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# Calorimetry

## Heat of Solution of Calcium Hydroxide

### Concepts

Calcium hydroxide is commonly known as slaked lime and is spread on soil to raise the pH of the soil (if the soil is too acidic).

In determining the heat of solution of ammonium nitrate and sulfuric acid, it was possible to directly dissolve the analyte in pure water and measure the heat for this process. Unlike ammonium nitrate and sulfuric acid, however, calcium hydroxide has a very low solubility and does not dissolve to any great extent when treated with water. Consequently, it is not possible to directly measure the heat of solution of calcium hydroxide.

Bases like Ca(OH)2 are very soluble in acids owing to the neutralization reaction, which removes hydroxide ion from the system.

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

H+ (aq) + OH- (aq)   →   H2O (l)

In performing a calorimetry experiment in which calcium hydroxide is dissolved in an acidic aqueous solution, one must account for both the heat of the solubility reaction (qsoln) and the heat of the neutralization reaction (qneut). The heat balance equation is

0 = qcal + qwater + qsoln + qneut

### Experiment

Objective:

• Determine the molar enthalpy of solution of calcium hydroxide.

Approach:

• Determine the molar enthalpy of neutralization using the Heat of Neutralization experiment.
• Using calorimetry, determine the molar enthalpy of solution by dissolving calcium hydroxide in an aqueous HCl solution and accounting for the heat of neutralization in the calculations.

The calorimeter is initially filled with 100.0 mL of 6 M aqueous HCl.

Enter the value for the mass of calcium hydroxide (FW 74.093) to use in this experiment.

During the course of the calorimetry experiment, the temperature is sampled at 0.5 sec intervals and automatically plotted on the graph. The temperature vs time data is listed at the right. You will need to perform extrapolations to the initiation time for the reaction (5.00 sec) to determine the initial and final temperatures for the experiment, just as was performed in the Heat of Solution of Sulfuric Acid experiment.

The heat capacity of the calorimeter is 183. J oC-1

The specific heat capacity of the aqueous solution is 4.184 J oC-1 g-1.

Perform the Heat of Neutralization experiment to determine the molar enthalpy of neutralization.

Use the data from this experiment and the molar enthalpy of neutralization to calculate the molar enthalpy of solution for calcium hydroxide. (Note that each mole of calcium hydroxide releases two moles of hydroxide ion, which are neutralized by the acid.)

Perform the experiment two or three times and compare the values you obtain.

How reproducible is the experimentally determined molar enthalpy of solution for calcium hydroxide? What factors limit the reproducibility?

Initial Baseline Final Baseline
Mass of Ca(OH)2: g

slope:     intercept:

slope:     intercept:

Heat of Combustion of Methane