Question

Of the compounds \(\mathrm{HF}, \mathrm{CH}_{4}, \mathrm{CH}_{3} \mathrm{OH}, \mathrm{N}_{2} \mathrm{H}_{4},\) and \(\mathrm{CHCl}_{3},\) hydrogen bonding is an important intermolecular force in (a) none of these; (b) two of these; (c) three of these; (d) all but one of these; (e) all of these.

Short answer

Three of these compounds can form important hydrogen bonds.

Step-by-step solution

Identify Potential Hydrogen Bonds

Identify the presence of potential hydrogen bonds in each compound. A compound can form hydrogen bonds if it contains a hydrogen bound to nitrogen (N), oxygen (O), or fluorine (F). Examine \(\mathrm{HF}\), \(\mathrm{CH}_{4}\), \(\mathrm{CH}_{3} \mathrm{OH}\), \(\mathrm{N}_{2} \mathrm{H}_{4}\), and \(\mathrm{CHCl}_{3}\) to see if they match these conditions.

Hydrogen Bond Analysis

\(\mathrm{HF}\) can form hydrogen bonds, as the hydrogen is bonded to fluorine. \(\mathrm{CH}_{4}\) cannot form hydrogen bonds as the hydrogen is connected to carbon not to a nitrogen, oxygen, or fluorine atom. \(\mathrm{CH}_{3} \mathrm{OH}\) can form hydrogen bonds, as the hydrogen is bonded to oxygen. \(\mathrm{N}_{2} \mathrm{H}_{4}\) can form hydrogen bonds, as the hydrogen is bonded to nitrogen. \(\mathrm{CHCl}_{3}\) cannot form hydrogen bonds, as the hydrogen is bonded to carbon, not nitrogen, oxygen, or fluorine.

Count molecules

Count the number of compounds identified in step 2 that can form hydrogen bonds. In this case, it's HF, \(\mathrm{CH}_{3} \mathrm{OH}\), and \(\mathrm{N}_{2} \mathrm{H}_{4}\), so three compounds.

Key concepts

Intermolecular Forces

In chemistry, one of the paramount concepts students must grasp is the nature of intermolecular forces. These are the attractions that exist between different molecules, as opposed to the forces holding atoms together within a molecule, known as intramolecular forces. A particularly strong type of intermolecular force is hydrogen bonding, often dominating the physical properties of a compound, such as its boiling point, melting point, and solubility.

Hydrogen bonds occur when a hydrogen atom, covalently bonded to a highly electronegative atom (namely nitrogen, oxygen, or fluorine), experiences an attractive force to another electronegative atom in a neighboring molecule. The resulting 'bond' isn't a true bond but a strong dipole-dipole interaction that is crucial for substances like water. Understanding the conditions for hydrogen bonding is essential in predicting a molecule's behavior in different environments. For instance, water's high boiling point can be attributed to the hydrogen bonds between individual water molecules, which require more energy to break during the phase transition from liquid to gas.

Chemical Bonding

Delving into the realm of chemical bonding, we distinguish between the interactions within molecules (intra) and those between them (inter). Intramolecular forces, or chemical bonds, include ionic, covalent, and metallic bonds. These are the bonds that hold atoms together to form molecules or compounds. Covalent bonds, specifically, involve the sharing of electron pairs between atoms.

When students come across molecules like \textbf{HF} (hydrogen fluoride) or \textbf{CH}\(_3\)\textbf{OH} (methanol), it's crucial to recognize that the hydrogen atoms are involved in polar covalent bonds with fluorine and oxygen, respectively. These electronegative atoms attract the shared electrons more strongly, creating a dipole moment. It's this polarity that allows for hydrogen bonds, a critical intermolecular force, to form with nearby molecules possessing lone pairs on their electronegative atoms.

Compound Analysis

In the realm of compound analysis, students must become adept at predicting molecular properties based on their structure. Examining each atom and its bonding helps determine if a molecule can engage in hydrogen bonding, which, as previously mentioned, is vital for many physical properties.

For compound analysis, students must look for the presence of hydrogen atoms attached to N, O, or F. By doing so, in our exercise example, we could easily exclude \textbf{CH}\(_4\) (methane) and \textbf{CHCl}\(_3\) (chloroform) since their hydrogens are bonded to carbon, which is not electronegative enough to facilitate hydrogen bonding. Through such analyses, students not only learn how to identify whether hydrogen bonding is present but also how to anticipate the implications of these interactions on the compound's macroscopic properties, aiding in the understanding of topics ranging from material science to biology.