Question

The table shown here lists the molar heats of vaporization for several organic compounds. Use specific examples from this list to illustrate how the heat of vaporization varies with (a) molar mass, (b) molecular shape, (c) molecular polarity, (d) hydrogen-bonding interactions. Explain these comparisons in terms of the nature of the intermolecular forces at work. You may find it helpful to draw out the structural formula for each compound.) $$\overline{)\begin{array}{ll}\text{ Compound }& \begin{array}{c}\text{ Heat of }\\ \text{ Vaporization (kJ/mol) }\end{array}\\ {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_3& 19.0\\ {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_3& 27.6\\ {\mathrm{CH}}_3{\mathrm{CHBrCH}}_3& 31.8\\ {\mathrm{CH}}_3{\mathrm{COCH}}_3& 32.0\\ {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Br}& 33.6\\ {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{OH}& 47.3\end{array}}$$

Short answer

The heat of vaporization of organic compounds varies with (a) molar mass, as seen in the comparison of propane (19.0 kJ/mol) and pentane (27.6 kJ/mol), due to stronger intermolecular forces with more atoms; (b) molecular shape, as observed in propane (19.0 kJ/mol) versus 2-bromopropane (31.8 kJ/mol), where the presence of bromine affects the intermolecular forces; (c) molecular polarity, demonstrated by acetone (32.0 kJ/mol) having stronger dipole-dipole interactions than 1-bromopropane (33.6 kJ/mol) due to its polar C=O bond; and (d) hydrogen-bonding interactions, as shown in the comparison of 1-propanol (47.3 kJ/mol) and 1-bromopropane (33.6 kJ/mol), where the presence of an OH group in 1-propanol contributes to higher heat of vaporization.

Step-by-step solution

(a) Molar Mass

Compare the heat of vaporization for compounds that vary in molar mass but have similar molecular structures. For example, consider propane ($C{H}_{3}C{H}_{2}C{H}_3$, 19.0 kJ/mol) and pentane ($C{H}_{3}C{H}_{2}C{H}_{2}C{H}_{2}C{H}_3$, 27.6 kJ/mol). These compounds have similar molecular structures but vary in molar mass. In general, the heat of vaporization increases with the molar mass of a compound because a higher mass indicates more atoms and stronger intermolecular forces.

(b) Molecular Shape

Observe the heat of vaporization for compounds that differ in their molecular shapes. For instance, we can compare propane ($C{H}_{3}C{H}_{2}C{H}_3$, 19.0 kJ/mol) and 2-bromopropane ($C{H}_{3}CHBrC{H}_3$, 31.8 kJ/mol). Even though both compounds have similar molar masses, they have different molecular shapes due to the presence of the bromine atom in 2-bromopropane, which affects the intermolecular forces and, in turn, the heat of vaporization.

(c) Molecular Polarity

Compare the effect of molecular polarity on heat of vaporization using acetone ($C{H}_{3}COC{H}_3$, 32.0 kJ/mol) and 1-bromopropane ($C{H}_{3}C{H}_{2}C{H}_{2}Br$, 33.6 kJ/mol). These two compounds have similar molar heats of vaporization, but acetone has higher molecular polarity due to its polar C=O bond, leading to stronger dipole-dipole interactions compared to the weaker dispersion forces in 1-bromopropane.

(d) Hydrogen-Bonding Interactions

Analyze the effect of hydrogen bonding on the heat of vaporization by comparing a compound with hydrogen bonding and one without it. For instance, let's consider 1-propanol ($C{H}_{3}C{H}_{2}C{H}_{2}OH$, 47.3 kJ/mol) and 1-bromopropane ($C{H}_{3}C{H}_{2}C{H}_{2}Br$, 33.6 kJ/mol). The presence of the OH group in 1-propanol allows for hydrogen bonding between molecules, which is a strong intermolecular force and contributes to the higher heat of vaporization compared to 1-bromopropane, where only dispersion forces and dipole-dipole interactions are present. In conclusion, the heat of vaporization of organic compounds varies based on factors like molar mass, molecular shape, molecular polarity, and hydrogen-bonding interactions. By analyzing specific examples from the given table, we can understand how these factors influence the strength of intermolecular forces and the energy required to vaporize the compounds.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces that hold molecules together in a liquid or solid state. These forces are significantly weaker than the bonds within a molecule, like covalent bonds, but are critical when it comes to phase changes such as vaporization. There are several types of intermolecular forces, including dispersion (or London dispersion) forces, dipole-dipole interactions, and hydrogen bonds.

• Dispersion Forces: These are the weakest intermolecular forces and exist between all molecules. They arise due to the temporary redistribution of electrons in a molecule, creating temporary dipoles that attract other molecules.
• Dipole-Dipole Interactions: These forces occur between polar molecules with permanent dipoles, where the positive end of one molecule is attracted to the negative end of another molecule.
• Hydrogen Bonds: This is a type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine and is in close proximity to a lone pair on another molecule.

The strength of these intermolecular forces affects a substance's heat of vaporization, which is the energy required to convert a substance from liquid to gas. A higher heat of vaporization indicates stronger intermolecular forces that must be overcome.

Molar Mass

Molar mass is the mass of one mole of a substance and is expressed in grams per mole (g/mol). It is directly related to the molecular weight of a compound, which is the sum of the atomic weights of all the atoms in the molecule. Molar mass can influence the heat of vaporization because heavier molecules tend to have stronger dispersion forces. As the molar mass increases, the electrons in the molecule are more spread out, leading to stronger temporary dipoles and, therefore, stronger intermolecular attractions that make it more energy-intensive to vaporize the substance.

For example, comparing the heat of vaporization for propane ($C{H}_{3}C{H}_{2}C{H}_3\text{, 19.0 kJ/mol}$) and pentane ($C{H}_{3}C{H}_{2}C{H}_{2}C{H}_{2}C{H}_3\text{, 27.6 kJ/mol}$), we can see that pentane, with a higher molar mass, requires more energy to vaporize due to the increased strength of intermolecular forces.

Molecular Polarity

Molecular polarity occurs when there is an uneven distribution of electrons in a molecule, causing it to have a positive end and a negative end, much like a magnet. This asymmetrical distribution of charge arises from differences in electronegativity between atoms within a molecule and the molecular geometry. Polar molecules exhibit stronger dipole-dipole interactions compared to nonpolar molecules, which mainly experience dispersion forces.

For instance, acetone ($C{H}_{3}COC{H}_3\text{, 32.0 kJ/mol}$) has higher molecular polarity due to its polar C=O bond, leading to stronger intermolecular forces compared to 1-bromopropane ($C{H}_{3}C{H}_{2}C{H}_{2}Br\text{, 33.6 kJ/mol}$), which has a lower polarity. The increased polarity affects the heat of vaporization as it requires more energy to break these interactions during the phase change from liquid to gas.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction and a significant factor in determining a compound's heat of vaporization. These bonds form when a hydrogen atom bonded to an electronegative atom (like oxygen, nitrogen, or fluorine) interacts with a lone pair of electrons on a nearby electronegative atom. Hydrogen bonds are stronger than other types of intermolecular forces and require more energy to break.

For example, 1-propanol ($C{H}_{3}C{H}_{2}C{H}_{2}OH\text{, 47.3 kJ/mol}$) can form hydrogen bonds due to the OH group, resulting in a substantially higher heat of vaporization than 1-bromopropane ($C{H}_{3}C{H}_{2}C{H}_{2}Br\text{, 33.6 kJ/mol}$), where such bonding is absent. This comparison demonstrates the significant impact that hydrogen bonding can have on the heat of vaporization, thus highlighting the remarkable energy required to separate molecules that are hydrogen-bonded.