Question

Electronegativity and Hydrogen Bonding The Pauling electronegativity is a measure of the affinity of an atom for the electron in a covalent bond. The larger the electronegativity value, the greater the affinity of the atom for an electron shared with another atom. $$ \begin{aligned} &\begin{array}{cc} \text { Abem } & \text { Electrenegativity } \\ \mathrm{H} & 2.1 \\ \mathrm{C} & 2.55 \\ \mathrm{~s} & 2.58 \\ \mathrm{~N} & 3.04 \end{array}\\\ &349 \end{aligned} $$ote that \(\mathrm{S}\) is directly beneath \(\mathrm{O}\) in the periodic table. a. Do you expect \(\mathrm{H}_{2} \mathrm{~S}\) to form hydrogen bonds with itself? With \(\mathrm{H}_{2} \mathrm{O}\) ? b. Water boils at \(100^{\circ} \mathrm{C}\). Is the boiling point for \(\mathrm{H}_{2} \mathrm{~S}\) higher or lower than for \(\mathrm{H}_{2} \mathrm{O}\) ? c. Is \(\mathrm{H}_{2} \mathrm{~S}\) a more polar solvent than \(\mathrm{H}_{2} \mathrm{O}\) ?

Short answer

a. No, \(\mathrm{H}_2\mathrm{S}\) does not form hydrogen bonds with itself or \(\mathrm{H}_2\mathrm{O}\). b. \(\mathrm{H}_2\mathrm{S}\) has a lower boiling point. c. \(\mathrm{H}_2\mathrm{S}\) is less polar than \(\mathrm{H}_2\mathrm{O}\).

Step-by-step solution

Identify Electronegativity Differences

To determine if \( \mathrm{H}_2\mathrm{S} \) can form hydrogen bonds with itself and with \( \mathrm{H}_2\mathrm{O} \), we need to analyze the electronegativity differences. Sulfur (S) has an electronegativity of 2.58, while Oxygen (O) is more electronegative (not listed but known to be around 3.44 based on periodic trends). For hydrogen bonds, hydrogen must be bonded to a significantly electronegative atom like O, F, or N. Since S has a lower electronegativity than these atoms, \( \mathrm{H}_2\mathrm{S} \) is less likely to form hydrogen bonds.

Analyze Hydrogen Bonding Possibility of H₂S

The presence of hydrogen bonding in a molecule generally depends on the presence of hydrogen atoms that are bound to highly electronegative atoms (F, O, or N), which \( \mathrm{H}_2\mathrm{S} \) does not have, as sulfur is less electronegative than oxygen. Therefore, \( \mathrm{H}_2\mathrm{S} \) is unlikely to form hydrogen bonds with itself or with \( \mathrm{H}_2\mathrm{O} \).

Determine Boiling Point Comparison

Boiling points are often higher for molecules that can form strong intermolecular forces, such as hydrogen bonds. Since \( \mathrm{H}_2\mathrm{O} \) forms hydrogen bonds due to oxygen's high electronegativity, its boiling point is high at \( 100^{\circ}\mathrm{C} \). \( \mathrm{H}_2\mathrm{S} \), lacking the ability to hydrogen bond effectively, will likely have a lower boiling point.

Evaluate Polarity

Molecular polarity is influenced by the electronegativity of atoms within the molecule and their spatial arrangement. \( \mathrm{H}_2\mathrm{O} \) is a highly polar molecule because of the electronegativity difference between H and O and its bent shape. \( \mathrm{H}_2\mathrm{S} \), although also bent, has a smaller electronegativity difference between H and S, making it less polar than \( \mathrm{H}_2\mathrm{O} \).

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of interaction that occurs between molecules. It happens when hydrogen is covalently bonded to highly electronegative elements like nitrogen (N), oxygen (O), or fluorine (F). These elements strongly attract electrons, making them pull electron density away from hydrogen. This creates a polar bond with a small positive charge on hydrogen, which can then interact with a lone pair of electrons on another molecule, forming a hydrogen bond.

Hydrogen bonds are much weaker than covalent bonds, but they are strong for intermolecular interactions. They play a crucial role in defining the properties of water, DNA, and proteins. For example, the boiling point of water is much higher than expected because of hydrogen bonding between water molecules, which holds them together tightly.

In the case of \( \mathrm{H}_2\mathrm{S} \), hydrogen atoms are bonded to sulfur, a less electronegative element than oxygen, making hydrogen bonds less feasible. This is why \( \mathrm{H}_2\mathrm{S} \) molecules do not exhibit hydrogen bonding as strongly as water.

Intermolecular Forces

Intermolecular forces are forces of attraction or repulsion between molecules. They are much weaker than the forces within a molecule, such as covalent bonds, but they are crucial for determining the physical properties of substances, such as boiling and melting points.

There are different types of intermolecular forces:

• **Van der Waals forces:** These include dispersion forces, which are the weakest and occur due to temporary shifts in electron density within a molecule.
• **Dipole-dipole interactions:** These occur between polar molecules where partial positive and negative charges attract each other.
• **Hydrogen bonding:** A stronger type of dipole interaction that occurs as previously discussed.

In the context of \( \mathrm{H}_2\mathrm{~S} \) and \( \mathrm{H}_2\mathrm{O} \), \( \mathrm{H}_2\mathrm{O} \) experiences strong hydrogen bonding due to the presence of oxygen, while \( \mathrm{H}_2\mathrm{S} \) mainly relies on the weaker dipole-dipole interactions because sulfur is less electronegative than oxygen.

Polarity

Polarity in molecules arises when there is an uneven distribution of electron density. This is caused by differences in electronegativity between the atoms in a molecule. When atoms with different electronegativities are bonded together, the more electronegative atom attracts electrons more strongly, creating a polar bond with partial charges.

The shape of the molecule also plays an essential role. For instance, water \( (\mathrm{H}_2\mathrm{O}) \) is a bent molecule, which allows its polar bonds to add up, resulting in a significant dipole moment that makes it polar.

On the other hand, \( \mathrm{H}_2\mathrm{S} \) is also bent but involves hydrogen and sulfur. Since sulfur is less electronegative than oxygen, the difference in electronegativity between hydrogen and sulfur is smaller compared to hydrogen and oxygen in water. Thus, \( \mathrm{H}_2\mathrm{S} \) is less polar than \( \mathrm{H}_2\mathrm{O} \). A molecule's polarity affects its interactions with other molecules and its solubility, boiling point, and other properties.

Boiling Point

The boiling point of a substance is the temperature at which it changes from a liquid to a gas. This transformation requires breaking the intermolecular forces that hold the liquid molecules together.

Substances with strong intermolecular forces, like hydrogen bonding, require more energy to overcome these forces, resulting in a higher boiling point. Take water as an example; its high boiling point of \( 100^{\circ} \mathrm{C} \) is due to strong hydrogen bonds between the water molecules.

In contrast, \( \mathrm{H}_2\mathrm{S} \), which cannot form hydrogen bonds efficiently, exhibits weaker intermolecular forces, thus requiring less energy to transition to the gaseous state. This means \( \mathrm{H}_2\mathrm{S} \) will have a much lower boiling point than water. Understanding boiling points helps us figure out the state of matter of substances under different conditions and their interactions with the environment.