Ksp of Calcium Hydroxide1
Author: J. M. McCormick
Last Update: Ocober 13, 2013
An equilibrium constant, K, is related to ΔG for a process through Eqn. 1. And as long as ΔH and ΔS are independent of temperature, Eqn. 1 can be combined with Eqn. 2 to determine ΔH and ΔS from the temperature dependence of K.
|ΔG = –RT ln K||(1)|
|ΔG = ΔH – TΔS||(2)|
In this exercise the equilibrium constant (Ksp) for the dissolution of Ca(OH)2 in water, Eqn. 3, will be studied as a function of temperature to determine ΔH0, ΔS0, and ΔG0 for the reaction.
Carbonate ion potentially interferes with this determination through the competitive formation of CaCO3. However as noted elsewhere,1 this has a negligible effect on the Ksp determination, and one which will be further minimized by using carbonate-free water to prepare the solutions. A more significant problem is the formation of the [Ca(OH)]+ ion pair in solution.2
Temperature Dependence of Ksp
Place 100.00 g carbonate-free water3 and ~2-3 g of Ca(OH)2 in a covered Erlenmeyer flask or a loosely-capped glass bottle. Place in water bath at the appropriate temperature, or heat on a hot plate, stirring occasionally, until equilibrium is achieved, which may be only a few minutes near 100 °C to 24 hr near room temperature. Note that there must be solid Ca(OH)2 present at all times; if it appears that of the Ca(OH)2 has dissolved, add more.
Quickly gravity filter each solution to remove excess Ca(OH)2 such that the temperature of the solution does not change appreciably during the filtration. Record the temperature of the solution and then allow it to cool to room temperature. Take a 10.00 mL aliquot of the filtered solution, add 25 mL distilled water and a 1 drop of bromothymol blue. Titrate with standardized 0.01 M HCl (click here for the standardization procedure).
Effect of Ionic Strength on Ksp
Prepare four 100-g solutions each with an ionic strength varying from about 0.001 to about 0.500 using KNO3 to adjust the ionic strength, I.4 You may need to use your results from the initial Ksp determination as an aid to selecting the lower value of the ionic strength. From these KNO3 solutions prepare saturated Ca(OH)2 solutions at room temperature, as described above. Filter and titrate each of these samples, as described above, to determine the Ksp of Ca(OH)2 at each ionic strength.
Results and Analysis
From the titration results determine the molality of OH– and Ca2+ in each solution, and from these the Ksp at a particular temperature and ionic strength. Repeat the Ksp calculation for the data taken at different temperatures, but use the Debye-Hückel limiting law, or other method, to calculate the activities of the ions in solution.4,5 Is there a significant difference between the Ksp values calculated including and neglecting activities?
Prepare a graph of the Ksp data (you choose the method so that you obtain the most accurate results possible) as a function of temperature to determine ΔH0, ΔS0, and ΔG0 at 25 °C for the reaction. Report all results at 95% confidence and perform an error analysis. Comment on the chemical meaning of the results.
Graph Ksp as a function of I and comment. Is there evidence for ion pair formation?
1. Euler, W. B.; Kirschenbaum, L. J. and Ruekberg, B. J. Chem. Educ. 2000, 77, 1039-1040. Click here for a PDF version of this article (Truman addresses and J. Chem. Educ. subscribers only).
2a. Chen, X.; Izatt, R. M. and Oscarson, J. L. Chem. Rev. 1994, 94, 467-517. Click here for a PDF version of this article (Truman addresses and Chem. Rev. subscribers only).
b. Stability Constants of Metal-Ion Complexes; Sillén, L. G. and Martell, A. S.; Eds.; Special Publication 17; The Chemical Society: London, 1964, p. 42.
c. Martell, A. E. and Smith, R. M. NIST Standard Database 46, Vers. 6.0; National Institute of Standards and Technology: Gaithersburg, MD, 2001.
d. Martell, A. E. and Smith, R. M. Critical Stability Constants, Vol. 4; Plenum Press: New York, 1974, p. 1.
3. Carbonate-free water is prepared by boiling high-purity water (either distilled or prepared by reverse osmosis followed by ion exchange and filtration) under inert gas purge and then cooling under inert gas.
4. Atkins, P. and de Paula, J. Physical Chemistry, 8th Ed.; W. H. Freeman: New York, 2006, p. 163-166.
5. Manov, G. G.; Bates, R. G.; Hamer, W. J. and Acree, S. F. J. Am. Chem. Soc. 1943, 65, 1765-1767. Click here for a PDF version of this article (Truman address and J. Am. Chem. Soc. subscribers only).