Question

Arrange the following in order of increasing dipole moment \(: \mathrm{H}_{2} \mathrm{O}, \mathrm{H}_{2} \mathrm{~S}, \mathrm{BF}_{3}\). (a) \(\mathrm{BF}_{3}<\mathrm{H}_{2} \mathrm{~S}<\mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{H}_{2} \mathrm{~S}<\mathrm{BF}_{3}<\mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{H}_{2} \mathrm{O}<\mathrm{H}_{2} \mathrm{~S}<\mathrm{BF}_{3}\) (d) \(\mathrm{BF}_{3}<\mathrm{H}_{2} \mathrm{O}<\mathrm{H}_{2} \mathrm{~S}\)

Short answer

The correct order of increasing dipole moment is: \(\mathrm{BF}_{3}<\mathrm{H}_{2} \mathrm{~S}<\mathrm{H}_{2} \mathrm{O}\), which is option (a).

Step-by-step solution

Determine Molecular Geometry and Polarity of BF3

Boron trifluoride (BF3) has a trigonal planar geometry with equal bonds to three fluorine atoms. Since the molecule is symmetrical, all the dipole moments of the individual B-F bonds cancel out, resulting in a net dipole moment of zero.

Evaluate the Molecular Geometry and Polarity of H2O

Water (H2O) has a bent molecular geometry due to the two lone pairs on the oxygen atom. This V-shaped structure ensures that the dipole moments of both O-H bonds do not cancel out but rather add up to give a net dipole moment, making water a polar molecule with a relatively high dipole moment.

Assess Molecular Geometry and Polarity for H2S

Similar to water, hydrogen sulfide (H2S) also has a bent molecular geometry. However, due to the less electronegative sulfur in comparison to oxygen, the S-H bonds are less polar. Therefore, the dipole moment of H2S is less than that of H2O.

Arrange the Molecules in Order of Increasing Dipole Moment

Given that BF3 has a dipole moment of zero, it is the least polar. H2S is less polar than H2O due to sulfur's lower electronegativity. Hence the order of increasing dipole moment is: BF3 < H2S < H2O.

Choose the Correct Option Based on Analysis

Based on the molecular geometry and polarity, the increasing order of dipole moments is BF3 (least polar), followed by H2S, and finally H2O (most polar). This corresponds to choice (a) \(\mathrm{BF}_{3}<\mathrm{H}_{2} \mathrm{~S}<\mathrm{H}_{2} \mathrm{O}\).

Key concepts

Molecular Geometry

Understanding the three-dimensional arrangement of atoms within a molecule is crucial for grasping concepts such as chemical reactivity, interaction, and physical properties. This arrangement, known as molecular geometry, plays a pivotal role in determining the behavior and characteristics of a molecule.

For example, consider water (H₂O), which has a 'bent' molecular geometry. This arrangement occurs because of the two lone pairs of electrons on the oxygen atom that push the hydrogen atoms downwards, resulting in a V-shape. This shape is critical as it directly influences the molecule's dipole moment and, hence, its polarity.

Molecular Polarity

Dipole moments are vector quantities that depend on both the magnitude of the charge separation and the distance between the charges. If a molecule has a symmetrical structure, like boron trifluoride (BF₃) with its trigonal planar shape, the individual bond dipole moments can cancel each other out, leading to a non-polar molecule. In contrast, asymmetrical molecules like water (H₂O) possess a net dipole moment, since the dipoles do not cancel. This concept is paramount when predicting the solubility and reactivity of molecules with other substances.

Electronegativity

Electronegativity refers to the tendency of an atom to attract a shared pair of electrons towards itself in a chemical bond. The greater the difference in electronegativity between atoms, the more polar the bond. Oxygen, for instance, is highly electronegative compared to hydrogen, resulting in a polar covalent bond in water (H₂O). On the other hand, sulfur's lower electronegativity in hydrogen sulfide (H₂S) leads to less polar bonds, thus contributing to a lower dipole moment than that of water.

Chemical Bonding

The type of chemical bonds formed between atoms is intimately linked to the molecule's geometry and polarity. The bonds in BF₃, H₂O, and H₂S involve the sharing of electrons between atoms, categorizing them as covalent bonds. However, differences in electronegativity and molecular geometry influence whether these bonds are non-polar or polar covalent. These characteristics help determine the physical and chemical properties of a compound, including the state of matter at room temperature and the compound's solubility in various solvents.