Chapter 9, Problem 9.50P

Calcium chloride is a salt used in a number of food and medicinal applications and in brine for refrigeration systems. Its most distinctive property is its affinity for water: in its anhydrous form it efficiently absorbs water vapor from gases, and from aqueous liquid solutions it can form (at different conditions) calcium chloride hydrate $\left(CaC{l}_2\cdot H_{2}O\right)$, dihydrate $\left(CaC{l}_2\cdot 2H_{2}O\right)$, tetrahydrate $\left(CaC{l}_2\cdot 4H_{2}O\right)$, and hexahydrate $\left(CaC{l}_2\cdot 6H_{2}O\right)$.

You have been given the task of determining the standard heat of the reaction in which calcium chloride hexahydrate is formed from anhydrous calcium chloride:

   $CaC{l}_2\left(s\right)+6H_{2}O\left(l\right)\to CaC{l}_2\cdot 6H_{2}O\left(s\right):\Delta H_r^{°}\left(kJ\right)=?$

By definition, the desired quantity is the heat of hydration of calcium chloride hexahydrate. You cannot carry out the hydration reaction directly, so you resort to an indirect method. You first dissolve 1.00 mol of anhydrous CaCl₂in 10.0 mol of water in a calorimeter and determine that 64.85 kJ of heat must be transferred away from the calorimeter to keep the solution temperature at 25°C. You next dissolve 1.00 mol of the hexahydrate salt in 4.00 mol of water and find that 32.1 kJ of heat must be transferred to the calorimeter to keep the temperature at 25°C.

1. Use these results to calculate the desired heat of reaction. (Suggestion: Begin by writing out the stoichiometric equations for the two dissolution processes.)
2. Calculate the standard heat of reaction in kJ for Ca(s), Cl₂(g) and H₂O reacting to form CaCl₂(aq, r= 10).

Speculate about why the standard heat of reaction in forming calcium chloride hexahydrate cannot be measured directly by reacting the anhydrous salt with water in a calorimeter.