Question

Water \(\left(\mathrm{H}_{2} \mathrm{O}\right)\) is more polar than hydrogen sulfide \(\left(\mathrm{H}_{2} \mathrm{~S}\right)\). Explain.

Short answer

Water is more polar due to oxygen's higher electronegativity and its molecular shape leading to a larger dipole moment.

Step-by-step solution

Define Polarity

Polarity in molecules is determined by the difference in electronegativity between the bonded atoms and the overall molecular shape. A polar molecule has regions of partial positive and partial negative charges.

Analyze Electronegativity

The electronegativity of oxygen is 3.44, while sulfur's electronegativity is 2.58. This higher electronegativity for oxygen compared to sulfur means that oxygen attracts the shared electrons in a bond more strongly, leading to a greater separation of charge.

Compare Molecular Shapes

Both water (H₂O) and hydrogen sulfide (H₂S) have bent molecular geometries due to the presence of lone pairs on the oxygen and sulfur atoms, respectively. However, the angle in H₂O is 104.5°, which is greater than the bond angle in H₂S of about 92° due to smaller, more electronegative oxygen causing a tighter angle.

Determine Net Dipole Moment

The greater electronegativity difference in H₂O translates to a larger dipole moment compared to H₂S. In water, the dipole moment is more pronounced, resulting in a stronger pull of electron density toward oxygen, making the molecule more polar.

Conclusion on Polarity

Combining the higher electronegativity of oxygen compared to sulfur and the shape's impact on the dipole moment, water (H₂O) is more polar than hydrogen sulfide (H₂S). The separation of charge due to high electronegativity difference and the geometry results in water having a significant dipole moment, thus being more polar.

Key concepts

Electronegativity

Electronegativity is a key concept in understanding the polarity of molecules. It refers to the tendency of an atom to attract a shared pair of electrons toward itself. In simple terms, the more electronegative an atom is, the more it "pulls" on electrons in a bond. Electronegativity values are assigned on a scale, and they vary across the periodic table.

For instance, oxygen, with an electronegativity of 3.44, is more electronegative than sulfur, which has a value of 2.58. This higher electronegativity means that oxygen attracts the bonding electrons more strongly than sulfur. As a result, the electrons spend more time closer to the oxygen atom, creating a partial negative charge on the oxygen and a partial positive charge on the hydrogen atoms in the water molecule.

• Higher electronegativity means stronger attraction of electrons
• Causes partial charges, contributing to molecule polarity
• Oxygen is more electronegative than sulfur

The difference in electronegativity between bonded atoms leads to the formation of what we know as polar bonds. Therefore, this difference is crucial in contributing to the overall polarity of the molecule.

Molecular Geometry

Molecular geometry describes the shape of a molecule and is determined by the arrangement of atoms and the electron pairs surrounding the central atom. The molecular shape affects how polar a molecule is by influencing the net distribution of electron density.

Molecules like water (H₂O) and hydrogen sulfide (H₂S) both have a bent structure due to the lone pairs located on their central atoms. In water, the bond angle is about 104.5°, which is larger than that in hydrogen sulfide, approximately 92°. This is because the electronegative oxygen holds its bond pairs closer together than sulfur.

• Bent shape due to lone pairs
• Water has a 104.5° bond angle
• Hydrogen sulfide has a 92° bond angle

The geometric shape dictates how the dipole moments (arising from differences in electronegativity) add up. In water, with its larger angle and more acute bent shape, these dipole moments reinforce each other more effectively, leading to a higher polarity overall.

Dipole Moment

A dipole moment is a measure of the separation of positive and negative charges in a molecule. It provides a quantitative assessment of a molecule's overall polarness. Dipole moments depend on both the difference in electronegativity between the atoms and the geometry of the molecule.

In water, the significant electronegativity difference and optimized bent geometry lead to a strong dipole moment, where the pull of electron density is maximally directed toward the oxygen atom. This results in a clear and effective separation of charges, denoting a strong polarity.

• Measures separation of charge
• Depends on both electronegativity and geometry
• Results in stronger polarity in water

Thus, the dipole moment of water is much larger than that of hydrogen sulfide. Despite having a similar shape, the stronger pull of electrons in water due to oxygen's higher electronegativity makes it more polar than hydrogen sulfide, where sulfur is less electronegative.