The boiling point of ethane (C2H6) and hydrogen chloride (HCl) are very close (-88.6 and -84.8°C, respectively). Which statement explains this observation? A. The nonpolar ethane exhibits polar character similar to HCl when boiled. B. The prediction that HCl is polar must be false. C. The London forces are significant in ethane due to the shape such that the intermolecular forces are comparable to those found in the polar HCl. D. Intermolecular forces (polar/nonpolar) do not influence properties such as boiling points; boiling points are predicted solely on molecular weight. E. The polar HCl has a lower boiling point than expected because it is an acid.
The correct answer and explanation is:
The correct answer is:
C. The London forces are significant in ethane due to the shape such that the intermolecular forces are comparable to those found in the polar HCl.
Explanation:
Boiling points depend largely on the strength of intermolecular forces (IMFs) between molecules. Stronger IMFs require more energy (higher temperature) to break, thus raising the boiling point.
Ethane (C₂H₆) is a nonpolar molecule made up of carbon and hydrogen atoms. It does not have a permanent dipole moment, so its primary intermolecular forces are London dispersion forces (a type of van der Waals force). These forces arise from temporary fluctuations in electron density creating instantaneous dipoles that induce dipoles in neighboring molecules. Although these forces are generally weaker than dipole-dipole interactions, they become significant in molecules with more electrons and particular shapes, which increase polarizability.
Hydrogen chloride (HCl) is a polar molecule with a permanent dipole moment because of the electronegativity difference between hydrogen and chlorine atoms. It exhibits dipole-dipole interactions along with London forces. Dipole-dipole forces are typically stronger than dispersion forces, which usually leads to higher boiling points for polar molecules than for nonpolar molecules of similar molar mass.
Why are the boiling points close?
- The molar masses of ethane (30 g/mol) and hydrogen chloride (36.5 g/mol) are fairly close, which affects London forces.
- Ethane’s shape and electron cloud distribution allow significant London dispersion forces.
- Though HCl has dipole-dipole forces, these aren’t so strong to raise its boiling point drastically above that of ethane.
- Thus, the overall strength of the London forces in ethane is comparable to the dipole-dipole forces in HCl, making their boiling points close.
Why the other options are incorrect:
- A: Ethane does not exhibit polar character when boiled; it remains nonpolar.
- B: HCl is indeed polar, and this is well established experimentally and theoretically.
- D: Molecular weight affects boiling points but intermolecular forces are a primary factor.
- E: HCl’s boiling point is not lowered because it is an acid; acidity does not directly influence boiling point.
In conclusion, the close boiling points are due to significant London forces in ethane being comparable to the dipole-dipole forces in HCl, making C the best explanation.