Question

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

Short answer

The attractive forces that need to be overcome are (a) Hydrogen bonding in ammonia, (b) London Dispersion Forces in Phosphorus, (c) Ion-Dipole interactions in CsI dissolved in HF, (d) Metallic bonds in Potassium metal.

Step-by-step solution

Identify the Force in Boiling Liquid Ammonia

Boiling requires breaking the forces that hold the particles together within the liquid state. Ammonia (\(NH_3\)) has hydrogen bonds due to the significant difference in electronegativity between the Nitrogen and Hydrogen atoms. Also, because Nitrogen has lone pair of electrons and Hydrogen is a small atom. Hence to boil liquid ammonia, Hydrogen bonds must be overcome.

Identify the Force in Melting Solid Phosphorus (\(P_4\))

Melting needs enough energy to overcome the forces keeping the solid structure intact. Phosphorus is a non-polar molecule and thus has London Dispersion Forces. To melt solid Phosphorus (\(P_4\)), the London Dispersion Forces need to be overcome.

Identify the Force in Dissolving CsI in liquid HF

Dissolving implies overcoming the forces that hold the solute molecule or ion intact, and also the solvent molecule. Here CsI is an ionic compound and HF is a Polar solvent. Hence, to dissolve CsI (Cesium Iodide) in liquid HF (Hydroflouric Acid), you need to overcome the ion-dipole interactions.

Identify the Force in Melting Potassium Metal

Melting solid metals implies the need to overcome the Metallic bonding which involves the sharing of free electrons among a structure of positively charged ions (cations). Therefore, to melt Potassium metal, Metallic bonds need to be overcome.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a strong type of intermolecular attraction that occurs between molecules when a hydrogen atom is bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine. The electronegative atom pulls the shared electrons closer, creating a partial negative charge. This leaves the hydrogen with a partial positive charge, allowing it to 'stick' to negatively charged parts of neighboring molecules.

In liquid ammonia (NH_3), boiling requires the breaking of these hydrogen bonds that hold the molecules together. Due to the electronegativity difference between nitrogen and hydrogen, and nitrogen's availability of a lone pair of electrons, ammonia is perfect for forming hydrogen bonds.

London Dispersion Forces

London dispersion forces, also known as Van der Waals forces, are the weakest type of intermolecular force. They originate from the temporary dipoles that occur when electrons in a molecule or atom are not evenly distributed, leading to temporary regions of positive and negative charge.

Even in non-polar molecules like Solid phosphorus (P4), these forces exist. When we talk about melting phosphorus, we refer to the necessity of overcoming the London dispersion forces to allow the solid to change into a liquid state.

Ion-Dipole Interactions

Ion-dipole interactions occur between an ion and a polar molecule, which has a partial positive and partial negative side due to an uneven distribution of electrons. This interaction is significant because it plays a crucial role in the solubility of ionic substances in polar solutions.

For example, dissolving cesium iodide (CsI) in liquid hydrogen fluoride (HF) involves overcoming the ion-dipole interactions. The ionic compound CsI dissolves because the Cs+ ions interact with the negative side of the polar HF molecules, and the I- ions with the positive side.

Metallic Bonding

Metallic bonding is a type of chemical bonding that occurs in metals, where electrons are not shared between individual atoms as in covalent bonding, nor transferred from one atom to another as in ionic bonding. Instead, electrons form a 'sea' around the metal cations. This bonding gives rise to the characteristic properties of metals such as electrical conductivity and malleability.

To melt a metal like potassium, heat needs to be supplied to overcome the strong attraction between the positively charged metal ions and the 'sea' of delocalized electrons, disrupting the metallic bond.