Question

You and a friend each synthesize a compound with the formula \(\mathrm{XeCl}_{2} \mathrm{F}_{2}\). Your compound is a liquid and your friend's compound is a gas (at the same conditions of temperature and pressure). Explain how the two compounds with the same formulas can exist in different phases at the same conditions of pressure and temperature.

Short answer

The two compounds with the same chemical formula, \(\mathrm{XeCl}_{2} \mathrm{F}_{2}\), can exist in different phases (liquid and gas) at the same temperature and pressure because they are isomers with different molecular structures. The differences in molecular structures result in different strengths and types of intermolecular forces acting between their molecules. The liquid compound has stronger intermolecular forces, leading to a higher boiling point, while the gaseous compound has weaker intermolecular forces, allowing it to turn into a gas more readily under the same conditions.

Step-by-step solution

Briefly introduce the concept of isomerism

Isomerism is a phenomenon where two or more compounds have the same chemical formula but different arrangements of atoms in their structures. These different structures give the compounds different properties, such as melting and boiling points. In this case, the two \(\mathrm{XeCl}_{2} \mathrm{F}_{2}\) compounds are isomers.

Discuss how molecular structure impacts the properties of substances

The differences seen in the physical properties of the two \(\mathrm{XeCl}_{2} \mathrm{F}_{2}\) compounds can be attributed to their different molecular structures. The arrangement of their atoms directly impacts the type and strength of intermolecular forces present between the molecules of the substance. Stronger intermolecular forces typically result in higher boiling points, as more energy is required to break these forces and transform the substance from a liquid (or solid) to a gas.

Identify the different types of intermolecular forces that exist in different isomers

Different isomers can exhibit different types of intermolecular forces (based on their structural arrangement), such as: 1. London dispersion forces: These are weak forces that exist between all molecules, and are dependent on the size of the electron cloud and the polarizability of the molecule. 2. Dipole-dipole forces: These forces exist between polar molecules due to the attraction between the unsymmetrical positive and negative ends of the polar molecules. 3. Hydrogen bonding: This is a type of dipole-dipole force that specifically occurs between hydrogen atoms bonded to highly electronegative atoms (such as nitrogen, oxygen, or fluorine) and the lone pairs of electrons on another highly electronegative atom.

Link the different molecular structures of isomers to their properties

In the case of the \(\mathrm{XeCl}_{2} \mathrm{F}_{2}\) compounds, the isomer that is a liquid at the given conditions must have stronger intermolecular forces than the one that is a gas. This means that the molecular structures of the two compounds must differ in a way that results in one having more pronounced intermolecular forces than the other. This could be due to a higher degree of atomic polarization, more surface area for London dispersion forces to act upon, or a more significant dipole moment due to the specific arrangement of the Xe, Cl, and F atoms.

Summarize the explanation

In summary, the two \(\mathrm{XeCl}_{2} \mathrm{F}_{2}\) compounds can exist in different phases (liquid and gas) at the same temperature and pressure due to their different molecular structures, which result in different strengths and types of intermolecular forces acting between their molecules. The compound that is a liquid has a higher boiling point due to stronger intermolecular forces, whereas the compound that is a gas has weaker intermolecular forces and turns into a gas more readily at the same conditions.

Key concepts

Intermolecular Forces

Understanding intermolecular forces is crucial when exploring why substances exhibit different physical states under the same environmental conditions. These forces are essentially the 'glue' that holds molecules together in different states of matter: solid, liquid, and gas.

Even molecules with identical chemical formulas can have varying intermolecular attractions, leading to different physical properties. The key types of intermolecular forces include London dispersion forces, dipole-dipole interactions, and hydrogen bonding.

• London dispersion forces are the weakest but present in all molecules regardless of polarity. They increase with heavier and larger atoms due to a more significant electron cloud.
• Dipole-dipole forces are stronger and occur in polar molecules with a partial positive and negative charge. Molecules align according to these charges, leading to an attractive force between them.
• Hydrogen bonding, a special type of dipole-dipole interaction, is considerably strong and occurs when hydrogen is directly bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine.

The balance between these forces determines if a molecule prefers a gaseous or a more condensed state under given conditions. For instance, a molecule with stronger dipole-dipole interactions and hydrogen bonding will likely be a liquid at room temperature, while one with primarily London dispersion forces may be a gas.

Molecular Structure

The particular way in which atoms are arranged in a compound greatly affects its intermolecular forces and, by extension, its physical properties. This is elegantly illustrated by isomerism, where isomers have the same molecular formula but differing structures.

The geometric shape of a molecule, the types of atoms involved, and their distribution within the structure have significant implications for the compound's polarity and the magnitude of intermolecular attractions. A linear molecule might have fewer points of contact with its neighbors, thus resulting in weaker intermolecular forces compared to a more branched structure with a greater surface area.

Molecular Polarity and its Implications

Symmetry within a molecule can neutralize dipoles, leading to nonpolar molecules that only exhibit London dispersion forces. On the other hand, asymmetry leads to a separation of electrical charge, resulting in a polar molecule with stronger intermolecular forces.

Understanding the molecular structure helps predict how the molecule behaves in different states and under varying conditions, providing insight into its liquidity, volatility, melting point, and boiling point.

Physical Properties of Substances

Physical properties such as melting point, boiling point, solubility, and viscosity are intimately connected to both the molecular structure and the intermolecular forces at play within a substance. These properties can tell us a lot about the nature of the substance and how it will react under different circumstances.

Substances with strong intermolecular forces generally have higher boiling points, as more energy is required to overcome these forces. Likewise, they'll have higher melting points for the same reason. In the case of solubility, like dissolves like; polar substances tend to dissolve well in polar solvents due to similar types of intermolecular forces.

Understanding Phase Changes

When analyzing why one compound might exist as a liquid while another with the same chemical formula is a gas, consider these properties. The one with a higher boiling point is likely to be liquid under the same conditions due to stronger intermolecular forces, which holds its molecules closer together and resists entering the gaseous state.

Ultimately, by examining the molecular structure, we can predict the physical properties and understand the behavior of substances in various phases, enhancing our ability to manipulate and utilize these compounds effectively.