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 points of H₂O and H₂S are 100°C (373.15 K) and -60°C (213.15 K), respectively, while their normal melting points are 0°C (273.15 K) and -82°C (191.15 K), respectively. Water (H₂O) has stronger intermolecular forces due to the presence of hydrogen bonding, in addition to dipole-dipole interactions and dispersion forces. Hydrogen sulfide (H₂S) only has dipole-dipole interactions and dispersion forces, which are weaker than hydrogen bonding.

Step-by-step solution

Look up the physical properties of H₂O and H₂S

First, we need to find the normal boiling points and normal melting points of H₂O and H₂S. These values can be obtained from chemistry books, the periodic table, or trusted online sources. For water (H₂O): - Normal boiling point: 100°C (373.15 K) - Normal melting point: 0°C (273.15 K) For hydrogen sulfide (H₂S): - Normal boiling point: -60°C (213.15 K) - Normal melting point: -82°C (191.15 K)

Compare physical properties to determine intermolecular forces

Now, we will compare the boiling points and melting points to determine which substance has stronger intermolecular forces. Higher boiling and melting points usually indicate stronger intermolecular forces in the substance. As we can see, water (H₂O) has a higher normal boiling point and melting point compared to hydrogen sulfide (H₂S). This implies that water has stronger intermolecular forces than hydrogen sulfide.

Identify intermolecular forces for each molecule

Now that we know which substance has stronger intermolecular forces, let's identify the types of intermolecular forces present in each molecule. For water (H₂O): - Hydrogen bonding: Water molecules have hydrogen atoms bonded to oxygen atoms, which have high electronegativity. This electronegativity difference creates a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atoms. Consequently, hydrogen bonding (a special type of dipole-dipole interaction) occurs between different water molecules. - Dipole-dipole interactions: Water molecules are also polar, which means they have regions with partial positive and negative charges. This leads to the attraction of oppositely charged regions from two separate molecules, called dipole-dipole interactions. - Dispersion forces: All molecules, whether polar or nonpolar, experience these weak attractions called dispersion forces (or London forces). However, dispersion forces are weaker compared to hydrogen bonding and dipole-dipole interactions. For hydrogen sulfide (H₂S): - Dipole-dipole interactions: H₂S has hydrogen atoms bonded to sulfur, which is also electronegative but not as electronegative as oxygen. This difference introduces a slight dipole, so dipole-dipole interactions can occur between hydrogen sulfide molecules. - Dispersion forces: Like all molecules, hydrogen sulfide experiences dispersion forces. In conclusion, water (H₂O) has stronger intermolecular forces than hydrogen sulfide (H₂S) due to the presence of hydrogen bonding in addition to other intermolecular interactions such as dipole-dipole and dispersion forces. Hydrogen sulfide (H₂S) only has dipole-dipole and dispersion forces, which are weaker than hydrogen bonding.