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. This allows your to determine ΔHº
and ΔSº 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, ΔGº, ΔHº
and ΔSº. 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
ΔGº =
-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
ΔGº = ΔHº - TΔSº, knowing ΔGº at two temperatures
allows you
to calculate ΔHº and ΔSº--two equations and two
unknowns. Ahhh, algebra. Yummy.
5 min screencast of 'concept' of experiment
Procedure
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 ΔHº and ΔSº 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