Question

When lithium iodide (LiI) is dissolved in water, the solution becomes hotter. a. Is the dissolution of lithium iodide cndothermic or exothermic? b. What can you conclude about the relative magnitudes of the lattice energy of lithium iodide and its heat of hydration? c. Sketch a qualitative energy diagram similar to Figure 14.6 for the dissolution of Lil. d. Why does the solution form? What drives the process?

Short answer

a. The dissolution of lithium iodide (LiI) in water is exothermic. b. The heat of hydration of LiI is greater than its lattice energy. c. The energy diagram would show an endothermic step (lattice energy) followed by a more substantial exothermic step (heat of hydration). d. The solution forms because the hydration process leads to a net release of energy, making it energetically favorable.

Step-by-step solution

Identifying the type of reaction

Since the solution becomes hotter when lithium iodide (LiI) is dissolved in water, it means the process is releasing heat into the surroundings. A chemical reaction that releases heat is known as an exothermic reaction.

Comparing lattice energy and heat of hydration

If the dissolution of LiI in water is exothermic, it implies that the energy released during the hydration of ions (heat of hydration) is greater than the energy required to separate the ions (lattice energy). This is because the overall reaction would not release heat unless the energy given off in the formation of hydrated ions exceeds the energy needed to break apart the ionic lattice.

Sketching the energy diagram

An energy diagram for the dissolution of LiI should show an initial increase in energy as the solid lattice is broken up (endothermic), followed by a greater decrease in energy as the ions are hydrated by water molecules (exothermic), leading to a net release of energy.

Understanding the formation of the solution

The solution forms because the process is energetically favorable; the system aims to achieve a lower energy state. The driving force for the dissolution process is the overall decrease in potential energy, which happens when the enthalpy change associated with the hydration of ions is greater than the enthalpy change to separate the ions in the solid lattice.

Key concepts

Lattice Energy

Lattice energy is a measure of the bond strength in ionic compounds. It represents the amount of energy required to separate one mole of a solid ionic compound into its individual gaseous ions. Essentially, it's the energy released when ions come together to form a crystalline lattice from being far apart.

When considering the dissolution of lithium iodide (LiI) in water, we first observe that energy must be absorbed to overcome the lattice energy and disrupt the ionic lattice. This part of the dissolution is endothermic, requiring an input of energy. The magnitude of lattice energy is influenced by the charge and size of the ions; for LiI, which has a small cation and a relatively large anion, the lattice energy is significant but not as high as in compounds with smaller or more highly charged ions.

Heat of Hydration

The heat of hydration is the amount of energy released when one mole of ions is surrounded by water molecules, effectively 'hydrating' the ions. This interaction is exothermic because the formation of new ion-dipole interactions between the ions and water molecules releases energy.

In the case of LiI, the hydration of lithium and iodide ions releases more energy than what is consumed to break apart the ionic lattice. This indicates that the heat of hydration has a greater magnitude compared to the lattice energy, leading to an overall exothermic dissolution process. The high heat of hydration for LiI can be largely attributed to the highly charged lithium ion, which interacts strongly with the polar water molecules.

Energy Diagrams

Energy diagrams visually represent the energy changes that occur during a chemical reaction. For the dissolution of LiI, an energy diagram would start with a certain energy level for the solid LiI representing the lattice energy. The diagram would show an initial upward curve (endothermic process) as the lattice energy is overcome to separate the ions.

Following this, a downward curve (exothermic process) would demonstrate the heat of hydration as ions interact with water molecules, ultimately ending at a lower energy level than the starting point. This change signifies a release of energy to the surroundings, characteristic of an exothermic reaction. If sketched, such an energy diagram would effectively illustrate the step where heat is absorbed (endothermic) and where more heat is released (exothermic) during the dissolution process.

Enthalpy Change

Enthalpy change, often denoted as (ΔHc), describes the heat change during a reaction at constant pressure. It's a crucial concept in understanding whether a process like dissolution is endothermic (positive (ΔHc)) or exothermic (negative (ΔHc)).

The enthalpy change for the dissolution of LiI is negative, indicating that the overall process is exothermic. This exothermic nature arises because the enthalpy change for the hydration of ions ((ΔHc) hydration) is more negative than the enthalpy change for separating the solid lattice ((ΔHc) lattice). The overall dissolving process is favored when the enthalpy change of the system decreases, which suggests the reaction proceeds to form a more stable arrangement of ions in the solution.