Problem 14 - Chemistry: The Central Science

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?

Step-by-Step Solution

Answer

NaCl dissolves in water due to the favorable ion-dipole interactions between its positively charged Na+ and negatively charged Cl- ions and the polar water molecules, which overcome the solute-solute and solvent-solvent interactions. In contrast, NaCl does not dissolve in the nonpolar solvent benzene, as the weak solute-solvent interactions are insufficient to overcome the solute-solute and solvent-solvent interactions. Factors that cause a cation to be strongly hydrated include high charge density, small size, and the polarizability of solvent molecules.

Step 1: Understand the energetics involved in dissolution

To explain why NaCl dissolves in water but not in benzene, we need to look at the solute-solute interaction, solvent-solvent interaction, and solute-solvent interaction. Dissolution occurs when the overall energetic change is favorable (negative). The solute-solvent interactions must be strong enough to overcome the energy required to break the solute-solute and solvent-solvent interactions.

Step 2: Analyze NaCl and water interaction

Water is a polar solvent with a bent shape and significant partial charges on its oxygen and hydrogen atoms. NaCl is an ionic compound that dissociates into positively charged Na+ and negatively charged Cl- ions when dissolved in water. The electrostatic interaction between these ions and the partial charges on the water molecules lead to a favorable (negative) energy change, as the ion-dipole interactions between Na+ or Cl- and water molecules are stronger than the solute-solute and solvent-solvent interactions. This allows NaCl to dissolve in water.

Step 3: Analyze NaCl and benzene interaction

Benzene, on the other hand, is a nonpolar solvent with a hexagonal shape and no significant partial charges on the carbon and hydrogen atoms. Due to the absence of partial charges on benzene molecules, the electrostatic interaction between ionic NaCl and benzene is weak. As a result, the solute-solvent interactions between NaCl and benzene are not strong enough to overcome the solute-solute and solvent-solvent interactions, which result in an unfavorable (positive) energy change. Thus, NaCl does not dissolve in benzene.

Step 4: Factors that cause a cation to be strongly hydrated

A cation is positively charged and can be strongly hydrated when it interacts with a polar solvent such as water. Some factors that cause a cation to be strongly hydrated are: 1. Charge density: Cations with a higher charge density (charge divided by volume) can form stronger interactions with solvent molecules due to the higher electrostatic force. This leads to stronger hydration. 2. Size of the cation: Smaller cations tend to have a higher charge density, leading to stronger hydration. 3. Polarizability of solvent molecules: A polar solvent like water, with significant partial charges, can interact more favorably with charged species, leading to stronger hydration. A nonpolar solvent, like benzene, does not interact as strongly with cations, resulting in weak or no hydration.