Question

Carbon diselenide \(\left(\mathrm{CSe}_{2}\right)\) is a liquid at room temperature. The normal boiling point is \(125^{\circ} \mathrm{C}\), and the melting point is \(-45.5^{\circ} \mathrm{C}\). Carbon disulfide \(\left(\mathrm{CS}_{2}\right)\) is also a liquid at room temperature with normal boiling and melting points of \(46.5^{\circ} \mathrm{C}\) and \(-111.6^{\circ} \mathrm{C}\), respectively. How do the strengths of the intermolecular forces vary from \(\mathrm{CO}_{2}\) to \(\mathrm{CS}_{2}\) to \(\mathrm{CSe}_{2}\) ? Explain.

Short answer

The intermolecular forces increase in strength from carbon dioxide to carbon disulfide to carbon diselenide, as indicated by their boiling points: CO₂ < CS₂ < CSe₂. However, the melting point trend (CO₂ < CSe₂ < CS₂) suggests that the intermolecular forces between CS₂ and CSe₂ are fairly comparable. This variation in strength can be attributed to differences in molecular size and electronegativity, with larger atoms and higher polarizability leading to stronger London dispersion forces.

Step-by-step solution

List down the boiling and melting points of each substance

: We will begin by listing the boiling and melting points of each substance: - Carbon dioxide (CO₂): Boiling point: -78.5°C, Melting point: -56.6°C - Carbon disulfide (CS₂): Boiling point: 46.5°C, Melting point: -111.6°C - Carbon diselenide (CSe₂): Boiling point: 125°C, Melting point: -45.5°C Please note that CO2 has a sublimation point at 1 atm pressure instead of boiling point due to its phase diagram. The given temperatures are approximations

Compare the boiling points

: The boiling point can provide a good indication of the strength of the intermolecular forces in a substance. A higher boiling point implies stronger intermolecular forces, while a lower boiling point implies weaker intermolecular forces. Using the boiling points we listed in step 1, we can compare the substances as follows: CO₂ < CS₂ < CSe₂

Compare the melting points

: Melting points can also be used as an indicator of the strength of intermolecular forces within a substance. However, please note that it's not always perfectly correlated with boiling points. We can still compare the substances using their melting points: CO₂ < CSe₂ < CS₂

Evaluate the difference in intermolecular forces between the substances

: From our comparison in steps 2 and 3, the boiling point trend of the substances is: CO₂ < CS₂ < CSe₂ This indicates that the intermolecular forces increase in strength from carbon dioxide to carbon disulfide to carbon diselenide. However, the melting points have slightly different trends: CO₂ < CSe₂ < CS₂ Considering both the boiling points and melting points, we can conclude that the strength of the intermolecular forces varies between these substances in the following way: CO₂ < CS₂ ≈ CSe₂ The intermolecular forces are weakest for carbon dioxide, stronger for carbon disulfide, and comparable in strength between carbon disulfide and carbon diselenide. This difference in strength can be attributed to the differences in molecular size and the presence of different types of electronegative atoms within the molecules. As the atom substitution changes (O to S to Se), the polarizability of the atoms increases leading to a higher London dispersion force between the molecules.

Key concepts

Boiling Point

Boiling point is a key characteristic that indicates the temperature at which a liquid turns into a vapor. Understanding the boiling point is crucial for comparing the intermolecular forces of different substances. In general, a higher boiling point suggests stronger intermolecular forces. This is because more energy is needed to overcome these forces to allow molecules to escape into the vapor phase. From the problem statement, we know the following boiling points:

• Carbon dioxide (\(CO₂\)): -78.5°C
• Carbon disulfide (\(CS₂\)): 46.5°C
• Carbon diselenide (\(CSe₂\)): 125°C

Here, carbon diselenide has the highest boiling point, indicating it has the strongest intermolecular forces, while carbon dioxide has the lowest, indicating the weakest intermolecular forces. This makes boiling points a reliable indicator for evaluating the strength of these forces across different compounds.

Melting Point

Melting point refers to the temperature at which a solid turns into a liquid. Unlike boiling points, melting points do not always directly correlate with the strength of intermolecular forces. However, they still provide useful information about the stability of a solid structure:

• Carbon dioxide (\(CO₂\)): -56.6°C
• Carbon disulfide (\(CS₂\)): -111.6°C
• Carbon diselenide (\(CSe₂\)): -45.5°C

The melting points show that carbon disulfide has the lowest melting point, which might indicate weaker lattice forces compared to carbon diselenide. Meanwhile, carbon dioxide sublimates instead of melting under normal atmospheric conditions, reflecting its weak intermolecular forces and a different solid-state phase transition. Melting points help understand the relative stability of solids but might not perfectly align with boiling point trends.

Carbon Disulfide

Carbon disulfide (\(CS₂\)) is an important compound with distinct physical properties as a liquid at room temperature. With a boiling point of 46.5°C and melting point of -111.6°C, it exhibits moderately strong intermolecular forces. The binding forces in carbon disulfide primarily come from London dispersion forces. These forces increase with the size and electron cloud of the molecule, making larger molecules like carbon disulfide exhibit higher boiling points than smaller ones like carbon dioxide.

• It is more stable in its liquid state compared to carbon dioxide.
• Its boiling point is reflective of stronger intermolecular interactions than those present in carbon dioxide.

Carbon disulfide's physical properties highlight the significance of molecular size and type in determining the intermolecular forces within different substances.

Carbon Dioxide

Carbon dioxide (\(CO₂\)) is a common compound known for its gaseous state at room temperature and atmospheric pressure. It has a sublimation point at -78.5°C, which means it transitions directly from a solid to a gas without becoming a liquid under normal conditions. Because of its simple molecular structure and low polarizability, carbon dioxide exhibits the weakest intermolecular forces among the substances discussed here.

• Its weak intermolecular forces are primarily due to its small size and linear molecular geometry.
• This makes carbon dioxide exist naturally as a gas under typical environmental conditions.

Despite its widespread presence in the atmosphere, the comparatively weak intermolecular forces in carbon dioxide reduce its boiling point and melting point compared to larger molecules like carbon disulfide and carbon diselenide.

Carbon Diselenide

Carbon diselenide (\(CSe₂\)) is a liquid at room temperature with significant intermolecular forces, as evidenced by its high boiling point of 125°C. Intermolecular forces in carbon diselenide are mainly due to its polarizable nature. The larger selenium atoms contribute to the higher London dispersion forces compared to sulfur in carbon disulfide or the oxygen in carbon dioxide.

• It has stronger intermolecular forces than both carbon dioxide and carbon disulfide.
• The higher energy required to separate its molecules is reflected in its higher boiling point.

Carbon diselenide serves as a good example of how increasing atomic size and polarizability can lead to stronger intermolecular forces, causing increased boiling and differing melting points.