Problem 27 - Chemistry: The Central Science

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? Explain. (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 due to their similar polar nature and hydrogen bonding capabilities. (b) The intermolecular attractions between water and glycerol molecules are hydrogen bonds, formed between the oxygen atom of the hydroxyl group in glycerol and the hydrogen atom of the water molecule, and vice versa.

Step 1: Understand the molecular structures of water and glycerol

Water has a molecular formula of H2O, and glycerol has a molecular formula of \(\mathrm{CH}_{2}(\mathrm{OH})\mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{OH}\). Both compounds contain oxygen and hydrogen atoms. In water, the oxygen atom is bound to two hydrogen atoms, while in glycerol, there are three hydroxyl groups \((\mathrm{OH})\), each connected to a carbon atom.

Step 2: Determine miscibility

A rule of thumb when considering whether two substances are miscible is "like dissolves like." This means that polar substances are more likely to mix well with other polar substances, and nonpolar substances are more likely to mix well with other nonpolar substances. Both water and glycerol are polar compounds, as they have oxygen and hydrogen atoms, which leads to hydrogen bonding. Since they are both polar substances, it can be expected that they would be miscible in all proportions.

Step 3: Explain miscibility

Water and glycerol are miscible in all proportions because they are both polar and exhibit hydrogen bonding. The hydrogen bonds between their molecules enable them to mix well, as they have similar interactions with each other.

Step 4: Identify intermolecular attractions

The specific intermolecular attractions occurring between water and glycerol molecules are hydrogen bonds. These hydrogen bonds are formed between the oxygen atom of the hydroxyl group in glycerol and the hydrogen atom of the water molecule. Additionally, there can be hydrogen bonding between the oxygen atom of the water molecule and the hydrogen atom of the hydroxyl group in glycerol. In summary: (a) Water and glycerol are expected to be miscible in all proportions due to their similar polar nature and hydrogen bonding capabilities. (b) The intermolecular attractions between water and glycerol molecules are hydrogen bonds, formed between the oxygen atom of the hydroxyl group in glycerol and the hydrogen atom of the water molecule, and vice versa.

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a unique and strong type of dipole-dipole attraction. It occurs when a hydrogen atom is directly bonded to a highly electronegative atom such as oxygen, nitrogen, or fluorine. In the case of water (H₂O) and glycerol (C₃H₈O₃), hydrogen bonding plays a significant role in their behavior and interactions.

• Water has hydrogen atoms bonded to oxygen, creating a polar molecule with areas of partial positive and negative charges due to the uneven distribution of electrons.
• Glycerol, with its three hydroxyl (-OH) groups, also supports strong hydrogen bonding. Each hydroxyl group in glycerol can form hydrogen bonds with water and other hydroxyl groups.

Hydrogen bonds are not actual chemical bonds but are strong enough to significantly influence the properties of compounds. They increase boiling and melting points, and in the case of water and glycerol, enable them to mix thoroughly, displaying complete miscibility.

Polar Molecules

Polar molecules are those in which there is an uneven distribution of electron density, leading to regions of positive and negative charge. This is typically due to the presence of polar bonds, where electrons are shared unequally between atoms. Water and glycerol are classic examples.

• In water, the oxygen atom draws the shared electrons from hydrogen towards itself, resulting in a partial negative charge on the oxygen and a partial positive charge on the hydrogen atoms.
• Glycerol, consisting of multiple hydroxyl groups, has similar polar characteristics where the electron-rich oxygen atoms contribute to its overall polarity.

These polar interactions make both water and glycerol compatible with substances that also have polar characteristics, leading to their miscibility. Polar substances tend to interact well through dipole-dipole interactions or hydrogen bonds, demonstrating the adage "like dissolves like."

Miscibility

Miscibility refers to the ability of two substances to mix and form a homogeneous solution. When two liquids are miscible, they dissolve in each other in all proportions. The concept of miscibility is vital when considering mixtures like water and glycerol.

• The high degree of polarity in both water and glycerol, supported by their capacity for hydrogen bonding, allows them to be miscible. Their similar polarity means that they prefer to interact with each other rather than separating.
• When dissolved, the molecules of these liquids freely intermingle due to their mutual attractions, forming a stable mixture that does not separate over time.

Unlike polar substances, nonpolar substances do not mix well with polar ones because they lack such interactions. The principle that "like dissolves like" highlights the importance of matching polarity and intermolecular forces to achieve miscibility.