Question

In which of the following processes is it necessary to break covalent bonds as opposed to simply overcoming intermolecular forces? (a) Decomposing HCl to \(\mathrm{H}_{2}\) and \(\mathrm{Cl}_{2}\) (b) Dissolving \(\mathrm{NaCl}\) in water (c) Freezing ethyl alcohol (d) Subliming iodine

Short answer

Answer: Covalent bonds need to be broken in process (a) Decomposing HCl to H2 and Cl2.

Step-by-step solution

Understand the difference between covalent bonds and intermolecular forces

Covalent bonds are the chemical bonds formed by the sharing of electron pairs between atoms in a molecule. These bonds are relatively strong and require a considerable amount of energy to be broken. Intermolecular forces, on the other hand, are weaker attractive forces between neighboring molecules or atoms that do not involve the sharing of electrons. Examples of intermolecular forces are hydrogen bonds and van der Waals forces. Overcoming intermolecular forces typically requires less energy than breaking covalent bonds.

Analyze each process

(a) Decomposing HCl to \(\mathrm{H}_{2}\) and \(\mathrm{Cl}_{2}\): In this process, we have to break the H-Cl covalent bond in order to form hydrogen and chlorine molecules. This does involve breaking covalent bonds. (b) Dissolving \(\mathrm{NaCl}\) in water: In this process, we have to break the ionic bond between \(\mathrm{Na}^{+}\) and \(\mathrm{Cl}^{-}\) (not covalent) and form ion-dipole interactions with water molecules. This does not involve breaking covalent bonds but overcoming intermolecular forces. (c) Freezing ethyl alcohol: In this process, the liquid ethyl alcohol molecules are transformed into a solid state by overcoming the hydrogen bonding (a type of intermolecular force) between alcohol molecules. This does not involve breaking covalent bonds. (d) Subliming iodine: In this process, iodine molecules are transformed directly from the solid phase to the gas phase. This involves overcoming the van der Waals forces (intermolecular forces) between the iodine molecules but does not involve breaking covalent bonds.

Identify the process that involves breaking covalent bonds

After analyzing each process, we can conclude that process (a) Decomposing HCl to \(\mathrm{H}_{2}\) and \(\mathrm{Cl}_{2}\) is the one that requires breaking covalent bonds, whereas the other processes only involve overcoming intermolecular forces.

Key concepts

Intermolecular Forces

To begin our exploration, let's delve into the world of intermolecular forces. These are the attractions that occur between molecules, which keep them close enough to form liquids or solids but are significantly weaker than the bonds holding an individual molecule together. Understanding intermolecular forces is crucial when we consider processes like freezing or sublimation.

There are several types of intermolecular forces, with some of the most common being hydrogen bonds, dipole-dipole interactions, and London dispersion forces, also known as van der Waals forces. For instance, hydrogen bonds are particularly strong intermolecular forces that occur when a hydrogen atom is bonded to a highly electronegative atom such as oxygen, nitrogen, or fluorine. These are essential in biological structures like DNA and proteins.

Chemical Bonds

Moving onto chemical bonds, these are the powerful interactions that hold atoms together within a molecule. Atoms can share or transfer electrons, leading to the formation of covalent or ionic bonds, respectively. A covalent bond is established when two atoms share one or more pairs of electrons, and this type of bond is highlighted in the breaking of HCl into \( \mathrm{H}_{2} \) and \( \mathrm{Cl}_{2} \) molecules. Breaking these bonds requires a significant amount of energy, indicating their strength and stability.

Understanding the differences between intermolecular forces and chemical bonds is fundamental in predicting the behavior of substances under various conditions. In a chemical reaction, breaking chemical bonds is often the key step that leads to the transformation of substances into new products.

Dissolving Ionic Compounds

Finally, let's look at dissolving ionic compounds such as \( \mathrm{NaCl} \) in water. When ionic compounds dissolve, the ionic bonds between the positive and negative ions must be overcome. Instead of breaking covalent bonds, water works to separate the ions due to its polar nature, which allows it to surround and stabilize the individual positive and negative ions in a process called solvation.

This interaction between ion and water molecules involves ion-dipole forces, where water's positive and negative ends address the opposing charges of the ions. This fascinating process is crucial to many bodily functions, industrial processes, and is the underlying principle by which many substances dissolve in solvents.