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Chapter 2: Problem 23

Explain why hydrogen fluoride (HF) has a higher boiling temperature than hydrogen chloride (HCl) \(\left(19.4^{\circ} \mathrm{C}\right.\) vs. \(\left.-85^{\circ} \mathrm{C}\right)\), even though HF has a lower molecular weight.

Short Answer

Expert verified
Answer: The higher boiling temperature of HF compared to HCl can be attributed to the presence of hydrogen bonding in HF. While both HF and HCl have dipole-dipole forces and dispersion forces due to their polar nature, HF has a stronger polar bond and hydrogen bonding due to the high electronegativity difference between hydrogen and fluorine. These stronger intermolecular forces in HF lead to its higher boiling temperature compared to HCl.

Step by step solution

01

Identify the relevant properties of HF and HCl

In order to compare the boiling temperatures of HF and HCl, we first need to identify the properties of the two molecules. The molecular weights of the two molecules are given in the problem. Beyond that, we should also consider properties such as electronegativity, bond strength, and intermolecular forces.
02

Compare electronegativities

Electronegativity is the ability of an atom to attract electrons towards itself. In a molecule, a higher difference in electronegativity between the two atoms leads to stronger and more polar bonds. Fluorine (F) is the most electronegative element (3.98) followed by chlorine (Cl) (3.16). Comparing electronegativity values, we can see that the electronegativity difference between hydrogen (H) and fluorine is larger than the difference between hydrogen and chlorine. This implies that the HF molecule is more polar than the HCl molecule.
03

Analyze intermolecular forces

Intermolecular forces play a crucial role in determining the boiling temperature of a substance. Since both HF and HCl are polar molecules, their intermolecular forces include dipole-dipole forces and dispersion forces, where dipole-dipole forces are stronger than dispersion forces.
04

Consider hydrogen bonding

An additional factor to consider is hydrogen bonding, which is a special type of dipole-dipole interaction that occurs between hydrogen and highly electronegative elements like fluorine, oxygen, or nitrogen. Due to the high electronegativity difference between H and F, hydrogen bonding occurs in HF, whereas such strong hydrogen bonding does not occur in HCl. Hydrogen bonding is a very strong intermolecular force and significantly affects boiling temperature.
05

Compare boiling temperatures

Now that we have analyzed the properties and intermolecular forces of HF and HCl, we can explain the difference in their boiling temperatures. Since hydrogen bonding is present in HF but not in HCl, the intermolecular forces in HF are stronger than those in HCl, even though HF has a lower molecular weight. This leads to HF having a higher boiling temperature than HCl.

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With regard to electron configuration, what do all the elements in Group VIIA of the periodic table have in common?

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