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

In short, NaCl dissolves in water but not in benzene because water is a polar solvent capable of forming strong interactions with NaCl ions, whereas benzene is a nonpolar solvent that cannot form these interactions. The dissolution process is energetically favorable in water due to the formation of a stable hydration shell around each ion. In contrast, the energy required to separate the NaCl ions is not compensated by new solute-solvent interactions in benzene. Factors that cause a cation to be strongly hydrated include a higher positive charge and smaller ion size, which lead to stronger interactions with water molecules.

Step-by-step solution

Understanding NaCl properties

NaCl is an ionic compound, consisting of positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). These ions are attracted to each other by strong electrostatic forces, known as ionic bonds.

Understanding water (solvent) properties

Water is a polar molecule, meaning it has a partial positive charge near the hydrogen atoms and a partial negative charge near the oxygen atom. This is due to the difference in electronegativity between oxygen and hydrogen, which creates an uneven distribution of electron density within the molecule. Also, water molecules are capable of forming hydrogen bonds.

Understanding benzene (solvent) properties

Benzene is a nonpolar molecule and its structure is composed of six carbon atoms arranged in a hexagonal ring, with each carbon atom bonded to a hydrogen atom. Unlike water, benzene does not have a significant difference in electronegativity between its atoms, and it cannot form hydrogen bonds.

Analyzing solute-solute, solvent-solvent, and solute-solvent interactions in water and benzene

When NaCl dissolves in a solvent, the ionic lattice breaks apart, and the individual ions interact with the solvent molecules. In the case of water as the solvent, the partially positive hydrogen atoms are attracted to the negatively charged chloride ions (Cl-), and the partially negative oxygen atoms are attracted to the positively charged sodium ions (Na+). This strong attraction between NaCl ions and water molecules results in the formation of a stable hydration shell around each ion, making the dissolution process energetically favorable. On the other hand, benzene does not have polar regions that can interact strongly with the NaCl ions. Therefore, it cannot form a stable interaction shell around the ions, and the energy required to separate the ions is not compensated by new solute-solvent interactions. This makes the dissolution of NaCl in benzene energetically unfavorable.

Factors causing strong hydration in cations

A cation's hydration strength depends on two primary factors: the charge of the ion and the size of the ion. 1. Charge of the ion: The higher the charge (the greater the positive charge) on the cation, the stronger its interaction with the negatively charged oxygen atoms in water molecules, resulting in a higher hydration strength. 2. Size of the ion: Smaller ions have a higher charge density, which means that their electric field is more concentrated. As a result, smaller cations form stronger interactions with water molecules, leading to increased hydration strength. In summary, cations with a higher positive charge and smaller size are more likely to be strongly hydrated in a polar solvent like water.

Key concepts

Ion-Dipole Interactions

Ion-dipole interactions are crucial for understanding why certain ionic compounds dissolve in specific solvents. These interactions occur between an ion, which may be either positively or negatively charged, and a polar molecule that has a partial charge. The strength of ion-dipole interactions is mainly determined by:

• the charge of the ion – a higher charge leads to stronger interaction, and
• the polarity of the molecule – a greater dipole moment in the molecule allows for more effective attraction.

When NaCl is placed in water, ion-dipole interactions are formed between the ions of NaCl and the water molecules. Here, the partially negative oxygen in water attracts the positively charged sodium ions (Na+), while the partially positive hydrogen atoms in water attract the negatively charged chloride ions (Cl-). This interaction results in the breaking of the ionic lattice and the dissolution of NaCl in water.
In contrast, benzene is a nonpolar solvent and lacks these partial charges. As a result, there are no sufficient ion-dipole interactions between benzene molecules and NaCl ions. Consequently, NaCl remains undissolved in benzene.

Polar vs Nonpolar Solvents

The polarity of a solvent greatly affects its ability to dissolve various substances. Polar solvents, like water, contain molecules with a net dipole moment due to differences in electronegativity among their atoms. This allows them to dissolve ionic compounds and other polar molecules effectively.
For instance, water, with its polar bonds, can easily dissolve NaCl by surrounding its ions and creating a hydration shell – a process guided by ion-dipole interactions. As a result, the ions are dispersed within the water, creating a homogeneous solution.
On the flip side, nonpolar solvents, like benzene, lack a significant dipole moment due to the uniform distribution of electron density across the molecule. This means nonpolar solvents are incapable of forming strong interactions with ionic compounds. Hence, substances like NaCl, which can form strong ionic bonds, won't dissolve in benzene.

• Polar solvents are excellent at dissolving ionic and polar substances.
• Nonpolar solvents dissolve nonpolar substances efficiently due to their mutual affinity.

Understanding the nature of solvents helps in predicting solubility outcomes when mixing different substances.

Hydration of Ions

The hydration of ions is a process where water molecules surround and interact with ions when a salt dissolves in water. This interaction helps stabilize ions in solution and is an endothermic process, typically driven by entropy.

• Charge and Size: The extent of hydration depends on the ion's charge and size. Ions with a higher charge attract water molecules more efficiently, creating a stronger hydration shell. Smaller ions, due to their higher charge density, attract water molecules more closely, leading to intense hydration.
• Thermodynamics: Hydration is more favorable when it compensates for the energy required to break the ionic bonds in the solute as well as any solvent-solvent interactions in water. This means the process releases enough energy to make overall dissolution spontaneous in polar solvents.

Hydration helps in understanding why certain cations dissolve readily in water. For example, calcium ions ( ext{Ca}^{2+} ext{) experience strong hydration and dissolve easily due to their high charge density. This also explains the behavior of transition metals, where their typically smaller ionic radii and high charge render them easily hydrated in aqueous solutions.