Question

Ethylene glycol, \(\mathrm{CH}_{2} \mathrm{OHCH}_{2} \mathrm{OH}\), and pentane, \(\mathrm{C}_{5} \mathrm{H}_{12}\), have approximately the same molar mass. Nevertheless, one of these compounds boils at \(198{ }^{\circ} \mathrm{C}\), and the other boils at \(36{ }^{\circ} \mathrm{C}\). Which compound boils at which temperature? Use an argument based on intermolecular forces to justify your choice.

Short answer

Ethylene glycol boils at \(198{ }^{\circ} \mathrm{C}\), while pentane boils at \(36{ }^{\circ} \mathrm{C}\). Ethylene glycol has stronger intermolecular forces due to hydrogen bonding, whereas pentane only has the weaker dispersion forces. As a result, ethylene glycol requires more energy to transition from liquid to gas, leading to its higher boiling point.

Step-by-step solution

Identify the intermolecular forces present in each compound

For ethylene glycol (CH2OHCH2OH) and pentane (C5H12), we will determine the types of intermolecular forces (IMF) present in each. Ethylene glycol (CH2OHCH2OH) is a polar molecule with -OH hydroxyl groups. These groups can form hydrogen bonds with other ethylene glycol molecules. Additionally, it has dispersion forces (also known as London dispersion forces or van der Waals forces) because all molecules exhibit these forces. Pentane (C5H12) is a nonpolar hydrocarbon. It cannot form hydrogen bonds or any polar interactions. Therefore, its only intermolecular force is dispersion forces.

Compare the strength of the intermolecular forces

Hydrogen bonding is a stronger intermolecular force than dispersion forces. This is because hydrogen bonds involve strong dipole-dipole attractions between molecules, while dispersion forces are caused by temporary dipoles that arise from the motion of electrons in a molecule. Ethylene glycol (CH2OHCH2OH) can form hydrogen bonds, which are stronger than the dispersion forces present in pentane (C5H12). Therefore, ethylene glycol has stronger intermolecular forces than pentane.

Correlate intermolecular forces to boiling points

A higher boiling point indicates stronger intermolecular forces holding the molecules together. Thus, the substance with stronger intermolecular forces requires more energy to overcome these forces and transition from liquid to gas. Since ethylene glycol has stronger intermolecular forces than pentane, it should have a higher boiling point.

Determine which compound boils at which temperature

From our analysis of their intermolecular forces, we determined that ethylene glycol has a higher boiling point than pentane. So, ethylene glycol corresponds to the higher boiling point of 198°C and pentane corresponds to the lower boiling point of 36°C.

Result

Ethylene glycol boils at 198°C, while pentane boils at 36°C. This is because ethylene glycol has stronger intermolecular forces (hydrogen bonding) than pentane (dispersion forces).

Key concepts

Boiling Point

The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the surrounding atmospheric pressure, causing the liquid to transition into a gas. As one of the key properties of matter, the boiling point helps us understand how a substance behaves under heat and the strength of the forces that hold its molecules together.

For example, ethylene glycol has a much higher boiling point than pentane. This is because boiling requires breaking the intermolecular forces between molecules, and ethylene glycol, with its ability to form hydrogen bonds, has more substantial forces to overcome. Higher boiling points are indicative of stronger intermolecular forces and the amount of energy required to change the phase of a substance from liquid to gas.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine. This bond creates a strong positive dipole on the hydrogen, which can then interact with negative dipoles on the same or different molecules.

In ethylene glycol, the -OH hydroxyl groups are capable of forming these strong hydrogen bonds, which significantly increases the substance's boiling point. These bonds are much stronger compared to the weaker dispersion forces in nonpolar molecules, which is why substances with hydrogen bonding generally require more heat energy to reach their boiling point.

Dispersion Forces

Dispersion forces, or London dispersion forces, are weak intermolecular attractions that arise from the temporary fluctuations in electron density in molecules. These momentary changes create instantaneous dipoles that can induce corresponding dipoles in nearby molecules leading to attraction.

All molecules have dispersion forces, but their strength increases with the size of the molecule or molar mass and the shape's ability to facilitate close interaction between molecules. Pentane, being a nonpolar hydrocarbon, only has these dispersion forces as its intermolecular force. This is why pentane has a relatively low boiling point because its molecules can be separated with much less energy compared to molecules that form hydrogen bonds.

Polarity of Molecules

Polarity in molecules arises from an uneven distribution of electrons, which creates a dipole with distinct partial positive and negative charges. Molecules with polar bonds often have higher melting and boiling points because of the strong intermolecular forces such as hydrogen bonding and dipole-dipole interactions.

For instance, the polarity of ethylene glycol results from the electronegativity difference between the hydrogen and oxygen atoms in its -OH groups, leading to a dipole moment. This polarity allows it to engage in stronger intermolecular attractions, in contrast to nonpolar molecules like pentane that lack such interactive capability.

Molar Mass

Molar mass is the mass of one mole of substance, expressed in grams per mole (g/mol). It's a reflection of the combined mass of all the atoms in a molecule. Although ethylene glycol and pentane have similar molar masses, their boiling points differ greatly due to the types of intermolecular forces acting within the compound.

While molar mass does influence the strength of dispersion forces—larger molecules, or those with higher molar masses, generally have stronger dispersion forces — it is not the only determinant of boiling point. The presence of other types of intermolecular forces, such as hydrogen bonding in the case of ethylene glycol, play a critical role too. Thus, understanding molar mass in conjunction with intermolecular forces provides valuable insight into the thermal properties of a substance.