Question

Consider the molecules \(\mathrm{HCl}\) and \(\mathrm{HBr}\). (a) Which molecule has the larger bond dipole moment? Explain why. (b) Which molecule is more polar? Explain why.

Short answer

(a) HCl has the larger bond dipole moment due to the larger difference in electronegativity between H and Cl (\(0.96 > 0.76\)). (b) HCl is more polar because it has a larger bond dipole moment compared to HBr.

Step-by-step solution

Understand bond dipole moments

A bond dipole moment is a measure of the polarity of a chemical bond between two atoms. It results from the difference in electronegativity between the two atoms. The greater the difference in electronegativity, the larger the bond dipole moment.

Compare electronegativity between HCl and HBr

Firstly, we need to understand the electronegativity values of the individual elements. Hydrogen (H) has an electronegativity of 2.20, while chlorine (Cl) has an electronegativity of 3.16 and bromine (Br) has an electronegativity of 2.96. We can calculate the electronegativity difference for each molecule as follows: HCl: \(|3.16 - 2.20| = 0.96\) HBr: \(|2.96 - 2.20| = 0.76\)

Determine bond dipole moments

Since HCl has a larger difference in electronegativity, it has a larger bond dipole moment compared to HBr. Therefore, the answer to part (a) is that HCl has the larger bond dipole moment.

Understand molecularity and polarity

Molecularity refers to the number of atoms in a molecule, while polarity refers to the distribution of the molecules' charges. In our case, both HCl and HBr are diatomic molecules, meaning they each have only two atoms.

Compare the polarity of HCl and HBr

Since both HCl and HBr are diatomic molecules, the polarity of each molecule is determined by their bond dipole moments. We have already established that HCl has a larger bond dipole moment than HBr. As a result, the HCl molecule is more polar than the HBr molecule. Therefore, the answer to part (b) is that HCl is more polar than HBr. In conclusion: (a) HCl has a larger bond dipole moment due to the larger difference in electronegativity between H and Cl. (b) HCl is more polar because it has a larger bond dipole moment compared to HBr.

Key concepts

Electronegativity

Electronegativity is a fundamental concept that describes how strongly an atom attracts electrons in a chemical bond. Different atoms have different electronegativity values, which determine how they interact when bonded to other atoms.

• Atoms with high electronegativity values, such as chlorine (Cl), tend to attract electrons more strongly.
• Atoms with lower electronegativity values, like hydrogen (H), have a weaker pull on electrons.

When two atoms with differing electronegativities form a bond, the electronegativity difference leads to an unequal sharing of electrons. This difference plays a crucial role in determining the bond dipole moment and overall molecular polarity.
For example, in \(\text{HCl}\), chlorine’s higher electronegativity compared to hydrogen results in a notable electronegativity difference of 0.96. This causes electrons to be more attracted to Cl, giving \(\text{HCl}\) a significant dipole moment. In contrast, the electronegativity difference for \(\text{HBr}\) is smaller at 0.76, leading to a lesser dipole moment.

Polarity of Molecules

Polarity in molecules is crucial for determining many of their physical and chemical properties, such as solubility and boiling points.

• A molecule is considered polar when there is an uneven distribution of electron density.
• Such an uneven distribution results in a dipole moment, where one end of the molecule is slightly more negative, and the other end is slightly more positive.

To determine if a molecule is polar, consider both the shape of the molecule and the differences in electronegativity between bonded atoms.
Diatomic molecules like \(\text{HCl}\) can display polarity based solely on the electronegativity difference, as there are only two atoms. Larger differences lead to more pronounced polar bonds, contributing to the molecule's polarity. This is why \(\text{HCl}\) has a larger dipole moment and is more polar than \(\text{HBr}\).

Diatomic Molecules

Diatomic molecules are the simplest type of molecules, consisting of only two atoms. Examples include common molecules like \(\text{H}_2\), \(\text{N}_2\), and halides such as \(\text{HCl}\) and \(\text{HBr}\).

• The bonding in diatomic molecules can be influenced heavily by the electronegativity of each atom.
• Diatomic molecules provide an excellent context to study basic chemical principles like electronegativity and bond polarity.

Each diatomic molecule has unique properties based on its constituent atoms.
In cases like \(\text{HCl}\) and \(\text{HBr}\), the lone pair on the heavier halogens contributes to their physical properties. The polarity and bond dipole in such cases arise directly from the two participating atoms, as these molecules are too small to involve complex interactions from additional atoms.