Problem 68 - General Chemistry

Question

The halogens form a series of compounds with each other, which are called interhalogens. Examples are bromine chloride (BrCl), iodine bromide (IBr), bromine fluoride (BrF), and chlorine fluoride (CIF). Which compound is expected to have the lowest boiling point at any given pressure? Explain.

Step-by-Step Solution

Answer

ClF is expected to have the lowest boiling point.

Step 1: Understanding Interhalogens

Interhalogens are compounds formed by two different halogens. They tend to exhibit properties that are intermediate between those of their constituent elements. Commonly, interhalogens involve one halogen atom that is larger than the other.

Step 2: Examining the Boiling Points of Halogen Compounds

Boiling points of molecular compounds are generally affected by intermolecular forces, particularly London dispersion forces and dipole-dipole interactions. In the case of diatomic molecules like interhalogens, the boiling point tends to increase with molecular mass and the strength of these forces.

Step 3: Determining Expected Boiling Points Based on Molecular Interactions

Smaller interhalogens (those with smaller molecular mass and weaker intermolecular forces) will typically have lower boiling points. Among BrCl, IBr, BrF, and ClF, ClF has the smallest molecular mass and weakest intermolecular forces, as fluorine and chlorine are the smallest and lightest halogens in the group.

Key Concepts

Halogens

Halogens are a group of elements found in Group 17 of the periodic table, consisting of fluorine, chlorine, bromine, iodine, and astatine. These elements are known for being highly reactive, especially with alkali and alkaline earth metals. The reactivity of halogens is due to their seven valence electrons, needing only one more to achieve a stable electron configuration, similar to noble gases.

In terms of their states at room temperature, halogens can be found in different forms: fluorine and chlorine gas, bromine is a liquid, and iodine is a solid. This variance is due to the differences in their molecular structures and intermolecular forces. Halogens contract in size and increase in electronegativity as you move up the group, with fluorine being the smallest and most electronegative of all. Understanding halogens is key to appreciating the behavior of compounds they form, including interhalogens.

Boiling Points

Boiling points represent the temperature at which a substance switches from a liquid to a gas. The boiling points of molecular compounds are influenced by molecular weight and the type of intermolecular forces present. In simpler terms, stronger interactions between molecules require more energy (higher temperature) to break and move from the liquid phase to gas.

For instance, in interhalogens, the boiling point can be predicted based on the molecular mass and the nature of the intermolecular forces. Therefore, a compound with higher molecular mass generally has a higher boiling point because of increased dispersion forces, unless other factors come into play, like specific structure-related interactions. This explains why an interhalogen like ClF, with its smaller mass, typically has a lower boiling point compared to others like IBr.

Intermolecular Forces

Intermolecular forces are the attractions between molecules. They are crucial because they influence many physical properties, including boiling points, solubility, and state (solid, liquid, gas). The three primary types of intermolecular forces are:

• London dispersion forces
• Dipole-dipole interactions
• Hydrogen bonds

In interhalogens, London dispersion forces and dipole-dipole interactions are most significant. London dispersion forces occur due to temporary fluctuations in electron density, resulting in temporary dipoles, more prevalent in heavier atoms with more electrons.

Dipole-dipole interactions are attractions between polar molecules. The strength of these forces depends on the molecule's mass and shape. Smaller, lighter interhalogens like ClF have weaker dispersion forces and thus lower boiling points, compared to larger, heavier molecules.

Molecular Mass

Molecular mass, or molecular weight, is the sum of the atomic masses of all atoms in a molecule. It's an essential factor in determining physical properties such as density, boiling point, and melting point of a substance.

Generally, heavier molecules exhibit stronger dispersion forces, leading to higher boiling points. This is because larger molecules have a greater surface area for interactions, which increases the strength of dispersion forces. In the case of the interhalogens listed, ClF is the smallest in molecular mass which correlates with having the lowest boiling point due to weaker forces holding its molecules together. Understanding how molecular mass influences these properties helps predict the behavior of chemical substances in varying conditions.