Question

Compare the physical and chemical properties of \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{H}_{2} \mathrm{~S}\).

Short answer

\(\mathrm{H}_2\mathrm{O}\) is a polar liquid with hydrogen bonding; \(\mathrm{H}_2\mathrm{S}\) is a less polar gas with a rotten egg smell.

Step-by-step solution

Identify the Substance Formulas

Let's first identify the chemical formulas of the substances we are comparing. Water is represented as \(\mathrm{H}_2\mathrm{O}\), and hydrogen sulfide is represented as \(\mathrm{H}_2\mathrm{S}\). Both compounds consist of hydrogen combined with a group 16 element.

Determine Physical Properties

Now, let's consider the physical properties of each compound. \(\mathrm{H}_2\mathrm{O}\) has a melting point of 0°C and a boiling point of 100°C. It is a colorless and odorless liquid at room temperature. \(\mathrm{H}_2\mathrm{S}\) has a lower melting point of about -83°C and a boiling point of -60°C, and it is a colorless gas with a distinct rotten egg smell at room temperature.

Examine Chemical Properties

For chemical properties, \(\mathrm{H}_2\mathrm{O}\) is a stable compound known for its ability to act as a solvent, facilitating numerous chemical reactions. It is polar due to the electronegativity difference between hydrogen and oxygen. \(\mathrm{H}_2\mathrm{S}\) is less stable, readily dissociating into hydrogen and sulfur under certain conditions. It is less polar compared to water because sulfur is less electronegative than oxygen.

Analyze Effects of Molecular Structure

Water forms hydrogen bonds because of its high polarity, leading to higher boiling and melting points. These interactions are responsible for its liquid state at room temperature. In contrast, hydrogen sulfide does not form strong hydrogen bonds, resulting in lower boiling and melting points and existing as a gas at room temperature.

Key concepts

Physical Properties

Physical properties help us understand how substances appear in nature. They involve characteristics like state (solid, liquid, gas), color, smell, melting and boiling points. These traits do not involve any change in the substance's chemical identity.
For example,

• Water (\(\mathrm{H}_{2}\mathrm{O}\)) is a colorless and odorless liquid at room temperature.
• Hydrogen sulfide (\(\mathrm{H}_{2}\mathrm{S}\)), on the other hand, is a colorless gas with a noticeable rotten egg smell at room temperature.

These differences arise from the molecular structure of each compound and its interactions with the environment.

Melting Point and Boiling Point

Melting and boiling points indicate the temperatures at which a substance changes state. The melting point is the temperature at which a solid turns into a liquid, while the boiling point is when a liquid turns into a gas.

• Water (\(\mathrm{H}_{2}\mathrm{O}\)) has a melting point of 0°C and a boiling point of 100°C. This means ice melts at 0°C, and water boils at 100°C under normal atmospheric pressure.
• Hydrogen sulfide (\(\mathrm{H}_{2}\mathrm{S}\)) has a much lower melting point of about -83°C and boils at around -60°C. It is primarily gas at room temperature.

These differences in temperature points are largely due to the distinct molecular interactions that occur within each substance.

Molecular Structure

The molecular structure of a compound describes the arrangement of atoms within a molecule. This plays a role in determining its physical and chemical properties.
For instance,

• Water's molecular structure is bent due to the two hydrogen atoms being bonded to oxygen in a V-shape at an angle of about 104.5°.
• Hydrogen sulfide also has a bent shape, but with a smaller angle than water because sulfur is less electronegative than oxygen.

These structural features influence the molecule's polarity and its ability to form interactions with other molecules.

Polarity and Hydrogen Bonding

Polarity refers to the uneven distribution of charges across a molecule. A molecule is polar if it has a positive and a negative pole.

• Water is highly polar due to the significant electronegativity difference between hydrogen and oxygen. This enables water molecules to form hydrogen bonds with each other, resulting in stronger intermolecular forces.
• In contrast, hydrogen sulfide has a lower polarity because sulfur is less electronegative than oxygen. This results in weaker intermolecular forces and the inability to form effective hydrogen bonds.

Hydrogen bonding, which is strong in water but weak or absent in hydrogen sulfide, affects the physical properties like the boiling points and states of matter at room temperature.