Calorimetry

Heat of Solution of Calcium Hydroxide

Lime is a common name for calcium oxide. 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). The term slaked simply means the lime was treated with water to convert the CaO to Ca(OH)₂.

In determining the heat of solution of ammonium nitrate, it was possible to directly dissolve the solid in pure water and measure the heat transfer for this process. Unlike ammonium nitrate, however, calcium hydroxide has low solubility in water. Consequently, it is not easy to directly measure the heat of solution of calcium hydroxide.

In this experiment, the heat of solution of calcium hydroxide will be measured indirectly. 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 flow when the solid dissolves ( q_soln = heat of solution ) and the heat flow of the neutralization reaction ( q_neut ). The heat balance equation is

0   =   q_cal   +   q_aq   +   q_soln   +   q_neut

The molar heat of neutralization is   ΔH_neut = - 55.83 kJ mole ^-1 . From this value and knowledge of the amount of hydroxide released, one can computer q_neut .

The other heat flow terms are associated with the change in temperature of objects:   q_cal for the calorimeter and q_aq for the aqueous solution. These heat flows are calculated from heat capacities and the temperature change for the system.

From this point, one can solve for q_soln and calculate the molar heat of solution (or molar enthalpy of solution), ΔH_soln. The moles of calcium hydroxide that dissolve are represented by n .

Hess's Law

Conceptually, there is another way one can use calorimetry data to obtain the heat of solution of calcium hydroxide. If one dissolves calcium hydroxide solid in an acid (such as HCl), the overall chemical reaction is

Ca(OH)₂ (s)   +   2 H ⁺   →   Ca²⁺ (aq)   +   2 H₂O (l)

The calorimetry heat balance equation for this reaction is

0   =   q_cal   +   q_aq   +   q_rxn

In this equation, q_rxn is the heat flow for the reaction where calcium hydroxide solid reacts with hydrogen ion to form water. q_rxn is related to the heat flows for the dissolution of Ca(OH)₂ and the neutralization of the released OH ^- ion.

q_rxn   =   q_soln   +   q_neut

The molar enthalpy of reaction, ΔH_rxn is defined as

Hess's Law states that the enthalpy change for a reaction does not depend upon the pathway by which the reaction occurs. Thus one could calculate ΔH_rxn from the direct reaction of Ca(OH)₂ and H ⁺ to form H₂O. Or one could envision the reaction as a two-step process, as shown at the top of the page, where Ca(OH)₂ dissolves to give Ca²⁺ and OH ^- followed by the reaction of OH ^- with H ⁺. Either way, one arrives at the same enthalpy change.

Therefore one can determine ΔH_rxn from the calorimetry data and then calculate ΔH_soln .

ΔH_rxn   =   ΔH_soln   +   2 ΔH_neut

Recall that the q are extensive properties and the ΔH are intensive properties.

Experiment

Objective

• Determine the molar enthalpy of solution of calcium hydroxide.

Procedure

1. The calorimeter is filled with 100. g of 6 M aqueous HCl
2. Choose the mass (between 1 and 10 g) of solid Ca(OH)₂ to be dissolved in the acid.
3. Click on the Reset button to set up the experiment.
4. Record the initial temperature, T_i , reported by the thermometer.
5. Click on the Start button to begin the experiment.
6. When the system has reached equilibrium, record the final temperature, T_f .
7. Calculate ΔH_rxn and ΔH_soln .

The solid calcium hydroxide is sealed in a glass ampule. To initiate the reaction, the "hammer" breaks the thin glass of the ampule, allowing the HCl solution to reach the Ca(OH)_s solid.

Information

• The calorimeter is initially filled with 100. g of 6 M aqueous HCl.
• The formula weight of calcium hydroxide is 74.093 g mole ^-1.
• The heat capacity of the calorimeter is 183 J °C ^-1.
• The specific heat capacity of the aqueous solution is   s_aq = 4.184 J °C ^-1 g ^-1
• The molar enthalpy of neutralization is ΔH_neut = - 55.83 kJ mole ^-1.

How reproducible is the experimentally determined ΔH_soln for calcium hydroxide?
What factors limit the reproducibility?