TEMPERATURE DEPENDENCE OF THE SOLUBILITY OF CA(OH)2

  experimentally determine how temperature influences the solubility equilibria of a salt with low solubility, Ca(OH)2.

  experimental determination of the solubility product constant, Ksp, at two temperatures will be employed to determine ΔHº, ΔSº, and ΔGº for this reaction:

Ca(OH)2 (s) ⇔ Ca2+(aq) + 2 OH-

(aq) (1)

(2)

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Ksp = Ca 2 +[ ] OH −[ ] 2 = 4S 3

S = the molar solubility of the compound (note: S = [Ca2+], but [OH-] = 2S).

Determination of Ksp at Room Temperature

  Retrieve ~50 mL room temp. sat’d Ca(OH)2 (aq)   Record the temperature of the solution.   Filter the solution using qualitative filter paper

  Carefully pipet 10 mL of the FILTERED saturated calcium hydroxide solution into an Erlenmeyer flask.   Add 25 mL of distilled water + 1-2 drops of

bromothymol blue indicator.   Titrate the solution with standard HCl (aq)

  Three replicates within 1% agreement

Determination of Ksp at High Temperature   Prepare ~100 mL high temp. sat’d Ca(OH)2 (aq).

  Boil ~100 mL of water (removes dissolved CO2)   Add ~2 g Ca(OH)2 (s). Stir while boiling the solution.   Record the temperature and decant ~50 mL and filter as

rapidly as possible

  Immediately pipet four 10 mL aliquots of the filtered solution into four clean containers   add 25 mL of water and bromothymol blue indicator

  Titrate the solution with standard HCl (aq)   Three replicates within 1% agreement

Calculate expt’l. Ksp values in lab!

  the volume of HCl (of known concentration) is used to calculate the quantity (moles), and concentration (M), of the OH- present in the 10.00 mL aliquot of saturated Ca(OH)2 (aq)

  The calcium ion concentration is one half of the hydroxide concentration.

  You might know what a “good value” of Ksp should be at room temp. (You will look this up, right?)

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Ksp = Ca 2 +[ ] OH −[ ] 2 = 4S 3

Note: most commonly the experiment yields results that are a bit high.

Variation of Ksp with Temperature

  Equilibrium constants vary with temperature.   the variation depends upon the relative magnitude

and sign of the standard enthalpy change (ΔH°)   the value of K also depends on the standard

entropy change (ΔS°) of the equilibrium.   The linear relationship that describes this

relationship is known as the van’t Hoff equation:

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lnK =−ΔH o

R

1T

+ΔSo

R

The rest of the math . . .

  A graph of lnK (y-axis) vs. 1/T (x-axis) has a slope related to the ΔHº and a y-intercept related to ΔSº for the equilibrium.

  The values of ΔHº and ΔSº can be used to find ΔGº at 25°C (298 K) with this equation:

  and Ksp at 25°C (298 K) since we know:

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ΔGo = ΔH o −TΔSo

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ΔGo = −RT lnKsp

y = 1847.6x - 16.715 R² = 0.99986

-12

-11.8

-11.6

-11.4

-11.2

-11

-10.8

-10.6

-10.4

-10.2 0.0024 0.0026 0.0028 0.003 0.0032 0.0034 0.0036

ln K

sp

1/T in K-1

Experimental vs. Accepted   Complete the report form and submit with your graph   Experimental:

  Use the slope to find ΔHº and the y-intercept to find ΔSº for the solubility equilibrium (rxn. 1)

  Use these values to find ΔGº and Ksp for the equilibrium at 25°C (298 K)

  Accepted (these should be tabulated at 25°C):   Use thermodynamic tables and the standard math to find ΔHº, ΔSº, and ΔGº for the solubility equilibrium (rxn. 1)

  You can also look up Ksp