Problem 56 - Chemistry: An Atoms-Focused Approach

Question

Which of the following pairs of substances is likely to be miscible? a. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (ethanol) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\) (diethyl ether) b. \(\mathrm{CH}_{3} \mathrm{OH}\) (methanol) and methyl amine \(\left(\mathrm{CH}_{3} \mathrm{NH}_{2}\right)\) c. \(\mathrm{CH}_{3} \mathrm{CN}\) (acetonitrile) and acetone \(\left(\mathrm{CH}_{3} \mathrm{COCH}_{3}\right)\) d. \(\mathrm{CF}_{3} \mathrm{CHF}_{2}\) (a Freon replacement) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (pentane)

Step-by-Step Solution

Answer

Answer: Pairs a, b, and c are likely to be miscible due to their similarities in polarity and functional groups.

Step 1: Identify the solubility rule

Birds of a feather flock together, so if the two substances in a pair have similar polarity, especially with the functional groups, then the pair of substances is likely to be miscible.

Step 2: Determine the polarity of the molecules in pair a

In pair a, both ethanol (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\)) and diethyl ether (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\)) have polar oxygen (O) atoms, which indicates that these molecules are likely to be miscible.

Step 3: Determine the polarity of the molecules in pair b

In pair b, both methanol (\(\mathrm{CH}_{3} \mathrm{OH}\)) and methyl amine (\(\mathrm{CH}_{3} \mathrm{NH}_{2}\)) have polar oxygen (O) and nitrogen (N) atoms, respectively, with hydrogen which makes them able to form hydrogen bonds. Due to the presence of these hydrogen bonding interactions, these molecules are likely to be miscible as well.

Step 4: Determine the polarity of the molecules in pair c

In pair c, we have acetonitrile (\(\mathrm{CH}_{3} \mathrm{CN}\)) and acetone (\(\mathrm{CH}_{3} \mathrm{COCH}_{3}\)). Both molecules have a polar carbon-nitrogen and carbon-oxygen bond, respectively. Due to the presence of polar bonds and similar functional groups (carbonyl and nitrile groups), these molecules are likely to be miscible.

Step 5: Determine the polarity of the molecules in pair d

In pair d, we have a Freon replacement (\(\mathrm{CF}_{3} \mathrm{CHF}_{2}\)) and pentane (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\)). The Freon replacement has polar C-F and C-H bonds, while pentane is a nonpolar hydrocarbon. Due to the difference in polarity, these two substances are unlikely to be miscible.

Step 6: Identify the likely miscible pair

Based on the analysis of molecular structures and polarity, pairs a, b, and c indicate miscible pairs due to their similarities in polarity and functional groups.

Key Concepts

Polarity

Polarity is a fundamental concept that helps explain whether two substances can mix well together, like oil and water or alcohol in water. It relates to the positive and negative charges in a molecule. When molecules have a balanced distribution of charge, they are nonpolar. However, when there is an uneven distribution of charge, creating partial positive or negative ends, those molecules are polar.

Polar substances tend to dissolve well in other polar substances, while nonpolar substances mix with other nonpolar substances. Think of this like how friends with similar interests tend to get along best. In the context of the exercise, we need to identify which of the provided pairs of substances have similar polarity, as this will indicate their ability to be miscible or mix well together. Examining the types of bonds and charges they have can help make this determination.

Functional Groups

Functional groups are specific groupings of atoms within molecules that determine the chemical behavior of those molecules. They play an important role in defining a molecule's polarity and reactivity features.

For example, in ethanol (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\)), the functional group is the hydroxyl group (\(-OH\)), which contributes to its polar nature. In the exercise, you also see the functional groups like ether (\(-O-\)) in diethyl ether, amine (\(-NH_{2}\)) in methyl amine, and carbonyl (\(\mathrm{C=O}\)) in acetone.

These groups often have atoms like oxygen or nitrogen, which are electronegative, meaning they pull electrons closer and thus create a polar bond. Recognizing these functional groups can readily aid in guessing whether molecules with different structures might interact and dissolve into each other.

Hydrogen Bonding

Hydrogen bonding is a special type of interaction that occurs when a hydrogen atom is attracted to a highly electronegative atom, such as oxygen, nitrogen, or fluorine. This creates a strong dipole-dipole attraction that influences how molecules interact.

In our examples from the exercise, methanol (\(\mathrm{CH}_{3} \mathrm{OH}\)) and methyl amine (\(\mathrm{CH}_{3} \mathrm{NH}_{2}\)) have hydrogen bonds due to the presence of \(-OH\) and \(-NH_{2}\) groups. Because of these hydrogen bonds, these molecules are more likely to mix with one another.

Hydrogen bonding is significant as it makes molecules "stick" to each other, explaining why water forms droplets and why it is a great solvent for many substances. This concept is key in understanding why certain pairs of substances in the exercise are miscible.