Question

a. Use the following data to calculate the enthalpy of hydration for calcium chloride and calcium iodide. $$\begin{array}{lll}{\mathrm{CaCl}_{2}(s)} & {-2247 \mathrm{k} / \mathrm{mol}} & {-46 \mathrm{kJ} / \mathrm{mol}} \\ {\mathrm{Cal}_{2}(s)} & {-2059 \mathrm{k} / \mathrm{mol}} & {-104 \mathrm{kJ} / \mathrm{mol}}\end{array}$$ b. Based on your answers to part a, which ion, \(\mathrm{Cl}^{-}\) or \(\mathrm{I}^{-},\) is more strongly attracted to water?

Short answer

The enthalpy of hydration for CaCl2 is 2201 kJ/mol and for CaI2 is 1955 kJ/mol. Therefore, the Cl- ion is more strongly attracted to water compared to the I- ion.

Step-by-step solution

Calculate the enthalpy of hydration for CaCl2

We are given: Enthalpy of solution (CaCl2) = -46 kJ/mol Lattice enthalpy (CaCl2) = -2247 kJ/mol Using the formula, we can find the enthalpy of hydration for CaCl2: Enthalpy of hydration (CaCl2) = Enthalpy of solution (CaCl2) - Lattice enthalpy (CaCl2) Enthalpy of hydration (CaCl2) = -46 kJ/mol - (-2247 kJ/mol) Enthalpy of hydration (CaCl2) = 2201 kJ/mol

Calculate the enthalpy of hydration for CaI2

We are given: Enthalpy of solution (CaI2) = -104 kJ/mol Lattice enthalpy (CaI2) = -2059 kJ/mol Using the formula, we can find the enthalpy of hydration for CaI2: Enthalpy of hydration (CaI2) = Enthalpy of solution (CaI2) - Lattice enthalpy (CaI2) Enthalpy of hydration (CaI2) = -104 kJ/mol - (-2059 kJ/mol) Enthalpy of hydration (CaI2) = 1955 kJ/mol

Compare the enthalpies of hydration and determine the ion more strongly attracted to water

We now have the enthalpies of hydration for both CaCl2 and CaI2: Enthalpy of hydration (CaCl2) = 2201 kJ/mol Enthalpy of hydration (CaI2) = 1955 kJ/mol Higher the enthalpy of hydration, stronger is the attraction between the ions and water. Since the enthalpy of hydration for CaCl2 is greater than that of CaI2, the Cl- ion is more strongly attracted to water as compared to the I- ion.

Key concepts

Lattice Enthalpy

Lattice enthalpy is a key concept when dealing with ionic compounds like calcium chloride and calcium iodide. It refers to the energy released when ions in a gaseous state come together to form a solid lattice. This process is exothermic, meaning it releases energy. Thus, lattice enthalpy values are typically negative.

The magnitude of lattice enthalpy depends on two main factors:

• The charge on the ions – Greater charges result in stronger attractions and larger lattice enthalpies.
• The size of the ions – Smaller ions can get closer together, resulting in stronger attractions and larger lattice enthalpies.

For compounds like CaCl2 and CaI2, these values vary due to the size and charge of the ions involved. Recognizing lattice enthalpy helps with understanding how much energy is needed or released when ionic compounds dissolve in water.

Calcium Chloride

Calcium chloride (CaCl2) is a salt commonly used for de-icing roads and as a drying agent due to its highly soluble nature. It consists of calcium ions (\( ext{Ca}^{2+}\)) and chloride ions (\( ext{Cl}^{-}\)).

In the context of solution chemistry, calcium chloride exhibits notable behavior:

• Its dissolution in water is exothermic, releasing heat and contributing to its lower enthalpy of solution.
• The strong attraction between \( ext{Ca}^{2+}\) and \( ext{Cl}^{-}\) ions helps in forming a stable ionic lattice which impacts its lattice enthalpy.

Overall, when calcium chloride dissolves, the energy change can be analyzed through its solution enthalpy and lattice enthalpy, showing how ions interact with water and each other.

Calcium Iodide

Calcium iodide (CaI2) combines calcium ions (\( ext{Ca}^{2+}\)) with iodide ions (\( ext{I}^{-}\)), forming another common ionic compound. It is often used in photography and medical applications.

The characteristics of calcium iodide in a solution are distinct due to its ionic size and bonds:

• Larger iodide ions compared to chloride ions result in a different lattice structure with a slightly lower lattice enthalpy.
• Dissolution of calcium iodide in water also produces an energy change, evidenced by its enthalpy of solution.

Understanding the energetic changes in calcium iodide helps explain its properties and reactivity when interacting with water. It shows that not all ionic compounds behave similarly, even if they share an ion like calcium.

Solution Enthalpy

Solution enthalpy, or the enthalpy of solution, represents the overall heat change when a substance dissolves in a solvent such as water. This value can be either positive or negative, depending on whether the process is endothermic or exothermic.

The enthalpy of solution involves two main steps:

• Breaking down the ionic lattice, which requires energy.
• Hydrating the ions, which releases energy.

For calcium chloride and calcium iodide, these interactions can be observed closely. Their enthalpy changes help in deciding which ions are more strongly attracted to water. A larger enthalpy of hydration suggests stronger attraction between the ions and water molecules. In practice, this is essential for determining the solubility and behavior of salts in aqueous solutions.