate ion, in the sample mixture. The original equilibrium expression, and balanced solubility equilibrium reaction, leads to a convenient way to express either ion in terms of the other, so that it is possible to define Ksp in terms of either the concentration of sodium ion, or concentration of borate ion (Chang, 2010). A series of substitutions, based on the original balanced solubility equilibrium equation, gives the desired Ksp, expression defined in terms of the borate ion only: [Na] = 2 [B4O5(OH)42]

K = [ (2 [B4O5(OH)42] ) ]2 [B4O5(OH)42]
Ksp = 4 [B4O5(OH)42]3 (3) Finding the concentration of borate ion, in any sample at any given temperature, leads directly to a value for solubility product (Ksp) at that temperature. The concentration of tetraborate (B4O5(OH)42) can be determined through titration. Tetraborate is a weak base, so it can be titrated with a strong acid (Brown, 2012). The reaction for the titration is written as follows:

B4O5(OH)42– (aq) + 2 H3O+ (aq) + H2O (l)  4 H3BO3 (aq) (4)
The volume of the acid used can then be used to calculate the concentration of tetraborate using the formula written in the next page.

Where Mtetraborate = concentration of tetraborate
(M x V)acid = the product of the concentration and volume of the acid used
Vtetraborate = volume of borax used
From the Ksp value, the Gibb’s energy can be calculated The relationship between the Ksp and Gibb’s energy is:
∆G° = -RT lnKsp (6)
where R is the gas constant (8.314 J/K.mol) and T is the absolute temperature (Brown, 2012). From the definition of Gibb’s energy the following can be written:
∆G° = ∆H° - T∆S° (7)
(8)
Clearly, ∆G° is a function of temperature. A plot of ∆G° vs T should yield a straight line with a slope of -∆S° and intercept of ∆H°. Likewise, a plot of ln K vs 1/T should also generate a straight line with a slope of -∆H°/R and intercept of ∆S°/R (Chaka & Madhugiri, n.d.). III. METHODOLOGY

A. Materials/Apparatuses/Chemicals
The materials/apparatuses/chemicals that were used in this experiment are as follows: analytical balance, volumetric flask, stirring rod, thermometer, beaker, hot plate, iron ring, iron stand, test tube,...

References: Brown, T. et.al. (2012). Chemistry: The Central Science. 12th ed. United States of America: Pearson Education, Inc.
Chaka, G. & Madhugiri S. (n.d.) Determination of Thermodynamic Quantities for a Chemical Reaction
Chang, R. (2010). Chemistry. 10th ed. 1221 Avenue of the Americans, New York: McGraw- Hill Companies, Inc.
Petrucci, Ralph H., et al. General Chemistry: Principles and Modern Applications. Upper Saddle River, NJ: Prentice Hall 2007.
Weisstein, E. W. (2014). Roundoff Error. Retrieved July 22, 2014 from the world wide web: http://mathworld.wolfram.com/RoundoffError.html
Experiment 17. Thermodynamics of Borax Solubility
Chemistry 212 Lab: Simultaneous Determination of Several Thermodynamic Quantities: K, ∆G°, ∆H°, and ∆S°
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