Problem 142 - Chemistry

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.

Step-by-Step Solution

Answer

The two compounds with the same chemical formula, XeCl2F2, can exist in different phases (liquid and gas) under the same temperature and pressure due to their different molecular structures. The square planar structure is nonpolar and has weaker intermolecular forces (only London dispersion forces), allowing it to exist as a gas. In contrast, the T-shaped structure is polar with stronger intermolecular forces (dipole-dipole interactions and London dispersion forces), causing it to exist as a liquid. Different molecular structures lead to variations in intermolecular forces, which in turn affect their phases under the given conditions.

Step 1: Consider the molecular structure of XeCl2F2

First, we need to know that XeCl2F2 is a compound formed by the central atom xenon (Xe) bonded to two chlorine (Cl) atoms and two fluorine (F) atoms. The electron configuration and repulsion between electron pairs determine the shape of the molecule.

Step 2: Identify the possible structures for XeCl2F2

There are two possible molecular structures for XeCl2F2. In one structure, the XeCl2F2 molecule has a square planar shape, with the Cl and F atoms arranged in an alternating pattern in the same plane around the Xe atom. In the other structure, the XeCl2F2 molecule has a T-shaped arrangement with two F atoms in the same plane as the Xe atom and one Cl atom above and one below the plane. These different arrangements lead to variations in intermolecular forces between the molecules, affecting their phase under the same temperature and pressure.

Step 3: Discuss the intermolecular forces involved

Intermolecular forces play a crucial role in determining the phase of a substance under a given temperature and pressure. There are three main types of intermolecular forces: London dispersion forces, dipole-dipole interactions, and hydrogen bonding. The stronger the intermolecular forces, the higher the boiling point of the substance, making it more likely to be in the liquid phase.

Step 4: Compare the two structures of XeCl2F2

For the square planar structure, the molecule is nonpolar as the dipole moments of the Xe-Cl and Xe-F bonds cancel each other out due to symmetry. Hence, only London dispersion forces are present in this variation of the compound, which are relatively weak, resulting in a lower boiling point and allowing the compound to exist as a gas. In the T-shaped arrangement, the molecule is polar because the two Xe-F bonds have a greater dipole moment directed toward the fluorine atoms, which doesn't cancel out with that of the Xe-Cl bonds. This leads to stronger dipole-dipole interactions between the XeCl2F2 molecules, in addition to the London dispersion forces. This increases the boiling point of the compound, causing it to exist as a liquid at the same temperature and pressure.

Step 5: Conclude the explanation

In conclusion, the two compounds with the same chemical formula, XeCl2F2, can exist in different phases (liquid and gas) under the same temperature and pressure, because of their different molecular structures. The square planar structure has weaker intermolecular forces (only London dispersion forces), allowing it to exist as a gas, while the T-shaped structure has stronger intermolecular forces (dipole-dipole interactions and London dispersion forces), causing it to exist as a liquid.

Key Concepts

Intermolecular Forces

Intermolecular forces are the attractions between molecules, influencing how substances behave in different phases like solids, liquids, and gases. These forces are what keep molecules together, and their strength varies depending on the molecular structure. There are three main types of intermolecular forces:

• London dispersion forces: These are weak forces caused by temporary shifts in the density of electrons in electron clouds. They are present in all molecules, whether polar or nonpolar, but are the only forces at work in nonpolar molecules.
• Dipole-dipole interactions: These occur between polar molecules, where the positive end of one induced dipole is attracted to the negative end of another.
• Hydrogen bonding: A special type of dipole-dipole interaction occurring when hydrogen is bonded to highly electronegative elements like nitrogen, oxygen, or fluorine, greatly enhancing these attractions.

In the case of XeCl\(_2\)F\(_2\), the intermolecular forces vary based on its molecular geometry. This variation influences whether the compound is a liquid or a gas under identical conditions.

Polarity and Molecular Shape

The polarity of a molecule is determined by its shape and the distribution of charge. A molecule is considered polar when there's an uneven distribution of electrons leading to a molecule having a positive and negative pole. This happens when there's an electronegativity difference between bonded atoms.

Molecular shape, dictated by the arrangement of atoms and repulsion of electron pairs around the central atom, impacts polarity. For instance:

• Square planar: Molecules like the square planar form of XeCl\(_2\)F\(_2\) can be nonpolar due to their symmetrical structure. The dipoles cancel each other out in such arrangements.
• T-shaped: This shape, as seen in another form of XeCl\(_2\)F\(_2\), results in a polar molecule because the asymmetrical distribution of atoms does not allow for complete cancellation of dipole moments.

Polarity significantly influences a molecule's physical properties, including its boiling point and phase state.

Phase Changes

Phase changes are the transitions between different states of matter: solid, liquid, and gas. These changes occur because of variations in temperature and pressure, affecting the energy and mobility of molecules. During a phase change:

• Melting: A solid absorbs heat, its particles gain energy, becoming a liquid.
• Boiling: A liquid's particles gain enough energy to become a gas.
• Condensing: A gas loses energy and turns back into a liquid.

The phase in which a substance exists under a particular set of conditions is determined by its intermolecular forces. Stronger forces lead to higher melting and boiling points, keeping a substance in the liquid or solid state at a given temperature and pressure. Hence, the different molecular structures of XeCl\(_2\)F\(_2\) mean it can exist as either a liquid or gas due to these changing intermolecular attractions.

Boiling Point

The boiling point is the temperature at which a liquid's vapor pressure equals the atmospheric pressure, causing it to turn into gas. This property is heavily influenced by intermolecular forces.

• Stronger intermolecular forces: Substances with strong forces, like dipole-dipole interactions or hydrogen bonds, require more energy (higher temperature) to break their bonds, resulting in a high boiling point.
• Weaker intermolecular forces: Substances dominated by London dispersion forces boil at lower temperatures due to less energy being needed to break these weak attractions.

In the case of XeCl\(_2\)F\(_2\), the molecular structure affects its boiling point significantly. The T-shaped, polar configuration has stronger forces, thus a higher boiling point, allowing it to remain liquid. Conversely, the nonpolar, square planar structure experiences weaker forces, implying a lower boiling point and leading it to exist as gas under the same conditions.