Question

Indicate whether each statement is true or false: \((\) a) \(\mathrm{NaCl}\) dissolves in water but not in benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\) because benzene is denser than water. (b) NaCl dissolves in water but not in benzene because water has a large dipole moment and benzene has zero dipole moment. (c) NaCl dissolves in water but not in benzene because the water-ion interactions are stronger than benzene-ion interactions.

Short answer

(a) False, (b) True, (c) True. The solubility of NaCl depends on the interactions between the solute and the solvent, not their densities. NaCl dissolves in polar solvents like water due to its large dipole moment, and strong water-ion interactions. It does not dissolve in nonpolar solvents like benzene with zero dipole moment and weak benzene-ion interactions.

Step-by-step solution

Statement A: NaCl dissolves in water but not in benzene because benzene is denser than water.

This statement is false because the solubility of NaCl in a solvent depends on the interactions between the solute (NaCl) and the solvent (water or benzene), not on the density of the solvent.

Statement B: NaCl dissolves in water but not in benzene because water has a large dipole moment and benzene has zero dipole moment.

This statement is true. NaCl is an ionic compound, so it dissolves in polar solvents like water, which have a large dipole moment. Benzene, being a nonpolar solvent with zero dipole moment, cannot dissolve NaCl due to lack of strong interactions between the ions of NaCl and benzene molecules.

Statement C: NaCl dissolves in water but not in benzene because the water-ion interactions are stronger than benzene-ion interactions.

This statement is also true. In order for NaCl to dissolve in a solvent, the solvent-solute interactions must be strong enough to overcome the ionic bonds in NaCl. In water, the strong water-ion interactions, due to hydrogen bonding, are capable of breaking the ionic bonds in NaCl and allowing it to dissolve. However, in benzene, the interactions between the benzene molecules and the ions in NaCl are weak, so NaCl does not dissolve.

Key concepts

Dipole Moment

A dipole moment is a measure of the polarity of a chemical bond within a molecule. It occurs when there is an unequal distribution of electrons between two atoms in a bond, leading to one end being slightly more positive and the other more negative. This polarity arises because different atoms have different electronegativities – the ability to attract electrons.

For instance, in water (\(\text{H}_2\text{O}\)), the oxygen atom is more electronegative than the hydrogen atoms. This makes the region around the oxygen atom more negatively charged, giving water a significant dipole moment. This characteristic is essential for water's ability to dissolve many substances, like ionic compounds.

• High dipole moment: indicates a strong polarity
• Zero dipole moment: means no polarity, as seen in molecules like benzene (\(\text{C}_6\text{H}_6\))

Ionic Compounds

Ionic compounds are formed when atoms transfer electrons to achieve a full outer shell of electrons, resulting in the formation of charged ions. These ions are held together by ionic bonds, which are the strong electrostatic forces of attraction between the oppositely charged ions.

Typical examples include common table salt, \(\text{NaCl}\), where sodium (\(\text{Na}^+\)) and chloride (\(\text{Cl}^-\)) ions bond together. In order for these compounds to dissolve, the solvent must be able to disrupt these strong ionic bonds. Water is particularly good at this due to its polar nature and ability to form strong ion-dipole interactions.

• Made up of charged ions
• Generally have high melting and boiling points
• Conduct electricity when molten or dissolved in water

Polar Solvents

Polar solvents, such as water, are liquids that have molecules with significant electric dipole moments. This means they have partially charged regions within the molecule, contributing to their high polarity. Such solvents are excellent at dissolving ionic compounds because they can surround and stabilize the ions.

Water is the quintessential polar solvent, known for its ability to dissolve a wide range of salts, acids, and other ionic substances. This is because water molecules can form hydrogen bonds and effectively compete with the ionic bonds in compounds like \(\text{NaCl}\), separating the ions and allowing them to disperse in solution.

• Can dissolve ionic and polar solutes
• Examples include water, methanol, and ethanol
• Effective at affecting reactions involving charged species

Nonpolar Solvents

Nonpolar solvents are liquids that consist of molecules without a significant dipole moment, meaning they lack the charged regions found in polar substances. This makes them ineffective at dissolving ionic compounds, such as \(\text{NaCl}\), which depend on strong ion-dipole interactions for dissolution.

Benzene is a classic example of a nonpolar solvent. It has a symmetrical ring structure and evenly distributed electron clouds, reducing the chance of forming dipoles.

• Poor at dissolving ionic substances but effective for nonpolar solutes
• Include solvents like benzene, hexane, and toluene
• Often used in organic chemistry to dissolve grease, oils, and other nonpolar compounds