Question

Name the kinds of attractive forces that must be overcome to (a) boil liquid ammonia, (b) melt solid phosphorus \(\left(\mathrm{P}_{4}\right),(\mathrm{c})\) dissolve CsI in liquid \(\mathrm{HF},\) (d) melt potassium metal.

Short answer

(a) Hydrogen bonds, (b) van der Waals forces, (c) ionic bonds and hydrogen bonding, (d) metallic bonds.

Step-by-step solution

Identify the forces in liquid ammonia

Liquid ammonia \(\text{NH}_3\) has hydrogen bonds due to the presence of nitrogen, which is highly electronegative and creates a strong attraction between its own hydrogen atoms and the nitrogen atoms of other ammonia molecules. To boil liquid ammonia, these hydrogen bonds need to be overcome.

Determine the forces in solid phosphorus

Solid phosphorus forms a tetrahedral \(\text{P}_4\) molecule with only weak van der Waals forces (also known as London dispersion forces) between the molecules because \(\text{P}_4\) is nonpolar. Melting solid \(\text{P}_4\) means overcoming these weak van der Waals forces.

Analyze the forces in CsI dissolving in liquid HF

When dissolving cesium iodide \(\text{CsI}\) in hydrofluoric acid (liquid \(\text{HF}\) ), ionic bonds within \(\text{CsI}\) must be broken, and new ion-dipole attractions between \(\text{Cs}^+\), \(\text{I}^-\), and \(\text{HF}\) molecules must form. This process requires overcoming the ionic bonds and forming hydrogen bonds and dipole interactions.

Explore the forces in potassium metal

Potassium metal forms a metallic bond characterized by a lattice of \(\text{K}^+\) ions surrounded by a sea of delocalized electrons. To melt potassium metal, these metallic bonds must be weakened to allow the potassium ions to move freely as a liquid.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special kind of attractive force that can occur when hydrogen is bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine. This type of bond is stronger than typical dipole-dipole interactions because of the small size of hydrogen and its strong attraction to the electronegative atom's electrons.

In hydrogen bonding, the hydrogen atom develops a partial positive charge, making it strongly attracted to the lone pair(s) of electrons on the electronegative atom in a nearby molecule. This is why in ammonia (\(\text{NH}_3\)), hydrogen bonds are significant. The nitrogen atom creates a partial negative charge due to its higher electronegativity, prompting the attraction.

• Ammonia's boiling point is significantly higher compared to other molecules of similar size solely due to hydrogen bonding.
• This makes hydrogen bonding critical in biological structures, such as the DNA double helix, where they help hold the strands together.

Van der Waals Forces

Van der Waals forces, including London dispersion forces, are a type of weak intermolecular attractive force that exists between all molecules, regardless of polarity. These forces arise due to temporary dipoles that occur when the electrons in a molecule are uneven in their distribution.

In nonpolar molecules like phosphorus tetramer (\(\text{P}_4\)), these forces are especially important even though they are weak. These temporary dipoles allow \(\text{P}_4\) to transition from a solid to a liquid state when these weak attractions are overcome.

• Van der Waals forces are crucial for understanding behaviors of nonpolar compounds.
• They help explain why larger nonpolar molecules, like phosphorous or complex hydrocarbons, have higher boiling or melting points than smaller ones.

Ionic Bonds

Ionic bonds are a form of electrostatic attraction occurring between positively and negatively charged ions. These bonds are strong and require significant energy to break. Ionic bonding is a result of the complete transfer of electrons from one atom (typically a metal) to another (a non-metal), forming cations and anions, respectively.

For instance, in cesium iodide (\(\text{CsI}\)), an electron is transferred from cesium (\(\text{Cs}^+\)) to iodine (\(\text{I}^-\)). This bond must be overcome to dissolve CsI in liquid hydrogen fluoride (\(\text{HF}\)), allowing ions to interact with HF's polar molecules.

• Ionic compounds typically have high melting and boiling points due to the strength of these interactions.
• When dissolved, ionic bonds turn into ion-dipole interactions, crucial for solubility in polar solvents like water or HF.

Metallic Bonds

Metallic bonding is a unique form of bonding found in metal elements, characterized by a lattice of metal cations surrounded by a "sea" of delocalized electrons. This electron sea allows metals to conduct electricity and heat effectively, as the electrons can move freely through the lattice.

In potassium (\(\text{K}\)), metallic bonds involve the shared delocalized electrons allowing the metal to exist in a solid state with flexibility and ductility. To melt potassium, the energy must allow these electrons and the resulting cations mobility for the substance to transition into a liquid.

• Metallic bonds are responsible for the malleability and ductility of metals.
• Differences in metallic bond strength can influence a metal's melting point.