Question

Use intermolecular forces to explain why \(\mathrm{NaCl}\) is soluble in water.

Short answer

\(\mathrm{NaCl}\) is soluble in water due to the ion-dipole intermolecular forces between the water molecules and the sodium chloride ions, which allows the water molecules to pull the \(\mathrm{NaCl}\) into its individual ions through a process called solvation.

Step-by-step solution

Identifying the types of Intermolecular forces

Sodium chloride (\(\mathrm{NaCl}\)) is an ionic compound while water (\(\mathrm{H_2O}\)) is a polar molecule. The strong attractive forces that exist between these two are known as ion-dipole forces.

Describing the Solvation Process

When \(\mathrm{NaCl}\) salt is added to water, the positive end of water (Hydrogen) is attracted towards the Chlorine ions while the negative end of water (Oxygen) is attracted to Sodium ions. This is mainly because opposite charges attract, according to Coulomb's law.

Explaining Solubility

The ion-dipole forces between the \(\mathrm{NaCl}\) and \(\mathrm{H_2O}\) are strong enough such that the kinetic energy of the water molecules can overcome the ionic bonds of the \(\mathrm{NaCl}\) lattice. The water thus 'pulls' the sodium and chloride ions from the \(\mathrm{NaCl}\) lattice and surrounds them, a process known as solvation. This results in the \(\mathrm{NaCl}\) dissolving into its individual ion components in water, hence \(\mathrm{NaCl}\) is soluble in water.

Key concepts

Solvation Process

The solvation process is the key to understanding why certain substances, like sodium chloride (NaCl), can dissolve in solvents such as water. This process involves the surrounding of solute particles—ions or molecules—by solvent molecules. When NaCl is introduced into water, water molecules, which are polar and have partial positive (hydrogen) and negative (oxygen) charges, interact with the Na+ and Cl- ions.

This interaction leads to the detachment of individual ions from the solid lattice structure of NaCl. As the solvent molecules continue to surround and isolate these ions, the solid salt gradually dissociates until fully dissolved. By appreciating this microscopic dance between solute and solvent, students can gain a tangible understanding of dissolution at the molecular level.

Ion-Dipole Forces

Ion-dipole forces are a type of intermolecular force that is particularly relevant when discussing the solubility of ionic compounds in polar solvents. These forces arise from the electrostatic attraction between a full charge on an ion and a partial charge on a polar molecule.

In the example of NaCl in water, the positively charged sodium ions (Na+) are attracted to the negatively charged end of the water molecules (the oxygen atom), while the negatively charged chloride ions (Cl-) are attracted to the positively charged end (the hydrogen atoms). This attraction helps to stabilize the ions in the solution, which is crucial for keeping the substance dissolved. Ion-dipole interactions are significant because they are generally stronger than other kinds of dipole interactions, such as dipole-dipole or London dispersion forces.

Solubility in Water

Solubility in water is determined by the balance between the forces holding the solute particles together and the forces acting between the solute and the solvent. For an ionic compound like NaCl, its solubility hinges on the ability of water to overcome the ionic bonds of the salt crystal.

Water is highly efficient at this because its polar nature results in strong ion-dipole interaction with the ions of NaCl. The 'breaking' of the salt into individual ions that are surrounded by water molecules is what we refer to as dissolution. Solubility is influenced by factors such as temperature, pressure, and the nature of the solvent, with water being known as the 'universal solvent' due to its efficacy in dissolving many substances.

Coulomb's Law

Coulomb's Law is essential for understanding the fundamental principles behind the solvation process and overall solubility. This law quantifies the electrostatic force between two charged particles, stating that the force is directly proportional to the product of the charges and inversely proportional to the square of the distance between them. In the context of dissolving NaCl in water, Coulomb's Law helps explain why the opposite charges of the ions and the polar ends of the water molecules attract one another so strongly. It emphasizes that the force of attraction between ions (such as Na+ and Cl-) and water molecules is much stronger when they are close together, which is what happens when salt dissolves in water.