Question

Which of the following compounds would you expect to have the highest boiling point? Why? a. \(\mathrm{H}_{2} \mathrm{O}\) b. \(\mathrm{H}_{2} \mathrm{~S}\) C. \(\mathrm{HCl}\)

Short answer

Water (H_2O) is expected to have the highest boiling point due to its ability to form hydrogen bonds, which are stronger than the dipole-dipole interactions present in H_2S and HCl.

Step-by-step solution

Consider Intermolecular Forces

The first step is to consider the types of intermolecular forces present in each substance. Stronger intermolecular forces result in higher boiling points because more energy is required to overcome these forces.

Identify the Types of Intermolecular forces

Hydrogen bonds are the strongest of the intermolecular forces present in these molecules. Determine which molecules can form hydrogen bonds. - H_2O can form hydrogen bonds due to its highly electronegative oxygen atoms and the presence of hydrogen.- H_2S has dipole-dipole interactions but cannot form hydrogen bonds because sulfur is less electronegative and does not create the significant polarity seen in water.- HCl can form dipole-dipole interactions due to the electronegativity difference between hydrogen and chlorine.

Compare Molecular Masses

Generally, boiling point increases with molecular mass when comparing substances with the same type of intermolecular forces. Compare the molecular mass of the compounds to make a final determination if the intermolecular forces are the same.

Determine the Compound with the Highest Boiling Point

Considering the type and strength of intermolecular forces, water has the strongest intermolecular forces due to its ability to form hydrogen bonds. Between H_2S and HCl, even though HCl has a higher molar mass, the difference in intermolecular forces trumps the mass difference. Therefore, water is expected to have the highest boiling point.

Key concepts

Boiling Point

The boiling point of a substance is the temperature at which it changes from a liquid to a gas. This phase transition occurs when the vapor pressure of the liquid equals the atmospheric pressure surrounding the liquid. A key factor affecting boiling point is the strength of intermolecular forces within the substance. Stronger intermolecular attractions require more energy - in the form of heat - to break, leading to a higher boiling point.

Substances with larger molecules typically have higher boiling points due to greater surface area allowing for increased London dispersion forces. However, molecular mass is not the only determinant. The type of intermolecular forces significantly influences boiling points as well, with hydrogen bonding generally resulting in much higher boiling points compared to just dipole-dipole interactions or dispersion forces.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs in molecules containing a hydrogen atom attached to a highly electronegative atom, such as nitrogen, oxygen, or fluorine. The large difference in electronegativity between the hydrogen atom and the electronegative atom creates a polar bond with a positive and a negative end.

When these polar molecules come close to each other, the positive hydrogen end of one molecule is attracted to the negative end of another, forming an intermolecular bond known as a hydrogen bond. Hydrogen bonding significantly increases the boiling point of a substance because these bonds are relatively strong and require additional energy to break. This is why water, with its ability to form multiple hydrogen bonds, has a high boiling point relative to other molecules of similar size.

Dipole-Dipole Interactions

Dipole-dipole interactions are another form of intermolecular force that arises in polar molecules, which have a permanent dipole moment due to the unequal sharing of electrons within their bonds. The positive end of one polar molecule is attracted to the negative end of another, leading to an attractive force between them.

These interactions are not as strong as hydrogen bonds but are stronger than London dispersion forces found in nonpolar molecules. The presence of dipole-dipole interactions also increases the boiling point of a substance, but not to the extent of hydrogen bonding. In cases like HCl, the dipole-dipole interactions contribute to a higher boiling point compared to molecules with only London dispersion forces, but without the significant boost that hydrogen bonds provide, as seen with water.