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, ΔG. This allows your to determine ΔH and ΔS for the specific chemical reaction

This experiment is a paragon of elegant simplicity and ingenious breadth of concept. This semester, you have studied equilibrium, free energy, enthalpy, and entropy and the relationships between them. In this experiment, you will use relatively simple measurements to determine the equilibrium constant, ΔG, ΔH and ΔS. So for the solution of a sparingly soluble compound, Ca(OH)2;
Ca(OH)2(s) <--> Ca2+(aq) + 2 OH-(aq)
In the first part of this experiment, you will take a sample of a saturated Ca(OH)2 solution at room temperature, note the temperature, filter it, take out a 10 mL aliquot of the filtered solution in a graduated pipet, transfer it to a clean 125 mL Erlenmeyer flask, add 25 mL of distilled water and titrate it with standardized hydrochloric acid, with a piece of white paper under the flask to improve visibility. (Don't forget to rinse the buret with the solution three times before filling it.) Why do you filter it? [Because solid will dissolve when acid is added.] When you add the bromothymol blue to the Ca(OH)2 solution, it will turn blue. When you add acid, it will begin to turn green. When it is "Stop-light Green," the pH is about 7, the expected endpoint for the reaction of a strong acid with a strong base. You may consider the endpoint to be when it turns yellow.(Note: strong refers to dissociation not concentration.) Record the initial volume of hydrochloric acid in your buret. Take your final volume of acid at this color. The difference between the buret's final volume and the initial volume is the volume of HCl you will fill in on your data sheet. Repeat the titration twice. Repeat it again if there is poor agreement of the results.
From the volume of hydrochloric acid, you can determine the moles of hydrochloric acid used to neutralize the Ca(OH)2. The reaction is
Ca2+(aq) + 2 OH-(aq) + 2 HCl(aq) ---> Ca2+(aq) + 2Cl-(aq) + 2 H2O(l)
So you can determine the concetrations of Ca2+ and OH- and calculate the Ksp for Ca(OH)2
Ksp = [Ca2+][OH-]2

The free energy is related to the equilibrium constant by the equation
ΔG = -RTln(Ksp)
where R = 8.314 J/mol•K and T is the temperature in degrees Kelvin.

In the next part of the experiment, you will titrate a saturated solution of Ca(OH)2 at a higher temp ( ~90oC). By the same method just discussed, you can calculate the Ksp and ΔG for this higher  temperature.
Because ΔG = ΔH - TΔS, knowing ΔG at two temperatures allows you to calculate ΔH and ΔS--two equations and two unknowns. Ahhh, algebra. Yummy.

5 min screencast of 'concept' of experiment 


1. 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 40 mL of this solution and record the temperature. Filter the solution using a long stem funnel. Pipet 10.00 mL (volumetric pipet) of the filtrate into a clean 125 mL Erlenmeyer flask and add 25 mL of distilled water and ~10 drops of bromothymol blue indicator. Titrate with the standard HCl solution (~0.02 M) until the yellow endpoint. Repeat twice more.
2. Find the solubility of calcium hydroxide of a hot solution. There will be a boiling hot Ca(OH)2 solution in the lab as well. Carefully (using a centrifuge) spin out ~12 mL, note the temperature from your thermometer and immediately draw off 10.00 mL of the solution, and place in a 125 mL flask. Process (read: add water, indicator, titrate, etc.) as with room T runs. You will only be doing one high T run. 

Lab Report: Short form memo. Last one of term? Your goal, remember is to get ΔH and ΔS of a chemical reaction, so you best include that equation in your report.

Pre-lab Question (worth 5 points):

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) Box your final answers for ΔH and ΔS

~MEO 03.26.08