Question

The lattice energy* of NaI is \(-686 \mathrm{~kJ} / \mathrm{mol}\), and the enthalpy of hydration is \(-694 \mathrm{~kJ} / \mathrm{mol}\). Calculate the enthalpy of solution per mole of solid NaI. Describe the process to which this enthalpy change applies.

Short answer

The enthalpy of solution for NaI can be calculated by adding the given lattice energy and enthalpy of hydration: \(-686 \mathrm{~kJ/mol} + (-694 \mathrm{~kJ/mol}) = -1380 \mathrm{~kJ/mol}\). This value represents the energy change when one mole of solid NaI dissolves in water, indicating an exothermic process as energy is released during dissolution.

Step-by-step solution

Understand the terms

The lattice energy is the energy required to break one mole of a solid ionic compound into gaseous ions. The enthalpy of hydration is the energy released when one mole of gaseous ions is solvated by water molecules.

Write the given values

The lattice energy of NaI is given as -686 kJ/mol and the enthalpy of hydration of NaI is given as -694 kJ/mol.

Calculate the enthalpy of solution

The enthalpy of solution is the sum of the lattice energy and the enthalpy of hydration. Therefore, we can calculate the enthalpy of solution using the given values: Enthalpy of solution = Lattice energy + Enthalpy of hydration Enthalpy of solution = (-686 kJ/mol) + (-694 kJ/mol)

Find the answer

Adding the values together, we get: Enthalpy of solution = -1380 kJ/mol

Describe the process

The enthalpy change of -1380 kJ/mol applies to the process of dissolving one mole of solid NaI in water. This value indicates that the dissolution of NaI in water is an exothermic process, as energy is released during the process.

Key concepts

Lattice Energy

The lattice energy is a critical concept in understanding ionic compounds and their interactions. Imagine a crystal structure where ions are neatly arranged and bound together in a repeating pattern. The lattice energy is the total amount of energy it takes to break apart this robust structure into individual gaseous ions. For a compound like sodium iodide (NaI), this energy is quite significant due to the strong electrostatic forces between the positively charged sodium ions and the negatively charged iodide ions.

The process of breaking these bonds is always endothermic, meaning it requires the absorption of energy. Students can think of lattice energy as a measure of an ionic compound's stability: the higher the lattice energy, the more stable the compound and the more energy it would take to disassemble the crystal lattice.

Enthalpy of Hydration

When these gaseous ions enticed by lattice energy encounter water, a new type of energy change comes into play called the enthalpy of hydration. This is the energy change associated with the solvation process, where water molecules surround and stabilize the ions. In the enthalpy of hydration, we observe the reverse of lattice energy: energy is released as water molecules, owing to their polarity, are attracted to and interact with ions. The negative sign of the enthalpy of hydration value indicates that it's an exothermic process.

For example, the hydration of Na+ and I- ions from sodium iodide (NaI) results in a substantial energy release. This process is critical to understand as it explains how ionic compounds dissolve in water and also greatly influences the solubility of different substances in aqueous solutions.

Exothermic Process

An exothermic process is a reaction or change that releases heat into its surroundings. The 'exo' prefix means 'outward', and thermic pertains to 'heat'. In chemistry, this is a fundamental concept that students must discern, as it underpins the behavior of substances during reactions.

When ionic compounds like NaI dissolve in water, the overall process is exothermic. The sum of the endothermic lattice energy and the exothermic enthalpy of hydration results in net energy release. This release of energy typically manifests as an increase in the temperature of the solution, a tangible effect students can observe during laboratory experiments. Understanding exothermic reactions is also important in various real-world applications, such as in the design of heat packs or even within industrial chemical processes.

Ionic Compounds

Ionic compounds are substances composed of positively charged ions (cations) and negatively charged ions (anions) that are held together by potent electrostatic forces known as ionic bonds. In a classic example, sodium iodide (NaI) comprises sodium ions (Na+) and iodide ions (I-). The formation of ionic compounds often results from the transfer of electrons from one atom to another, leading to a full outer shell for each ion – a stable electron configuration.

Students must appreciate the unique properties of ionic compounds: high melting points, solid at room temperature, and the ability to dissolve in water due to their ionic nature. The dissolution process involves the disruption of the ionic lattice and interaction with water molecules, key factors in understanding the enthalpy changes discussed in lattice energy and the enthalpy of hydration.