Problem 103 - Chemistry The Molecular Science

Question

Consider this information regarding two compounds (common names are used). Orpiment: yellow solid; density \(=3.49 \mathrm{~g} / \mathrm{cm}^{3}\) at \(25^{\circ} \mathrm{C} ;\) melting point \(=573 \mathrm{~K}\); slightly soluble in hot water; soluble in basic solution. Zeaxanthin: orange-red solid; density \(=0.93 \mathrm{~g} / \mathrm{cm}^{3}\) at \(25^{\circ} \mathrm{C} ;\) melting point \(=489 \mathrm{~K} ;\) insoluble in water, soluble in benzene. (a) Is orpiment an ionic or molecular compound? Explain your answer. (b) Is zeaxanthin an ionic or molecular compound? Explain your answer.

Step-by-Step Solution

Answer

Orpiment is a molecular compound; zeaxanthin is a molecular compound.

Step 1: Analyze Physical Properties of Orpiment

Orpiment is a yellow solid with a density of \(3.49 \ \text{g/cm}^3\) at \(25^{\circ} \text{C}\). It has a melting point of \(573 \ \text{K}\) and is slightly soluble in hot water and soluble in basic solution. These properties suggest it doesn't form the strong electrostatic forces typical of ionic bonds, which would usually make it highly soluble in water.

Step 2: Determine Nature of Bonding in Orpiment

Ionic compounds typically have high melting points, are soluble in water, and are insoluble in non-polar solvents. Since orpiment is only slightly soluble in hot water and soluble in a basic solution, it indicates more covalent character rather than ionic, suggesting a molecular compound.

Step 3: Analyze Physical Properties of Zeaxanthin

Zeaxanthin is an orange-red solid with a low density of \(0.93 \ \text{g/cm}^3\) at \(25^{\circ} \text{C}\) and a melting point of \(489 \ \text{K}\). It is insoluble in water but soluble in benzene, which is a non-polar solvent, indicating it likely has non-polar or weakly polar covalent bonds.

Step 4: Determine Nature of Bonding in Zeaxanthin

Ionic compounds are typically soluble in water but not in non-polar solvents like benzene. Since zeaxanthin is insoluble in water and soluble in benzene, it suggests that it is a molecular compound rather than ionic.

Key Concepts

Ionic Compounds

When it comes to chemical bonding, ionic compounds stand out due to their distinctive properties and bond formation. Ionic compounds are made up of positively and negatively charged ions, which are attracted to each other by strong electrostatic forces. These forces are what bind the ions together, forming a stable ionic lattice structure.

Typically, ionic compounds showcase the following characteristics:

• High melting and boiling points: The strong attraction between the oppositely charged ions requires significant amounts of energy to break, resulting in high melting and boiling points.
• Solubility in water: Most ionic compounds are soluble in water, as the polar nature of water molecules can disrupt the ionic bonds, allowing the compound to dissolve.
• Insolubility in non-polar solvents: Ionic compounds generally do not dissolve in non-polar solvents like benzene because these solvents do not provide the necessary interactions to separate the ions from each other.
• Electrical conductivity when dissolved: Once dissolved in water or melted, ionic compounds can conduct electricity because the ions are free to move and carry an electric current.

In the context of the exercise, orpiment has properties suggesting it's not an ionic compound, as it's slightly water-soluble and soluble in basic solution rather than extensively water-soluble.

Molecular Compounds

Molecular compounds, also known as covalent compounds, feature atoms bonded together by the sharing of electrons. Unlike ionic compounds, molecular compounds consist of discrete molecules rather than a continuous lattice of ions. This sharing of electrons occurs because the atoms seek to achieve a stable electron configuration, usually resembling that of the nearest noble gas.

Here are some defining features of molecular compounds:

• Lower melting and boiling points: The forces holding molecules together, known as Van der Waals forces or London dispersion forces, are much weaker than ionic bonds, resulting in lower melting and boiling points.
• Varied solubility: Molecular compounds' solubility in water or other solvents depends on the polarity of their molecules. Polar molecular compounds may dissolve in water, while non-polar molecules dissolve more readily in non-polar solvents like benzene.
• Non-conductive nature: Because they don't have free-moving charged particles, molecular compounds do not conduct electricity in solid or liquid form.

In our exercise, both orpiment and zeaxanthin exhibit properties consistent with molecular compounds. Orpiment's slight solubility in hot water and zeaxanthin's solubility in benzene along with lower melting points when compared to typical ionic compounds, support this conclusion.

Solubility Properties

Understanding solubility properties can help in identifying the nature of a compound, whether it is ionic or molecular. Solubility refers to a compound's ability to dissolve in a particular solvent, a process significantly influenced by the nature of the compound's bonds and the solvent's polarity.

Factors affecting solubility include:

• "Like dissolves like": Polar solvents like water are more likely to dissolve polar substances or ionic compounds, while non-polar solvents such as benzene dissolve non-polar substances.
• Temperature: It can influence solubility. Typically, an increase in temperature results in increased solubility of solids in liquids. This can explain why orpiment is slightly soluble in hot water.
• Chemical reactions: In some cases, compounds are soluble due to reactions with the solvent. For example, orpiment's solubility in a basic solution could involve a chemical reaction altering its solubility.

In the exercise, the solubility properties of orpiment and zeaxanthin provide valuable clues. Orpiment's slight solubility in hot water and better solubility in a basic solution compared to zeaxanthin's lack of water solubility but solubility in benzene highlight their molecular nature.