Problem 27 - Chemistry: The Central Science in SI Units, Global Edition

Question

Consider water and glycerol, \(\mathrm{CH}_{2}(\mathrm{OH}) \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{OH}\). (a) Would you expect them to be miscible in all proportions? (b) List the intermolecular attractions that occur between a water molecule and a glycerol molecule.

Step-by-Step Solution

Answer

(a) Yes, water and glycerol are expected to be miscible in all proportions, as both are polar molecules with hydroxyl groups that follow the "like dissolves like" principle. (b) The intermolecular attractions between water and glycerol molecules include hydrogen bonding and dipole-dipole interactions.

Step 1: Analyze the molecular structure of water and glycerol

For water, the molecular formula is H₂O, which has two hydrogen atoms bonded to one oxygen atom with a bent molecular shape. It's a polar molecule with a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom. Glycerol has the molecular formula CH₂(OH)CH(OH)CH₂(OH). It contains three hydroxyl groups (OH) and has a linear molecular shape. The presence of hydroxyl groups makes glycerol a polar molecule as well.

Step 2: Determine miscibility based on molecular structure

Since both water and glycerol are polar molecules, they should be miscible according to the "like dissolves like" principle. Polar molecules generally dissolve in polar solvents, and water is considered a universal polar solvent. (a) So, we would expect water and glycerol to be miscible in all proportions.

Step 3: Identify the intermolecular attractions between water and glycerol

(b) There are two main types of intermolecular attractions that can occur between water and glycerol molecules: 1. Hydrogen bonding: Due to the presence of polar O-H groups in both water and glycerol, they can form hydrogen bonds with each other. The oxygen atom in water can form a hydrogen bond with a hydrogen atom from the hydroxyl group in glycerol. Similarly, the oxygen atom in glycerol can form a hydrogen bond with a hydrogen atom of water. 2. Dipole-dipole interactions: Both water and glycerol are polar molecules, which means that they have a positive and a negative end (a dipole moment). The positive end of one molecule can attract the negative end of another, leading to dipole-dipole interactions between water and glycerol molecules.

Key Concepts

Miscibility

Miscibility refers to the ability of two substances to mix and form a homogeneous solution. This property depends largely on molecular polarity. According to the 'like dissolves like' rule, molecules with similar polarities tend to be miscible. This occurs because similar types of molecules interact more favorably with one another, minimizing energy in the mixture.
In the case of water and glycerol, both substances are polar. Water is everywhere in our lives, enjoying the moniker of a universal polar solvent. Glycerol, with its three hydroxyl (OH) groups, is also polar. Because both molecules exhibit polarity, they can readily mix. Hence, water and glycerol are expected to be fully miscible in any proportion. Note that this is why glycerol can be easily used in water-based products in everyday items like skincare solutions and cough syrups.
Overall, understanding miscibility requires grasping the idea that molecules with similar polar characteristics tend to dissolve in each other - creating uniformity and stability in solutions.

Hydrogen bonding

Hydrogen bonding is a strong type of dipole-dipole attraction that occurs when a hydrogen atom is bound to a highly electronegative atom like oxygen, nitrogen, or fluorine. This bond is not as strong as a covalent bond but is significant enough to influence the physical properties of compounds.
Water molecules are classics in forming hydrogen bonds due to their structure: each water molecule can form up to four hydrogen bonds, contributing to water's high boiling point and surface tension.
When mixed, water and glycerol can form such hydrogen bonds extensively:

• The oxygen atom in water can connect with the hydrogen from glycerol's hydroxyl groups.
• Similarly, the oxygen in glycerol can bond with hydrogen from water.

These interactions are responsible for the high level of miscibility between the two substances. Additionally, these bonds make mixtures of water and glycerol more viscous compared to water alone, as they create a network of tightly linked molecules.

Dipole-dipole interactions

Dipole-dipole interactions occur between molecules that have permanent dipole moments. These are molecules where there is an unequal distribution of electron density, creating regions of positive and negative charge.
Both water and glycerol are polar, meaning they have a separation of charge that allows them to interact through dipole-dipole interactions. The positive end of one polar molecule will attract the negative end of another, reinforcing their interaction and alignment.

• Water features a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom.
• Glycerol's hydroxyl groups also exhibit polarity, aligning the slightly negative oxygen with the slightly positive hydrogen of nearby molecules.

These interactions are a key component in understanding the miscibility and the resulting physical properties of a water-glycerol solution. They not only allow the two substances to dissolve in one another but also influence how they respond under different conditions, such as temperature changes.