Problem 101 - General Chemistry: Principles and Modern Applications

Question

The bond energies of \(\mathrm{Cl}_{2}\) and \(\mathrm{F}_{2}\) are 243 and \(159 \mathrm{kJ} \mathrm{mol}^{-1},\) respectively. Use these data to explain why \(\mathrm{XeF}_{2}\) is a much more stable compound than \(\mathrm{XeCl}_{2} .[\text { Hint: Recall that Xe exists as a monatomic gas. }]\).

Step-by-Step Solution

Answer

\(\mathrm{XeF}_{2}\) is more stable than \(\mathrm{XeCl}_{2}\) because the F-F bonds in \(\mathrm{XeF}_{2}\) are weaker and require less energy to break compared to the Cl-Cl bonds in \(\mathrm{XeCl}_{2}\). The bond energy for F-F is 159 kJ/mol which is less than the bond energy for Cl-Cl which is 243 kJ/mol. Hence, compounds with lower bond energies are more stable.

Step 1: Consider Bond Energies

The given bond energies are 243 kJ/mol for \(\mathrm{Cl}_{2}\) and 159 kJ/mol for \(\mathrm{F}_{2}\). This means that it requires 243 kJ of energy to break a single mole of Cl-Cl bonds and 159 kJ of energy to break a single mole of F-F bonds.

Step 2: Determine the Stability

The lower the bond energy, the less energy it requires to break the bond and hence the less stable the molecule. Hence, a compound with F-F bonds would be less stable than a compound with Cl-Cl bonds because F-F has a lower bond energy.

Step 3: Apply to \(\mathrm{XeF}_{2}\) and \(\mathrm{XeCl}_{2}\)

The bond energies of F-F and Cl-Cl suggest that \(\mathrm{XeF}_{2}\) must be more stable than \(\mathrm{XeCl}_{2}\), because the F-F bonds in \(\mathrm{XeF}_{2}\) are weaker (lower bond energy) than the Cl-Cl bonds in \(\mathrm{XeCl}_{2}\). Hence, \(\mathrm{XeF}_{2}\) is more stable because it contains the weaker F-F bonds, which require less energy to break.

Key Concepts

Chemical Stability

Chemical stability refers to a molecule's resistance to change or decompose under standard conditions.
In essence, a stable molecule requires a significant amount of energy to have its bonds broken.
Stability is largely determined by bond energies, which measure the strength of chemical bonds.

• Higher bond energies mean stronger bonds, thus greater stability.
• Lower bond energies indicate weaker bonds, leading to less stability.

When comparing compounds, it is crucial to assess their bond energies to predict their stability. Here, we consider the bond energies of chlorine ( ext{Cl} ) and fluorine ( ext{F} ). The bond energy of ext{Cl}_{2} is significantly higher than that of ext{F}_2 , suggesting that molecules containing chlorine tend to be more stable than those with fluorine due to stronger bonds.
Still, other factors such as molecular structure and environmental conditions can influence chemical stability.

Molecular Compounds

Molecular compounds consist of atoms connected by covalent bonds, where pairs of electrons are shared between atoms.
These compounds typically form between nonmetals, showcasing a variety of chemical properties depending on their composition.

• Covalent bonds are characterized by their specific bond energies, which impact the compound's physical and chemical properties.
• The type of atoms involved and the arrangement of atoms within the molecular structure influence the overall stability and reactivity of the compound.

In both X ext{eF}_{2} and X ext{eCl}_{2} , xenon shares electrons with fluorine and chlorine.
These compounds are examples of stable molecular compounds derived from noble gases, showcasing unique attributes due to the sharing of electrons between different atoms.

XeF2 and XeCl2 Stability

Stability in molecules like ext{XeF}_{2} and ext{XeCl}_{2} can be elucidated through their respective bond energies.
Xenon difluoride contains weaker ext{F-F} bonds compared to the ext{Cl-Cl} bonds in xenon dichloride.
This is counter to the general concept that weaker bond energies should imply less stable compounds.
However, due to the monatomic nature of xenon, and the way bonds are formed with these halogens, ext{XeF}_{2} appears more stable.

• The lower bond energy for fluorine suggests that bonds between xenon and fluorine are easier to form and harder to break, thus enhancing stability.
• Conversely, xenon bonds with chlorine need more energy both to form and to break, explaining the reduced stability of ext{XeCl}_{2} .

This can be attributed to factors like bond polarity and the relative size of the halogen atoms, impacting the final stability profile of the compound.