Thermodynamics of Ca(OH)2

The purpose of this experiment: to measure the solubility of calcium hydroxide by titration with hydrochloric acid at two different temperatures and to use these solubilities to estimate Ksp, ΔGº, ΔHº, and ΔSº.

Background

The solubility of hydroxides is easily determined by titration. From the molar solubility, the solubility equilibrium constant, Ksp, can be calculated thus the ΔGº for the dissolution of the material can be determined. Using ΔGº at two different temperatures and noting that the ΔHº and ΔSº are relatively invariant over small temperature ranges, the enthalpy and entropy changes can also be estimated. With knowledge of these constants, ΔGº and Ksp can be calculated at any temperature between 0 °C and 100 °C.

Reaction
The solubility of an ionic substance in a solvent can be thought of as a reaction where the solid dissociates into ions in solution. In this experiment, the solubility of calcium hydroxide will be determined, as suggested in the reaction shown below.

Ca(OH)2(s) Ca2+(aq) + 2OH-(aq)

Once equilibrium has been established and the solid removed, the concentration of hydroxide ion can be easily determined via an acid/base titration. Hydrochloric acid will be used to find the concentration of OH-(aq).

HCl(aq) + OH-(aq)  H2O(l) + Cl-(aq)

Note that the molar solubility of Ca(OH)2 is half of the OH- concentration found by titration.

Ksp = [Ca2+][OH-]2

The calcium ion concentration is half the hydroxide ion concentration determined by the titration.

The Gibbs Free Energy, ΔGº can then be determined at two temperatures, and then ΔH° and ΔS° can be determined. We assume that ΔH° and ΔS° do not change significantly with temperature which is usually true for this small a temperature change. Using the free energies, ΔG° found for two different temperatures, ΔH° and ΔS° are found by solving for two unknowns in two equations.

Procedure

1. Record the concentration of the standard HCl provided in the laboratory.

2. Find the solubility of calcium hydroxide in water at room temperature. A solution of calcium hydroxide will be stirring at room temperature in the laboratory. Draw off approximately 30 mL of this solution and record the temperature. Filter the solution through a piece of filter paper placed in a funnel. Do NOT wet down the filter paper with DI water. If you want to wet it down, use some of your calcium hydroxide solution.  Pipet 3.0 mL of the filtrate into a clean test tube and a few drops of phenolthalein indicator. Measure the exact mass of the test tube. Using a pipet, add standardized (~0.1M) HCl dropwise to the solution until the pink solution disappears. Measure the new mass and determine the mass (then volume) of HCl added. Repeat thrice more.

3. To prepare a 100 °C saturated calcium hydroxide solution, bring 100 mL of distilled water to a boil in a 250 mL beaker. After the water has been boiling for several minutes, add about 2 g of Ca(OH)2 to the water and keep it near boiling with occasional stirring until needed.

4. Find the solubility of calcium hydroxide in the hot solution. On a hot plate, bring your hot solution to a gentle boil for about two minutes, the fill a small test tube with the solution. Give the test tube to your instructor who will run it through the centrifuge. When done, measure the temperate of the solution and quickly draw off a 2.0 mL sample and place it in a reaction tube. Set up your other reaction tubes at the same time. Add a few drops of phenolthalein to each tube, and titrate with the HCl solution. When the solution is cool, titrate with the standard HCl solution until the pink disappears. Repeat thrice more.

Lab Report: Full boat including introduction, experimental, results, and discussion sections.

Pre-lab Question:

Consider the solubility of Sr(OH)2 as given below. Two solutions of Sr(OH)2 are allowed to equilibrate: one at 0 °C and the other at 25 °C. A 10.00 mL aliquot (a fancy term for 'portion') of each solution is titrated with 0.2000 M HCl. 3.37 mL of the acid are required for the 0 °C solution; 62.90 mL for the 25 °C solution. From this information, calculate the amount of hydroxide ion present originally from the Sr(OH)2. (VHCl → molHCl → molOH → [OH]) You can then determine the [Sr+2] in the original solution. You can then determine the value of Ksp for Sr(OH)2. Using ∆G = -RTlnKeq, you can calculate ∆G for the reaction at that temperature. Doing the whole calculation again for the other temperature will give another value of ∆G. The equation ∆G = ∆H - T∆S can be setup twice (once for each temperature) and the values of ΔH° and ΔS° for the reaction can be determined (2 equations, 2 unknowns)

~MEO 04.07.04 08:43