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? \(\mathrm{CH}_{3} \mathrm{OH} \quad \mathrm{CH}_{3} \mathrm{SH}\)

Short answer

Methanol \(\mathrm{CH}_{3}OH\) is the liquid at room temperature due to its stronger intermolecular hydrogen bonding.

Step-by-step solution

Understand Molecular Structure

Analyze the molecular structure of both substances: Methanol \(\mathrm{CH}_{3}OH\), and Methanethiol \(\mathrm{CH}_{3}SH\). Both are similar in size and shape, but differ in their functional groups - an alcohol group \(–OH\) in Methanol and a thiol group \(–SH\) in Methanethiol.

Consider Intermolecular Forces

Understand that the physical state of a substance at room temperature is largely determined by the strength of its intermolecular forces. The stronger the intermolecular forces, the more likely the substance is to be a liquid at room temperature.

Compare Intermolecular Forces

Recognize that the alcohol group \(–OH\) in Methanol can participate in hydrogen bonding, a strong type of intermolecular force. The thiol group \(–SH\) in Methanethiol is less electronegative and less able to participate in hydrogen bonding, resulting in weaker intermolecular forces.

Determine the Physical States

Conclude that Methanol \(\mathrm{CH}_{3}OH\), with its ability to form hydrogen bonds, will have stronger intermolecular forces and thus, it is more likely to be a liquid at room temperature. Methanethiol \(\mathrm{CH}_{3}SH\) will have weaker intermolecular forces, making it more likely to be a gas at room temperature.

Key concepts

Hydrogen Bonding

Hydrogen bonding is an incredibly strong type of intermolecular force that plays a crucial role in determining the physical properties of compounds.

At its core, hydrogen bonding occurs when a hydrogen atom, which is covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, experiences an attraction to another electronegative atom in a neighboring molecule. The result is a sort of 'bridge' between molecules, which has significant effects on the substance's boiling and melting points, solubility, and overall state at room temperature.

For instance, in the comparative study of Methanol \(\mathrm{CH}_{3}OH\) and Methanethiol \(\mathrm{CH}_{3}SH\), it's the hydrogen bonded to the oxygen in Methanol that creates an extensive network of hydrogen bonds. This network is strong enough to keep the molecules close together, thus making Methanol a liquid at room temperature, as opposed to Methanethiol which cannot form such bonds readily and remains a gas.

Physical States of Matter

The physical states of matter—solid, liquid, and gas—are determined by both the kinetic energy of the molecules and the strength of intermolecular forces between them.

When substances are at room temperature, solids have the strongest intermolecular forces holding the molecules tightly in place, while liquids have enough energy for the molecules to slide past one another but not enough to completely escape their neighbors' pull. Gases, conversely, have particles with sufficient kinetic energy to overcome intermolecular forces almost entirely, allowing them to roam freely and occupy any given volume.

Comparing Methanol and Methanethiol, we observe that Methanol's molecules are closely bound by hydrogen bonds, so although they have kinetic energy at room temperature, they aren't energetic enough to overcome these forces, and thus Methanol remains a liquid. Methanethiol, lacking strong intermolecular forces, easily transitions to the gas phase where its molecules are far apart and move independently.

Molecular Structure

Molecular structure is fundamental to understanding how substances interact and what physical state they assume under normal conditions. The shape of a molecule, its size, and the types of atoms involved all influence how molecules fit together and what types of intermolecular forces can exist between them.

In our example, Methanol and Methanethiol are akin in molecular size and shape, yet their functional groups differ drastically, the former having an alcohol group \(–OH\) and the latter sporting a thiol group \(–SH\). This subtle distinction is pivotal because it accounts for Methanol's capacity to form hydrogen bonds due to the highly electronegative oxygen as opposed to Methanethiol with sulfur, which is less suited to such bonding.

By appreciating the nuances in molecular structure, students can better predict and understand behaviors of substances, such as which molecular compositions will result in higher boiling points or solubility in water, and how they might interact within biological systems or in industrial applications.