Question

What is an unusual thermodynamic property of calcium hydroxide?

How does this property make it possible to perform this particular experiment?

*this is a prelab for thermo and ksp

Answer 1

Calcium hydroxide, Ca(OH)₂, is an ionic solid that is only slightly soluble in water.

A calcium hydroxide solution is also referred to as limewater. A saturated solution of calcium hydroxide has the solid in equilibrium with its ions as shown below:

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

Recall that a saturated solution is a solution that contains the maximum amount of dissolved solute possible at a given temperature. (The solution contains undissolved solute in equilibrium with the solution.)

Since calcium hydroxide is only slightly soluble in water, it is a difficult base to classify. It is often assumed that since calcium hydroxide has a low solubility that it is a weak base. But, don’t forget that it contains hydroxides ions, which automatically makes it a strong base! In fact, the pH of a saturated calcium hydroxide solution is about 12.4. Thus we can classify a saturated solution of calcium hydroxide as a dilute solution of a strong base.

In this type of equilibrium, the equilibrium constant is called the solubility product, and is represented by the symbol K_sp. Whenever you see the symbol K_sp it refers to a solubility equation, written with the solid to the left of the equilibrium sign, and the dissolved products to the right. The K_sp for this reaction will be:

K_sp = [Ca²⁺][OH^-]²

(The solid state is not included in a K_sp expression since it is a pure substance and cannot be expressed as a concentration).

Note that the rate of dissolving is equal to the rate of precipitation in a saturated solution equilibrium.

Note that substances that have a large K_sp value have a higher solubility (more dissolved ions); and substances with a small K_sp value have a lower solubility (few dissolved ions).

The Ksp provides us information on the Thermodynamic properties of the equilibrium between solid calcium hydroxide and its ions in solution.

Ca(OH)2 ↔ Ca2+(aq) + 2 OH−(aq).

Once the Ksp is determined, you can calculate the free energy with the following equation:

ΔG° = − RT ln Ksp.

After determining ΔG°, we can use that value to calculate ΔH° and ΔS° with the following expression:

ΔG° = ΔH° − TΔS°.