Use the data to calculate the heats of hydration of lithium chloride and sodium chloride. Which of the two cations, lithium or sodium, has stronger ion–dipole interactions with water? Why?

Key Concepts

Heats of Hydration

The heat of hydration refers to the energy change that occurs when ions are surrounded by water molecules. It is an exothermic process for most ions, as energy is released when ion-dipole interactions form between the ions and water. The magnitude of the heat of hydration is influenced by the charge and size of the ion, with smaller, highly charged ions typically having larger heats of hydration.

Ion-Dipole Interactions

Ion-dipole interactions are attractive forces between an ion and the polar molecules of a solvent, such as water. These interactions are crucial for the solvation process, where ions become surrounded by solvent molecules. The strength of these interactions depends on the charge of the ion and the dipole moment of the solvent, with smaller cations like lithium typically forming stronger interactions due to their higher charge density.

Comparative Cation Properties

When comparing lithium and sodium cations, it is essential to consider their ionic radii and charge density. Lithium ions (Li+) are smaller than sodium ions (Na+), leading to a higher charge density. This results in stronger ion-dipole interactions with water for lithium, as the smaller size allows for closer proximity to the dipoles of water molecules, enhancing the overall interaction strength.