Problem 25 - Introductory Chemistry Atoms First

Question

Draw a picture that shows how three polar \(\mathrm{HBr}\) molecules in the gas phase would attract one another. What kind of intermolecular force is involved?

Step-by-Step Solution

Answer

The three polar HBr molecules in the gas phase will attract one another through dipole-dipole interactions due to their partial charges. The arrangement can be represented as: H - Br : - - : H - Br : - - : H - Br where the ":" represents the attractive forces between the partially positive hydrogen atom and the partially negative bromine atom.

Step 1: Identify the polarity of HBr molecule

Hydrogen bromide (HBr) is a diatomic molecule consisting of a hydrogen atom (H) and a bromine atom (Br). Since the electronegativity difference between hydrogen and bromine is significant (\(\chi_{H} = 2.20\) and \(\chi_{Br} = 2.96\)), it results in a polar covalent bond and a polar HBr molecule. The polarity of the bond creates a partial positive charge (\(\delta^{+}\)) on the hydrogen atom and a partial negative charge (\(\delta^{-}\)) on the bromine atom.

Step 2: Understand the concept of intermolecular forces

Intermolecular forces are forces that exist between individual molecules. These forces are weaker than the forces responsible for holding the atoms together within a molecule (covalent or ionic bonds). Some common examples of intermolecular forces are hydrogen bonding, dipole-dipole interactions, and London dispersion forces.

Step 3: Sketch the arrangement of three HBr molecules, showing their attractions

As HBr is a polar molecule, we expect dipole-dipole interactions to occur between individual HBr molecules. These occur when the partial positive charge (\(\delta^{+}\)) of one HBr molecule is attracted to the partial negative charge (\(\delta^{-}\)) of another HBr molecule. Arrange the molecules in such a way that the partially positive hydrogen atom of one molecule is near the partially negative bromine atom of another molecule. The interaction between the three HBr molecules can be drawn as: H - Br : - - : H - Br : - - : H - Br In this sketch, ":" represents the attractive forces between the partially positive hydrogen atom and the partially negative bromine atom.

Step 4: Determine the type of intermolecular force involved in the interaction

The intermolecular force involved in the attraction between these polar HBr molecules is the dipole-dipole interaction, which results from the positive and negative partial charges on the hydrogen and bromine atoms, respectively.

Key Concepts

Dipole-Dipole Interactions

Dipole-dipole interactions are a type of intermolecular force that occurs between polar molecules. These interactions happen when the positive end of one polar molecule is attracted to the negative end of another polar molecule. This kind of attraction is common in molecules where the difference in electronegativity between atoms creates partial charges.

The strength of dipole-dipole interactions depends on two main factors:

• The magnitude of the dipole moment: Larger dipole moments lead to stronger interactions.
• The distance between dipoles: Shorter distances allow stronger attractions.

In the case of HBr, the partial positive charge on hydrogen is attracted to the partial negative charge on bromine in another HBr molecule, forming these dipole-dipole connections. This is crucial for understanding how certain substances behave in different phases, such as gases or liquids.

Polarity of HBr Molecules

Hydrogen bromide ( HBr) is a classic example of a polar molecule due to the significant difference in electronegativity between its constituent atoms, hydrogen (H) and bromine (Br). The electronegativity values generally guide how electrons are distributed in a bond.

For HBr:

• Hydrogen has an electronegativity of about 2.20.
• Bromine has an electronegativity of about 2.96.

The difference in electronegativity ( ≈ 0.76) creates a polar covalent bond where electrons are more closely aligned with bromine.

This causes hydrogen to have a partial positive charge ( δ^{+} ) and bromine a partial negative charge ( δ^{-} ), giving rise to a molecular dipole. Understanding the polarity here is key as it dictates how HBr will interact with other molecules around it.

Molecular Structure of Hydrogen Bromide

The molecular structure of hydrogen bromide is quite straightforward, being a diatomic molecule composed of just two atoms, hydrogen and bromine. Despite its simplicity, the arrangement has notable implications for its chemical properties and interactions.

The linear structure means:

• Hydrogen is at one end and bromine is at the other.
• The bond between them is polar due to unequal sharing of electrons.

In the gas phase, these properties allow HBr to form dipole-dipole attractions, lining up so that the δ^{+} hydrogen of one molecule is oriented towards the δ^{-} bromine of another.

These properties collectively influence how the gas behaves, how it dissolves in various solvents, and how it reacts with other compounds. Its molecular structure might be simple, but it is a foundation for understanding many of its chemical behaviors.