Problem 10 - Chemistry The Molecular Nature of Matter

Question

Which would give a stronger ion-dipole interaction with water molecules: \(\mathrm{Al}^{3+}\) or \(\mathrm{Na}^{+}\)

Step-by-Step Solution

Answer

\(\mathrm{Al}^{3+}\) would have a stronger ion-duplicate interaction with water molecules compared to \(\mathrm{Na}^{+}\), due to its higher charge and smaller size.

Step 1: Understanding Ion-Dipole Interactions

Ion-dipole interactions are a type of intermolecular force that occurs between an ion and a polar molecule with a dipole moment, such as water. The strength of an ion-dipole interaction depends on the magnitude of the ion's charge and the ion's ability to polarize neighboring water molecules.

Step 2: Comparing the Charges

To determine which ion would have a stronger interaction with water, compare the charges of the ions. A higher charge leads to stronger interaction because it can polarize the water molecules more. Here, the \(\mathrm{Al}^{3+}\) ion has a +3 charge, whereas the \(\mathrm{Na}^{+}\) ion has a +1 charge.

Step 3: Analyzing Ion Size

Size of the ion also plays a role in the strength of the ion-dipole interaction. Smaller ions (with the same charge) polarize water molecules more strongly because the charge density is higher, and therefore they interact more strongly. Aluminium ion (\(\mathrm{Al}^{3+}\)) is smaller than a sodium ion (\(\mathrm{Na}^{+}\)) when considering commonly found isoelectronic species with noble gas electron configurations.

Step 4: Concluding the Stronger Interaction

Based on the charge and size of the ions, \(\mathrm{Al}^{3+}\) would have a stronger ion-dipole interaction with water molecules than \(\mathrm{Na}^{+}\), as it has a greater charge and is typically smaller in size, leading to a higher charge density.

Key Concepts

Intermolecular Forces

The forces that hold molecules together in solids and liquids are known as intermolecular forces. These forces are significantly weaker than the bonds within molecules, known as intramolecular forces, such as covalent bonds. However, intermolecular forces play a crucial role in determining the physical properties of substances, including boiling and melting points, solubility, and viscosity.

Polar Molecules

When it comes to polar molecules, the distribution of electrical charge across the molecule is uneven. Molecules with polar covalent bonds have regions of positive and negative charge due to differences in electronegativity between bonded atoms. Water is the quintessential polar molecule, possessing a bent shape that allows for a partially negative charged oxygen atom, and partially positive charged hydrogen atoms. This uneven charge distribution enables water to form hydrogen bonds and interact effectively with other charged particles and polar molecules.

Charge Density

Charge density is a measure of electric charge per unit area or volume. In the context of ions, it refers to the ratio of the charge of the ion to the size of the ion. Ions with higher charge and smaller radii have high charge densities and can exert stronger electric fields. This leads to a greater ability to polarize and attract nearby polar molecules, intensifying intermolecular forces such as ion-dipole interactions.