The difference in physical states (liquid or gas) between water (H₂O) and hydrogen sulfide (H₂S) at room temperature and pressure can be attributed to the differences in the strength and type of intermolecular forces acting between their molecules.
Hydrogen Bonding:
Water (H₂O) is a polar molecule with a bent molecular geometry. The oxygen atom is more electronegative than the hydrogen atoms, creating a partial negative charge on the oxygen and partial positive charges on the hydrogens. This polarity allows water molecules to form hydrogen bonds between them.
Hydrogen bonding is a strong intermolecular force that occurs when a hydrogen atom bonded to an electronegative atom (such as oxygen) is attracted to another electronegative atom in a nearby molecule. The hydrogen bonds between water molecules result in a cohesive and stable structure, contributing to the liquid state at room temperature.
Dipole-Dipole Interactions:
Hydrogen sulfide (H₂S) is also a polar molecule due to the electronegativity difference between sulfur and hydrogen. However, hydrogen sulfide molecules exhibit weaker dipole-dipole interactions compared to the hydrogen bonding in water.
While dipole-dipole interactions contribute to the cohesion of H₂S molecules, they are not as strong as the hydrogen bonds in H₂O.
Boiling Points:
The boiling point of a substance is influenced by the strength of intermolecular forces. Water has a higher boiling point than hydrogen sulfide due to the stronger hydrogen bonding between water molecules.
Water boils at 100 degrees Celsius (212 degrees Fahrenheit) at standard atmospheric pressure, while hydrogen sulfide is a gas at room temperature and pressure and boils at -60.7 degrees Celsius (-77.3 degrees Fahrenheit).
Polarity and Molecular Size:
The difference in molecular size also plays a role. Hydrogen sulfide has larger sulfur atoms compared to the oxygen atoms in water, and this affects the overall molecular size and shape.
Additionally, the molecular shape of H₂S, which is linear, results in weaker intermolecular forces compared to the bent shape of H₂O, contributing to the differences in their physical states.
In summary, the primary factor determining why water is a liquid at room temperature while hydrogen sulfide is a gas is the stronger hydrogen bonding between water molecules compared to the dipole-dipole interactions in hydrogen sulfide. The presence of hydrogen bonding enhances the cohesive forces between water molecules, resulting in a higher boiling point and the liquid state of water under standard conditions.