Problem 37 - General Chemistry: Principles and Modern Applications

Question

Use Lewis structures and other information to explain the observation that (a) \(\mathrm{H}_{2} \mathrm{S}\) is a gas at room temperature, whereas \(\mathrm{H}_{2} \mathrm{O}\) is a liquid. (b) \(\mathrm{O}_{3}\) is diamagnetic.

Step-by-Step Solution

Answer

H2S is a gas at room temperature, and H2O is a liquid due to the differing strength of their intermolecular forces. H2O has strong hydrogen bonding, whereas H2S has weaker dipole-dipole interactions. O3 is diamagnetic because all its electrons are paired in its molecular orbitals, giving it no net magnetic moment.

Step 1: Drawing Lewis Structures

First, we draw the Lewis structures of the molecules in question. Lewis structures, also known as Lewis-dot diagrams, show the bonding relationship between atoms of a molecule. They also highlight the lone pairs of electrons. The Lewis structure for H2S is H-S-H with two lone pairs of electrons around S. The Lewis structure for H2O is H-O-H with two lone pairs of electrons around O. The Lewis structure for O3 shows a resonance structure with one of the double bonds shifting among the three oxygen atoms.

Step 2: Compare Intermolecular Forces in H2S and H2O

Intermolecular forces are responsible for the physical states of the molecules. In H2O, due to the presence of two hydrogen bonds between the molecules, there is a stronger intermolecular force. This high intermolecular force causes H2O to be in the liquid state at room temperature. In H2S, the intermolecular force comes only from dipole-dipole interactions and dispersion forces, which are considerably weaker than hydrogen bonds. This causes H2S to be in the gas state at room temperature.

Step 3: Analyze O3's Magnetic Properties

A molecule is diamagnetic when all the electrons are paired. In this case, the O3 molecule has all the electrons paired in its molecular orbitals. This means that it doesn't have unpaired electrons to create a net magnetic moment. Therefore, O3 is diamagnetic, which means it doesn't get attracted into a magnetic field.

Key Concepts

Understanding Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between molecules. They play a crucial role in determining the physical properties of substances, such as boiling and melting points. There are several types of intermolecular forces:

• Dispersion Forces: Also known as London dispersion forces, these are weak and arise due to temporary dipoles induced when electrons move around a nucleus. Every molecule experiences these forces, but they are often too weak to cause significant effects on smaller molecules.
• Dipole-Dipole Interactions: These occur in polar molecules where there is a separation of positive and negative charges, creating a dipole. Molecules like H₂S, which is polar, experience dipole-dipole interactions.
• Hydrogen Bonding: A stronger type of dipole-dipole interaction specifically occurring when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. Water (H₂O) exhibits hydrogen bonding, greatly affecting its physical state.

H₂O, with its ability to form hydrogen bonds, has stronger intermolecular forces compared to H₂S, which relies on weaker dipole-dipole and dispersion forces. This is why at room temperature, water is a liquid and hydrogen sulfide is a gas. The strength and type of intermolecular forces directly influence the matter's state and the energy needed to change that state.

Magnetic Properties and Diamagnetism

Magnetic properties in molecules are determined by their electron configurations. Whether a molecule is attracted or repelled by a magnetic field provides insights into its electronic structure. Let's decode some terms:

• Diamagnetism: Occurs when all electrons are paired in molecular orbitals. Diamagnetic substances are repelled by magnetic fields because they don’t have unpaired electrons. O₃ (ozone) is a classic diamagnetic molecule, as seen in its molecular orbital arrangement where all electrons are paired.
• Paramagnetism: Occurs in substances with one or more unpaired electrons, leading them to have a net magnetic moment. These substances are attracted by magnetic fields.

In the ozone molecule's Lewis structure, each oxygen has paired electrons, ensuring no net magnetic moment. It was observed through molecular orbital theory that all electrons remain in pairs, confirming its designation as diamagnetic. Understanding the electron pairing and magnetic properties can ultimately explain why certain substances behave the way they do in a magnetic field.

The Role of Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction. Its presence in certain molecules has profound effects on their physical and chemical properties.

• What Makes It Special: Hydrogen bonds result from the strong attractive force occurring between a hydrogen atom covalently bonded to a highly electronegative atom and another electronegative atom with a lone pair of electrons. The common atoms involved include oxygen, nitrogen, and fluorine.
• Effect on Properties: Due to the strength of hydrogen bonds, substances like water exhibit unique properties such as high boiling and melting points compared to other molecules of similar size. This is why water remains liquid at room temperature.

In contrast, substances like H₂S, with no hydrogen bonds, have much lower boiling points and thus exist in a gaseous state when compared to water. Hydrogen bonding influences everything from solubility to the three-dimensional structures of complex biological molecules, making it a key concept in understanding molecular interactions.