Problem 18 - Chemistry and Chemical Reactivity

Question

Place the following four compounds in order of increasing boiling point: (a) \(\mathrm{SCl}_{2}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{6}\) (b) \(\mathrm{NH}_{3}\) (d) Ne

Step-by-Step Solution

Answer

Ne < C₂H₆ < SCl₂ < NH₃

Step 1: Identify Intermolecular Forces

To determine the order of boiling points, first identify the types of intermolecular forces present in each compound. - \(\text{SCl}_{2}\) has dipole-dipole interactions and London dispersion forces.- \(\text{C}_{2}\text{H}_{6}\) has only London dispersion forces.- \(\text{NH}_{3}\) has hydrogen bonding, dipole-dipole interactions, and London dispersion forces.- Ne, being a noble gas, has only very weak London dispersion forces.

Step 2: Estimate Boiling Points by Intermolecular Force Strength

Generally, stronger intermolecular forces correlate with higher boiling points. Rank the forces from strongest to weakest:1. Hydrogen Bonding (strongest)2. Dipole-Dipole Interactions3. London Dispersion Forces (weakest)Based on this:- \(\text{NH}_{3}\) with hydrogen bonding should have the highest boiling point.- \(\text{SCl}_{2}\) with dipole-dipole interactions will have a moderate boiling point.- \(\text{C}_{2}\text{H}_{6}\) with only London dispersion forces will have a lower boiling point.- Ne with only weak London dispersion forces should have the lowest boiling point.

Step 3: Arrange Compounds by Boiling Points

Using the analysis of intermolecular forces, arrange the compounds in order of increasing boiling point.Ne < \(\text{C}_{2}\text{H}_{6}\) < \(\text{SCl}_{2}\) < \(\text{NH}_{3}\)

Step 4: Final Order Confirmation

Confirm that the relative order makes sense based on the presence and strength of intermolecular forces. Hydrogen bonding in \(\text{NH}_{3}\) is the strongest, while Ne, with the weakest London dispersion forces, correctly places it at the lowest boiling point among the provided compounds.

Key Concepts

Boiling Point

The boiling point of a substance is the temperature at which it changes from a liquid to a gas. It is a crucial aspect of understanding a compound's physical properties. The boiling point depends significantly on the strength of the intermolecular forces present in the substance. Stronger intermolecular forces lead to higher boiling points because more energy is required to break these forces and convert the liquid into vapor.
Consider these key points when thinking about boiling points:

• Substances with high boiling points usually have strong intermolecular forces.
• Those with weaker intermolecular forces tend to have lower boiling points.
• The presence and type of intermolecular forces greatly influence the boiling point of a compound.

This not only helps in categorizing substances but also in predicting how they might behave under different conditions.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules that have permanent dipoles, meaning there is an uneven distribution of electrons. This creates a positive end and a negative end within the molecule, which can then attract other molecules with a similar dipole formation.
Understanding dipole-dipole interactions is easier if you remember these points:

• They exist in polar molecules where there is a significant difference in electronegativity between the atoms.
• This type of interaction contributes to higher boiling points compared to nonpolar molecules with only London dispersion forces.
• They are stronger than London dispersion forces but weaker than hydrogen bonds.

In practice, compounds like \(\text{SCl}_{2}\) exhibit these forces, explaining its moderate boiling point compared to other molecules with different types of intermolecular forces.

Hydrogen Bonding

Hydrogen bonding is one of the strongest types of dipole-dipole interactions. It occurs when a hydrogen atom covalently bonded to a very electronegative atom, such as nitrogen, oxygen, or fluorine, experiences an attraction to another electronegative atom.
Hydrogen bonding has distinct characteristics:

• It significantly increases boiling points due to its strong interaction strength.
• It is commonly found in water (H2O), ammonia (NH3), and other polar molecules.
• Hydrogen bonds are essential for the structure of DNA and proteins, highlighting their biological importance.

In the original exercise, ammonia (\(\text{NH}_3\)) exhibits hydrogen bonding, giving it the highest boiling point among the listed compounds.

London Dispersion Forces

London dispersion forces are the weakest of the intermolecular forces. They arise from momentary fluctuations in electron density which create a temporary dipole in atoms or nonpolar molecules. These temporary dipoles can induce further dipoles in surrounding molecules, leading to attraction.
Here are some key features of London dispersion forces:

• They are present in all molecules, whether polar or nonpolar, but are the only forces in nonpolar molecules.
• Their strength increases with the size and molar mass of the molecules, as larger atoms have more electrons which can form temporary dipoles.
• Among the given compounds, \(\text{C}_{2}\text{H}_{6}\) and Ne rely solely on London dispersion forces, explaining their relatively low boiling points.

Understanding these forces is vital to grasping why such molecules have the physical properties they do, including how they behave in various states of matter.