Problem 14 - Chemistry The Central Science In Si Units

Question

Indicate whether each statement is true or false: \((\mathbf{a}) \mathrm{NaCl}\) dissolves in water but not in benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\) because benzene is denser than water. (b) \(\mathrm{NaCl}\) 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.

Step-by-Step Solution

Answer

(a) False, (b) True, (c) True.

Step 1: Understand solubility principles

Solubility is often determined by the 'like dissolves like' principle. Polar solvents, like water, dissolve polar solutes or ionic compounds due to electrostatic interactions. Nonpolar solvents, like benzene, dissolve nonpolar substances.

Step 2: Examine statement (a)

Statement (a) claims that NaCl dissolves in water but not in benzene because benzene is denser than water. Density is not a key factor in solubility, rather it is the type of interactions between solute and solvent. Therefore, this statement is False.

Step 3: Analyze statement (b)

Statement (b) suggests NaCl dissolves in water due to water's large dipole moment and not in benzene because benzene has a zero dipole moment. This is in line with the 'like dissolves like' principle since NaCl is ionic (polar) and dissolves in polar solvents. Thus, this statement is True.

Step 4: Evaluate statement (c)

Statement (c) states that NaCl dissolves in water instead of benzene due to stronger water-ion interactions compared to benzene-ion interactions. Water, being polar, forms strong electrostatic interactions with NaCl ions, unlike the nonpolar benzene. Therefore, this statement is True.

Key Concepts

Ionic Compounds

Ionic compounds are composed of positively and negatively charged ions. These ions are held together by strong electrostatic forces known as ionic bonds. This characteristic makes ionic compounds typically solid at room temperature with high melting and boiling points. When placed in a suitable solvent, such as water, the ions dissociate and disperse throughout the solvent.

• This is why substances like table salt (NaCl) easily dissolve in water, allowing the sodium (Na⁺) and chloride (Cl^-) ions to separate and interact with the solvent.
• However, in nonpolar solvents like benzene, ionic compounds usually do not dissolve because the nonpolar molecules cannot effectively interact with the charged ions to separate them.

Understanding the nature of ionic compounds is essential in predicting their solubility in different solvents.

Polar and Nonpolar Solvents

Solvents can be classified based on their polarity, which influences their ability to dissolve different substances. Polar solvents, like water, possess a significant dipole moment where there is a partial positive charge on one end and a partial negative charge on the other. This allows them to effectively interact with ionic and other polar substances.

• Water is a prime example of a polar solvent due to its bent shape and unequal charge distribution, making it excellent at dissolving ionic compounds such as NaCl.
• Conversely, nonpolar solvents, such as benzene, lack this distinct charge separation.
• Benzene molecules do not have significant positive or negative ends, which hinders their ability to interact with ionic or polar solutes.

Understanding these solvent properties provides insight into why certain substances dissolve in specific solvents.

Dipole Moment

The dipole moment is a measure of the polarity of a molecule. It reflects the separation of charge within the molecule due to differences in electronegativity between atoms. In polar molecules, such as water, the presence of a dipole moment signifies that the molecule has a partial positive end and a partial negative end. This property is crucial in dissolving ionic compounds and other polar substances.

• A molecule with a large dipole moment can interact with ions effectively, surrounding them and separating them in the solution.
• This is why NaCl easily dissolves in water, which has a significant dipole moment, allowing for strong electrostatic interactions with the ions.
• Benzene, having zero dipole moment, lacks this ability, thus unable to dissolve ionic compounds efficiently.

This concept helps us understand why the polarity of a solvent is essential in the dissolution process.

Electrostatic Interactions

Electrostatic interactions play a crucial role in the solubility of compounds. These interactions occur between charged particles, such as between ions in an ionic compound and the polar molecules of a solvent like water.

• The strength of these interactions greatly influences whether a compound dissolves in a particular solvent.
• In the case of NaCl in water, the polar water molecules surround the Na⁺ and Cl^- ions, stabilizing them in solution through electrostatic attraction.
• In contrast, nonpolar molecules like benzene cannot engage in these interactions effectively, as they lack the necessary charge separation.

Electrostatic interactions are vital in understanding the dissolution process of ionic compounds and their preference for polar solvents.