Question

One of these two substances is a liquid at room temperature and the other one is a gas. Which one is the liquid and why? $$ \mathrm{CH}_{3} \mathrm{OCH}_{3} \quad \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} $$

Short answer

\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) is the liquid because it has hydrogen bonding, which leads to a higher boiling point than \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) that only displays van der Waals forces.

Step-by-step solution

Analyze Molecular Structures

Identify the functional groups in both molecules. The first compound, \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\), is an ether with a simple structure of two methyl groups attached to an oxygen atom. The second compound, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\), is an alcohol, which has a hydroxyl group (OH) attached to an ethyl group.

Consider Intermolecular Forces

Determine the types of intermolecular forces that are present in both compounds. Ethers have relatively weak intermolecular forces, primarily van der Waals (dispersion) forces, as they do not have hydrogen bonding. Alcohols, on the other hand, have hydrogen bonds due to the presence of the hydroxyl group.

Compare Boiling Points

Understand that hydrogen bonding generally leads to a higher boiling point because more energy is required to overcome these bonds during the phase change from liquid to gas. Hence, substances with hydrogen bonds are often liquids at room temperature as opposed to substances with only van der Waals forces.

Determine the Liquid at Room Temperature

Since \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) has hydrogen bonding and a higher boiling point, it is the compound that is a liquid at room temperature. The ether, \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\), is likely to be the gas due to its weaker intermolecular forces and lower boiling point.

Key concepts

Boiling Point Comparison

The boiling point of a substance can be quite informative when it comes to assessing its physical state at room temperature. It is a direct reflection of the strength of intermolecular forces present within the substance - stronger intermolecular forces lead to higher boiling points.

Consider a liquid: for it to transition to a gas, its molecules must acquire enough energy to overcome the attractions holding them together. If these attractive forces are weak, less energy (in the form of heat) is needed for this transition, resulting in a lower boiling point. On the flip side, substances with strong intermolecular forces, like hydrogen bonds, require significantly more heat to boil, which corresponds to a higher boiling point.

When we compare the boiling points of substances, we're essentially gauging the tenacity of their intermolecular forces. This information can then be used to predict whether a substance will be a liquid or a gas at room temperature, which is typically around 25°C (77°F).

Hydrogen Bonding

Hydrogen bonding is a specific, highly significant type of intermolecular attraction that accounts for the unique properties of various compounds, most notably water. This bond occurs when hydrogen is attached to a highly electronegative atom such as oxygen, nitrogen, or fluorine, creating a partial positive charge on the hydrogen.

The positively charged hydrogen atom is then attracted to the lone pairs of electrons on electronegative atoms in nearby molecules, forming a hydrogen bond. These bonds are considerably stronger than other van der Waals forces, but still weaker than covalent or ionic bonds within molecules.

Properties Influenced by Hydrogen Bonding

• High boiling and melting points
• Unusual density behavior, like ice floating on water
• Higher surface tension

Hydrogen bonding contributes crucially to the three-dimensional structure of complex molecules like DNA and proteins, and it's essential for understanding the behavior of many biological and chemical substances.

Van der Waals Forces

Van der Waals forces encompass a range of intermolecular attractions, including dipole-dipole interactions, London dispersion forces, and dipole-induced dipole forces. These forces are significantly weaker than hydrogen bonds and are present in all molecules, regardless of whether or not they can form hydrogen bonds.

London dispersion forces, the weakest of the van der Waals forces, arise due to temporary fluctuations in electron density within molecules, which produce temporary dipoles that attract one another. These forces are present in all molecules but are the only type of intermolecular force in nonpolar substances.

Examples and Significance

• Nonpolar molecules' (like fats and oils) solubility in similar substances
• Determining the melting and boiling points of various gases
• Understanding the properties of noble gases

Realizing the prevalence and implications of van der Waals forces helps explain why substances like noble gases can exist as gases at room temperature and why certain materials are insoluble in water.

Molecular Structure Analysis

Scrutinizing the molecular structure of a substance reveals much about its physical properties and the types of intermolecular forces at play. The presence of functional groups, such as hydroxyl (-OH) or methyl (-CH3), dictates the manner in which molecules will interact with each other.

For instance, a hydroxyl group might indicate the possibility of hydrogen bonding, while a series of carbon-hydrogen chains suggests that van der Waals interactions could dominate. The structure can even determine the shape of the molecule, which influences how tightly it can pack in a solid state and how it interacts in a liquid state.

Additionally, the presence of polar groups can increase a molecule's ability to interact with polar solvents like water, affecting its solubility and reactivity. By analyzing molecular structures, we can predict and explain a wide range of physical and chemical properties of substances in various states and environments.