Water is a liquid at STP. Hydrogen sulfide, H2S, a heavier molecule, is a gas under the same conditions.Explain, using diagrams, why this is so.
To understand why water is a liquid and hydrogen sulfide is a gas at standard temperature and pressure (STP), we need to consider the molecular structures and intermolecular forces involved.
Water (H2O) has a bent molecular structure, which means that the oxygen atom is bonded to two hydrogen atoms at an angle of approximately 104.5 degrees. Due to the strong electronegativity of the oxygen atom, the shared electrons in the O-H bond are pulled closer to the oxygen atom, creating a partial negative charge (δ-) on the oxygen and partial positive charges (δ+) on the hydrogen atoms.
The diagram of the water molecule (H2O) can be represented as follows:
δ-
|
H - O - H
| |
δ+ δ+
The presence of these partial charges leads to a type of intermolecular force called hydrogen bonding, which occurs between the positively charged hydrogen atom of one water molecule and the negatively charged oxygen atom of another water molecule.
Hydrogen sulfide (H2S), on the other hand, also has a bent molecular structure similar to water. However, sulfur is less electronegative than oxygen, resulting in weaker polarization of the H-S bond compared to the O-H bond. Therefore, hydrogen sulfide only exhibits van der Waals forces, specifically dipole-dipole interactions between the positive end of one molecule and the negative end of another.
The diagram of the hydrogen sulfide molecule (H2S) can be represented as follows:
δ+
|
H - S - H
| |
δ- δ+
Based on the differences in the intermolecular forces, we can explain why water is a liquid and hydrogen sulfide is a gas at STP:
1. Hydrogen bonding in water: The stronger hydrogen bonding in water molecules leads to higher intermolecular attractions, requiring more energy to overcome these forces and convert water from a liquid to a gas. This results in water being a liquid at STP.
2. Dipole-dipole forces in hydrogen sulfide: The dipole-dipole forces in hydrogen sulfide molecules are not as strong as hydrogen bonding in water. Thus, hydrogen sulfide molecules do not require as much energy to overcome these forces, allowing hydrogen sulfide to exist as a gas at STP.
Therefore, the differences in the molecular structure, electronegativity, and intermolecular forces between water and hydrogen sulfide contribute to their different states under the same conditions of temperature and pressure.