Question

Why are some molecular solids (such as sugar or ethyl alcohol) soluble in water, while other molecular solids (such as petroleum) are insoluble in water? What structural feature(s) of some molecular solids may tend to make them soluble in water?

Short answer

Molecular solids, such as sugar and ethanol, are soluble in water due to their polar nature and the presence of polar functional groups like hydroxyl (-OH). These groups can interact and form hydrogen bonds with the polar water molecules. In contrast, nonpolar molecular solids like petroleum are insoluble in water because they lack polarity and cannot form hydrogen bonds with water molecules. Overall, the solubility of molecular solids in water is determined by their polarity and the presence of polar functional groups that can interact with the polar solvent, water.

Step-by-step solution

Understanding Polarity and Solubility

Solubility depends on the interactions between the solute (the substance to be dissolved) and the solvent (the substance in which it is being dissolved). In the case of molecular solids and water, we will be considering the effect of polarity on solubility. A polar molecule has a positive and a negative end, or a dipole, due to the difference in electronegativity between its atoms. This results in an uneven distribution of charge across the molecule.

Understanding the role of polarity in water

Water is a polar molecule, with the oxygen atom being more electronegative than the hydrogen atoms. This creates a partial negative charge on the oxygen and partial positive charges on the hydrogen atoms, resulting in a dipole. Water molecules can form hydrogen bonds with other polar molecules or ions, which makes them highly soluble in water.

Solubility of molecular solids in water

Polar molecular solids, like sugar (sucrose) and ethyl alcohol (ethanol), can form hydrogen bonds with water molecules, resulting in a good solubility in water. The polar groups in these molecules, such as the hydroxyl groups (-OH) in sugar and alcohol, can interact with water molecules and dissolve easily. On the other hand, nonpolar molecular solids, like petroleum, are insoluble in water. Nonpolar molecules don't have a significant difference in electronegativity between their atoms, and therefore don't have any strong dipole that can interact with the dipole of water molecules. As a result, they don't form hydrogen bonds with water and remain insoluble.

Structural features that promote solubility

Structural features that promote solubility in water include the presence of polar functional groups, such as hydroxyl (-OH), carboxyl (-COOH), and amino (-NH2) groups. These groups can form hydrogen bonds with water molecules, which facilitates the dissolution of the molecular solid in water. Additionally, the overall polarity of the molecular solid also plays a role in its solubility, as polar molecules will be more inclined to interact with the polar solvent (water).

Key concepts

Molecular Polarity

Molecular polarity plays a crucial role in determining the solubility of substances. A molecule is considered polar when it has a net dipole moment, which means there is an uneven distribution of electron density. This occurs due to differences in electronegativity between atoms.

• In a polar molecule, one end carries a partial positive charge, while the other end carries a partial negative charge.
• This creates a dipole, which can interact with other dipoles or charged particles.

Water, a classic example, is polar due to the difference in electronegativity between hydrogen and oxygen. This property allows water to dissolve other polar substances effectively by forming interactions with them. Thus, polar molecules tend to be soluble in polar solvents, forming stronger intermolecular interactions.

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular attraction that significantly influences solubility. It occurs when a hydrogen atom, covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, experiences attraction to another electronegative atom with a lone pair of electrons.

• These bonds are strong intermolecular forces and can greatly enhance solubility.
• In water, which can both donate and accept hydrogen bonds, this type of interaction is common.

When a substance can form hydrogen bonds with water, it is more likely to dissolve. For example, sugar and ethyl alcohol form hydrogen bonds with the water, making them soluble, whereas nonpolar substances like petroleum cannot engage in hydrogen bonding with water, rendering them insoluble.

Functional Groups

Functional groups are specific groupings of atoms within molecules that have distinct chemical properties and reactivities. They play a significant role in determining the solubility and overall chemical behavior of a molecule. For solubility in water, polar functional groups are especially important:

• Hydroxyl groups (-OH) are common polar groups that can engage in hydrogen bonding.
• Carboxyl groups (-COOH) and amino groups (-NH2) are also effective at forming hydrogen bonds.

These groups enhance the molecule's polarity, making it more amenable to dissolve in polar solvents like water. Molecules with such functional groups can interact with water through hydrogen bonding and dipole interactions, facilitating their solubility.

Solvents and Solubility

The interaction between solvents and solutes dictates solubility. A solvent is a liquid that dissolves a solute to form a solution. The principle of "like dissolves like" means polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes.

• Water is a polar solvent, excellent at dissolving other polar substances due to its ability to form hydrogen bonds.
• Nonpolar solvents, like oils, are better at dissolving nonpolar substances, lacking the ability for such interactions.

Understanding this principle helps in predicting the solubility of substances. For example, polar substances, like sugary compounds, dissolve well in water, while nonpolar compounds such as oils do not. This solubility behavior is critical in diverse applications ranging from biology to industrial chemistry.