Question

In order for a liquid to boil, the intermolecular forces in the liquid must be overcome. Based on the types of intermolecular forces present, arrange the expected boiling points of the liquid states of the following substances in order from lowest to highest: \(\mathrm{NaCl}(l), \mathrm{He}(l), \mathrm{CO}(l), \mathrm{H}_{2} \mathrm{O}(l)\).

Short answer

The order of the boiling points of the liquid states of the substances from lowest to highest is: \(\mathrm{He}(l), \mathrm{CO}(l), \mathrm{H}_{2}\mathrm{O}(l), \mathrm{NaCl}(l)\).

Step-by-step solution

Identify the type of intermolecular forces in each substance

We will start by identifying the forces that are present in each of the given substances. 1. \(\mathrm{NaCl}(l)\): Ionic bond (between sodium and chloride ions) 2. \(\mathrm{He}(l)\): Dispersion forces (weakest intermolecular force, present in all substances, but dominant in noble gases) 3. \(\mathrm{CO}(l)\): Polar covalent bond (between carbon and oxygen atoms) and dispersion forces 4. \(\mathrm{H}_{2}\mathrm{O}(l)\): Hydrogen bond (strongest type of dipole-dipole interaction) and dispersion forces

Arrange the substances by boiling point

Now we will arrange the substances in order of increasing boiling point, based on the strength of their intermolecular forces. 1. Dispersion forces are the weakest, so \(\mathrm{He}(l)\) will have the lowest boiling point. 2. Among polar covalent bonds, hydrogen bonds are stronger, so the boiling point of \(\mathrm{H}_{2}\mathrm{O}(l)\) will be higher than that of \(\mathrm{CO}(l)\). 3. Ionic bonds are the strongest, so \(\mathrm{NaCl}(l)\) will have the highest boiling point. Therefore, the order of the boiling points of the liquid states of the substances from lowest to highest is: \(\mathrm{He}(l), \mathrm{CO}(l), \mathrm{H}_{2}\mathrm{O}(l), \mathrm{NaCl}(l)\).

Key concepts

Intermolecular Forces

Intermolecular forces are the forces that act between molecules, dictating the physical properties like boiling and melting points. These forces are not as strong as the forces within a molecule, such as covalent or ionic bonds, but they are crucial in determining how molecules interact with one another.
Some common types of intermolecular forces include:

• Dispersion forces: Present in all substances, though usually the weakest form of interaction.
• Dipole-dipole interactions: Occur between polar molecules.
• Hydrogen bonds: A special, stronger type of dipole-dipole interaction.
• Ionic bonds: More of an intramolecular force in salts but still impactful between ions in a substance.

Understanding intermolecular forces helps us explain why certain substances have higher boiling points than others. The stronger the intermolecular forces, the more energy is required to break them, leading to a higher boiling point.

Ionic Bonds

Ionic bonds occur when electrons are transferred from one atom to another, resulting in positively and negatively charged ions. These oppositely charged ions are held together by strong electrostatic forces. For example, in sodium chloride (\(\mathrm{NaCl}\)), sodium donates an electron to chlorine, creating strong bonds that require high energy to break apart.
This strong attraction results in very high boiling and melting points, as seen with \(\mathrm{NaCl}(l)\), which reflects the significant amount of energy needed to separate the ions and allow the substance to transition to a gaseous state.
Ionic bonds are a hallmark of salts and exhibit the strongest forces among typical intermolecular interactions.

Hydrogen Bonds

Hydrogen bonds are a special type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom, such as oxygen, nitrogen, or fluorine. This configuration creates a significant dipole, where the hydrogen atom is slightly positive and the other atom is negative. Water (\(\mathrm{H}_2\mathrm{O}(l)\)) is an excellent example, where hydrogen bonds create a network of interactions that require a considerable amount of energy to break.
Due to these strong interactions, substances with hydrogen bonds generally exhibit higher boiling points than those with only weaker dipole-dipole interactions or dispersion forces. These bonds are highly directional, giving water and similar substances unique properties, such as a higher boiling point and specific heat capacity.

Dispersion Forces

Dispersion forces, also known as London forces, are the weakest intermolecular forces and arise from temporary fluctuations in electron distribution within atoms and molecules. These forces are present in all molecules, whether polar or nonpolar, but are the only intermolecular force acting in noble gases like helium (\(\mathrm{He}(l)\)).
In temporary states, the fluctuating electron clouds create instantaneous dipoles that attract nearby molecules. The effect of these forces is typically short-lived and quite weak, thus only leading to very low boiling points.

• For example, helium, which relies solely on dispersion forces, has a very low boiling point.

Despite being the weakest, dispersion forces significantly influence substances with large atoms or molecules due to the larger likelihood of electron fluctuation, contributing to slightly higher boiling and melting points compared to very small atoms or simple molecules.