Question

Look up and compare the normal boiling points and normal melting points of \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{H}_{2} \mathrm{S}\) . Based on these physical properties, which substance has stronger intermolecular forces? What kinds of intermolecular forces exist for each molecule?

Short answer

The normal boiling point of \(\mathrm{H}_2\mathrm{O}\) is 100 °C (373 K) while \(\mathrm{H}_2\mathrm{S}\) is -60.4 °C (212.8 K). The normal melting point of \(\mathrm{H}_2\mathrm{O}\) is 0 °C (273 K) while \(\mathrm{H}_2\mathrm{S}\) is -85.5 °C (187.7 K). As \(\mathrm{H}_2\mathrm{O}\) has higher boiling and melting points, it has stronger intermolecular forces than \(\mathrm{H}_2\mathrm{S}\). \(\mathrm{H}_2\mathrm{O}\) has dipole-dipole interactions and hydrogen bonding, while \(\mathrm{H}_2\mathrm{S}\) has weaker dipole-dipole interactions and no hydrogen bonding, resulting in the difference in intermolecular force strengths.

Step-by-step solution

Look up the normal boiling and melting points

For this step, we need to search for the normal boiling and melting points of \(\mathrm{H}_2\mathrm{O}\) and \(\mathrm{H}_2\mathrm{S}\). - The normal boiling point of \(\mathrm{H}_2\mathrm{O}\) is 100 °C (373 K) and its normal melting point is 0 °C (273 K). - The normal boiling point of \(\mathrm{H}_2\mathrm{S}\) is -60.4 °C (212.8 K) and its normal melting point is -85.5 °C (187.7 K).

Compare boiling and melting points

Based on the values from step 1, we can compare the boiling and melting points between the two substances. \(\mathrm{H}_2\mathrm{O}\) has a higher boiling and melting point compared to \(\mathrm{H}_2\mathrm{S}\).

Determine stronger intermolecular forces based on boiling and melting points

Generally, higher boiling and melting points are an indication of stronger intermolecular forces in a substance. As the boiling and melting points of \(\mathrm{H}_2\mathrm{O}\) are higher than those of \(\mathrm{H}_2\mathrm{S}\), we can conclude that the intermolecular forces in \(\mathrm{H}_2\mathrm{O}\) are stronger.

Identify the types of intermolecular forces present in each molecule

The three main types of intermolecular forces are: London dispersion forces, dipole-dipole interactions, and hydrogen bonding. \(\mathrm{H}_2\mathrm{O}\) is a polar molecule due to the difference in electronegativities between Hydrogen and Oxygen atoms. As a result, \(\mathrm{H}_2\mathrm{O}\) has dipole-dipole interactions. In addition, Oxygen, being a highly electronegative atom, forms a strong hydrogen bond with a Hydrogen from another \(\mathrm{H}_2\mathrm{O}\) molecule. \(\mathrm{H}_2\mathrm{S}\) is also a polar molecule, meaning it has dipole-dipole interactions. However, while sulfur is more electronegative than hydrogen, sulfur's electronegativity is significantly lower than oxygen's, thus, forming weaker dipole-dipole interactions. Additionally, sulfur does not form hydrogen bonding like oxygen. In conclusion, it is the hydrogen bonding in \(\mathrm{H}_2\mathrm{O}\) that makes its intermolecular forces stronger compared to the dipole-dipole interactions in \(\mathrm{H}_2\mathrm{S}\).

Key concepts

boiling point

The boiling point of a substance is the temperature at which it transitions from a liquid to a gas.
Boiling points give insights into the strength of the intermolecular forces holding the molecules together.
Substances with higher boiling points have stronger intermolecular forces.
For example:

• Water (\( \text{H}_2\text{O} \)) has a boiling point of 100 °C.
• Hydrogen sulfide (\( \text{H}_2\text{S} \)) boils at -60.4 °C.

Comparing these values, \( \text{H}_2\text{O} \) has a significantly higher boiling point than \( \text{H}_2\text{S} \), indicating stronger intermolecular forces at play.

melting point

The melting point is the temperature at which a substance changes from a solid to a liquid.
It's another indicator of the strength of intermolecular forces.
Higher melting points suggest stronger forces between the molecules.Some examples:

• The melting point of water (\( \text{H}_2\text{O} \)) is 0 °C.
• Hydrogen sulfide (\( \text{H}_2\text{S} \)) melts at -85.5 °C.

Clearly, water's higher melting point compared to \( \text{H}_2\text{S} \) signals that the intermolecular forces in water are more potent.

hydrogen bonding

Hydrogen bonding is a specific type of intermolecular force that occurs when hydrogen is bonded to highly electronegative atoms like oxygen or nitrogen.
This results in a strong attraction between molecules.
In the case of \( \text{H}_2\text{O} \):

• The oxygen atom is highly electronegative, creating a strong dipole.
• The hydrogen atoms are strongly attracted to oxygen atoms in neighboring molecules, facilitating hydrogen bonding.

This bond is much stronger than typical dipole-dipole interactions.
It significantly contributes to both the boiling and melting points of water.

dipole-dipole interactions

Dipole-dipole interactions occur between polar molecules.
Molecules align such that positive ends of one dipole attract the negative ends of another.
This force is stronger than London dispersion forces but weaker than hydrogen bonds.In both \( \text{H}_2\text{O} \) and \( \text{H}_2\text{S} \):

• The molecules are polar.
• They have an uneven distribution of electric charge, leading to dipole moments.

While both exhibit dipole-dipole interactions, \( \text{H}_2\text{O} \)'s interactions are reinforced by its hydrogen bonding, making it substantially stronger.

polar molecules

A polar molecule is one with an uneven distribution of electrons across its structure, leading to a dipole moment.
This occurs because of differences in electronegativity between the atoms.For example:

• Water (\( \text{H}_2\text{O} \)) is polar as the oxygen atom is more electronegative than hydrogen, pulling electron density towards itself.
• Hydrogen sulfide (\( \text{H}_2\text{S} \)) is also polar, but with weaker interactions due to sulfur's lower electronegativity compared to oxygen.

These polar characteristics affect the type and strength of the intermolecular forces, influencing their physical properties like boiling and melting points.