Question

Which substance in each of the following pairs would you expect to have the higher boiling point: (a) Ne or \(\mathrm{Xe},(\mathrm{b}) \mathrm{CO}_{2}\) or \(\mathrm{CS}_{2},(\mathrm{c}) \mathrm{CH}_{4}\) or \(\mathrm{Cl}_{2},(\mathrm{~d}) \mathrm{F}_{2}\) or \(\mathrm{LiF}\) (e) \(\mathrm{NH}_{3}\) or \(\mathrm{PH}_{3}\) ? Explain why.

Short answer

Xe, CS2, Cl2, LiF, and NH3 have higher boiling points, respectively.

Step-by-step solution

Assessing Intermolecular Forces for Ne and Xe

Neon (Ne) and xenon (Xe) are noble gases. Their boiling points are primarily determined by London dispersion forces, which increase with molecular size. Since xenon is larger and has more electrons compared to neon, it experiences stronger dispersion forces, leading to a higher boiling point. Thus, Xe has a higher boiling point than Ne.

Evaluating Intermolecular Forces for CO2 and CS2

Both carbon dioxide (CO2) and carbon disulfide (CS2) are linear, nonpolar molecules. Their boiling points are determined mainly by their molecular weights and the corresponding dispersion forces. CS2 has a higher molecular weight than CO2, resulting in stronger dispersion forces and hence, a higher boiling point. CS2 has a higher boiling point than CO2.

Comparing CH4 and Cl2

Methane (CH4) and chlorine (Cl2) both experience dispersion forces. Cl2 has a greater molecular mass, which means it has stronger dispersion forces compared to CH4. Thus, Cl2 has a higher boiling point than CH4.

Determining Boiling Point for F2 and LiF

Fluorine (F2) is a diatomic molecule held together by weak dispersion forces, while lithium fluoride (LiF) is an ionic compound with strong ionic bonds. Ionic bonds are much stronger than dispersion forces, which results in a much higher boiling point for LiF compared to F2. Hence, LiF has a higher boiling point.

Analyzing NH3 and PH3

Ammonia (NH3) and phosphine (PH3) both have dipole-dipole interactions and can engage in hydrogen bonding due to the presence of N-H and P-H bonds, respectively. However, hydrogen bonding is significantly stronger in NH3 because nitrogen is more electronegative than phosphorus. Consequently, NH3 has a higher boiling point than PH3.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles, such as atoms, molecules, or ions. These forces are crucial when determining properties like boiling points, as stronger intermolecular forces lead to higher boiling points. There are several types of intermolecular forces, each with varying strengths:

• Dipole-dipole interactions occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another.
• London dispersion forces are weak forces that arise from temporary fluctuations in electron density in atoms and molecules, which we'll discuss in more detail.
• Hydrogen bonding is a special case of dipole-dipole interaction that occurs when hydrogen is bonded to electronegative atoms like nitrogen, oxygen, or fluorine.
• Ionic bonds are not technically intermolecular forces but rather are strong attractions between ions, which result in extremely high boiling points.

Understanding these forces helps in predicting and comparing boiling points of different substances easily.

London Dispersion Forces

London dispersion forces, also known as dispersion forces or van der Waals forces, are the weakest type of intermolecular force. They result from temporary shifts in the distribution of electrons around an atom or molecule, creating a temporary dipole. These forces are present in all molecules, whether polar or nonpolar. However, they are the only forces present in nonpolar substances like noble gases (e.g., neon and xenon) and simple molecules (e.g., \(\text{CO}_2\) and \(\text{CS}_2\)).Why Size MattersThe strength of London dispersion forces increases with larger atomic or molecular sizes due to more electrons being involved, creating stronger temporary dipoles. This means that a substance like xenon, which is larger than neon, has stronger dispersion forces and thus a higher boiling point. Similarly, larger molecules like \(\text{Cl}_2\) will have higher boiling points than smaller ones like \(\text{CH}_4\).Impact on Boiling PointsSince dispersion forces are the primary driver for nonpolar molecules, those with greater molecular weights and sizes typically have higher boiling points due to increased London dispersion forces.

Hydrogen Bonding

Hydrogen bonding is a type of strong dipole-dipole attraction that occurs when hydrogen is directly bonded to small and highly electronegative atoms such as nitrogen, oxygen, or fluorine. These bonds are significantly stronger than regular dipole interactions, making substances with hydrogen bonds have relatively high boiling points.Case of Ammonia ( \(\text{NH}_3\) ) vs. Phosphine ( \(\text{PH}_3\) )Ammonia can form hydrogen bonds because its structure includes nitrogen-hydrogen bonds. Nitrogen is highly electronegative, pulling electrons towards itself, which leads to the formation of strong hydrogen bonds with nearby ammonia molecules. On the other hand, phosphine has weaker dipole interactions because phosphorus is less electronegative, resulting in weaker bonding. Thus, \(\text{NH}_3\) has a higher boiling point than \(\text{PH}_3\) due to stronger hydrogen bonding.The presence and strength of hydrogen bonds in a molecule can significantly elevate its boiling point compared to molecules with only regular dipole interactions.

Ionic Bonds

Ionic bonds are the electrostatic attractions between oppositely charged ions. They occur when one atom donates an electron to another atom, resulting in positive and negative ions that are held together very tightly. These bonds are much stronger than any intermolecular forces found in molecular compounds, which is why ionic compounds generally exhibit higher boiling points.Example: Lithium Fluoride ( \(\text{LiF}\) ) vs. Fluorine ( \(\text{F}_2\) )In the case of \(\text{LiF}\), the bond between lithium and fluoride ions is ionic, involving strong attractive forces. This results in a particularly high boiling point. On the other hand, \(\text{F}_2\) only involves weak London dispersion forces holding the nonpolar molecules together. This distinction showcases why ionic substances like \(\text{LiF}\) have much higher boiling points than similar-sized nonpolar molecules.Understanding ionic bonds and recognizing them in compounds can help explain why such substances often have markedly higher boiling points than covalent compounds.