Question

(a) Considering the energetics of solute-solute, solventsolvent, and solute- solvent interactions, explain why \(\mathrm{NaCl}\) dissolves in water but not in benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right) .\) (b) What factors cause a cation to be strongly hydrated?

Short answer

NaCl dissolves in water because strong solute-solvent interactions between NaCl ions and water molecules overcome solute-solute and solvent-solvent interactions. In benzene, weak solute-solvent interactions are insufficient to break NaCl's ionic bonds. A cation is strongly hydrated when it has a higher charge, smaller size, and greater polarizability, which increase its electrostatic attraction to water molecules.

Step-by-step solution

1. Understanding the Interactions

To explain the solubility of NaCl in water and benzene, we need to discuss three types of interactions: solute-solute interactions, solvent-solvent interactions, and solute-solvent interactions. * Solute-solute interactions: NaCl is an ionic compound, which means it consists of positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). These ions are held together by strong ionic bonds. * Solvent-solvent interactions: In both water and benzene, the molecules are attracted to each other by intermolecular forces. Water has strong hydrogen bonding, while benzene has weaker dispersion forces. * Solute-solvent interactions: These interactions occur between the solute (NaCl) and the solvent (water or benzene).

2. NaCl in Water

When NaCl is added to water, the positive and negative ends of the water molecules are attracted to the respective oppositely charged ions of NaCl. These interactions are strong enough to break the ionic bonds in NaCl, causing it to dissolve. Moreover, the energy released during the process of solvation (formation of solute-solvent interactions) compensates for the energy required to break solute-solute and solvent-solvent interactions. Therefore, NaCl dissolves in water.

3. NaCl in Benzene

When NaCl is added to benzene, the much weaker dispersion forces between benzene molecules are unable to break the ionic bonds in NaCl. Additionally, the energy released during the solvation process in benzene is not sufficient to compensate for the energy required to break solute-solute and solvent-solvent interactions. Therefore, NaCl does not dissolve in benzene.

4. Factors for Strong Cation Hydration

There are several factors that cause a cation to be strongly hydrated: * Charge: Cations with a higher charge have a stronger electrostatic attraction to the negative ends of water molecules, resulting in stronger hydration. * Size: Smaller cations have a greater charge density, which means the charge is concentrated over a smaller area. This increases the electrostatic attraction between the cation and water molecules, leading to stronger hydration. * Polarizability: Cations with a higher polarizability can induce dipoles in nearby water molecules, further increasing the strength of solute-solvent interactions and resulting in stronger hydration. In conclusion, NaCl dissolves in water due to strong solute-solvent interactions, while it does not dissolve in benzene because of weak solute-solvent interactions. Factors that cause a cation to be strongly hydrated include higher charge, smaller size, and greater polarizability.

Key concepts

Solute-Solvent Interactions

In the world of chemistry, understanding why some substances dissolve and others do not is crucial. When we talk about solute-solvent interactions, we're looking at the forces between particles. For

• Solute-Solute Interactions: These are the forces holding the particles of a solute together. In NaCl, strong ionic bonds hold the sodium (\(\text{Na}^+\)) and chloride (\(\text{Cl}^-\)) ions together.
• Solvent-Solvent Interactions: These are the forces between solvent molecules themselves. In water, these forces are strong due to hydrogen bonds; in benzene, they are weaker dispersion forces.
• Solute-Solvent Interactions: This is the magic that makes dissolving possible. When NaCl is added to water, water molecules interact with sodium and chloride ions, breaking the ionic bonds and forming new interactions.

The strength of these interactions and the energy changes involved determine if a solute dissolves.

Hydration of Cations

Hydration is the process where water molecules surround and interact with ions. For cations, strong hydration is influenced by a few key factors:

• Charge: Cations with a higher positive charge attract water molecules more strongly. The electrostatic attraction is intense, leading to stronger hydration.
• Size: Smaller cations result in a higher charge density, concentrating the charge and attracting water molecules effectively.
• Polarizability: This refers to the ability of a cation to distort the electron cloud of water molecules, enhancing the attraction further.

These factors all enhance the interaction between cations and water, facilitating the hydration process.

Polarizability

Polarizability is an essential property of ions that affects solubility and interaction with solvents. It describes how easily the electron cloud of an ion can be distorted.

• Effect on Hydration: Higher polarizability means ions can induce stronger dipoles in water molecules, reinforcing the solute-solvent interactions.
• Size and Charge Relation: Generally, larger ions have greater polarizability, but the effect is balanced by the ion's charge density. Stronger interaction can result from both size and charge.

In essence, polarizability is a dynamic quality influencing how molecules attract each other in a solution.

Ionic Compounds

Ionic compounds like NaCl are formed from the attraction between oppositely charged ions. These compounds behave distinctively in solvents depending on various factors.

• Ionic Bonds: The strong electrostatic forces hold ions tightly together. Dissolving requires breaking these bonds.
• Solubility in Water: Water, being polar, can surround and separate these ions effectively, facilitating solubility.
• Insolubility in Nonpolar Solvents: In something like benzene, the lack of strong interactions doesn't compensate for breaking the ionic bonds, preventing dissolution.

Understanding ionic compounds' behavior is crucial for predicting solubility outcomes.

Intermolecular Forces

Intermolecular forces are the forces between molecules that determine their physical properties, like boiling and melting points, and solubility. These include:

• Dispersion Forces: Present in all molecules, but dominant in nonpolar ones like benzene.
• Dipole-Dipole Interactions: Occur in polar molecules where positive and negative poles align.
• Hydrogen Bonds: A strong type of dipole-dipole interaction present in water, interacting powerfully with polar ionic solutes.

The nature and strength of these forces influence how substances mix or separate, affecting everything from solubility to state changes.