Question

Which of the following statements is wrong? (1) HCl molecule in the vapour state is an example of polar covalent bond. (2) Among \(\mathrm{CH}_{2} \mathrm{Cl}_{2}, \mathrm{BF}_{3}, \mathrm{NF}_{3}\) and \(\mathrm{ClO}_{2}\), only \(\mathrm{BF}_{3}\) has zero dipole moment. (3) The dipole moment of \(\mathrm{NH}_{3}\) is less than \(\mathrm{NCl}_{3}\). (4) Anhydrous IICl is a bad conductor of clectricity but aqueous IIC1 is a good conductor.

Short answer

Statement (3) is wrong; \(\text{NH}_3\) has a greater dipole moment than \(\text{NCl}_3\).

Step-by-step solution

- Analyze Statement 1

Statement (1) says that HCl in the vapor state is an example of a polar covalent bond. HCl is known to be a polar molecule due to the difference in electronegativity between H and Cl, so this statement is correct.

- Analyze Statement 2

Statement (2) indicates that only \(\text{BF}_3\) among \(\text{CH}_2 \text{Cl}_2, \text{BF}_3, \text{NF}_3, \text{and ClO}_2\) has zero dipole moment. \(\text{BF}_3\) is indeed nonpolar and has a zero dipole moment due to its symmetrical trigonal planar structure, making this statement correct.

- Analyze Statement 3

Statement (3) asserts that the dipole moment of \(\text{NH}_3\) is less than \(\text{NCl}_3\). Considering the molecular geometry and the relative electronegativity differences, \(\text{NH}_3\) has a larger dipole moment (1.47 D) compared to \(\text{NCl}_3\) (0.6 D). Therefore, this statement is incorrect.

- Analyze Statement 4

Statement (4) states that anhydrous HCl is a bad conductor of electricity while aqueous HCl is a good conductor. This is known to be true since HCl ionizes in water to form ions that conduct electricity, making this statement correct.

Conclusion

The incorrect statement is (3), as the dipole moment of \(\text{NH}_3\) is greater than that of \(\text{NCl}_3\).

Key concepts

Polar Covalent Bond

A polar covalent bond is a type of chemical bond where a pair of electrons is unequally shared between two atoms. These atoms have different electronegativities, which causes the electrons to spend more time closer to one atom than the other. This uneven electron distribution results in a molecule with a partial positive charge on one side and a partial negative charge on the other. For example, in a hydrogen chloride (HCl) molecule, the chlorine atom is more electronegative than the hydrogen atom, making HCl a polar molecule.

Polar covalent bonds are crucial because they create dipole moments. These dipoles can influence how the molecules interact with each other and with external electric fields. Polar compounds generally display higher solubility in polar solvents like water.

Understanding polar covalent bonds helps explain many chemical phenomena, such as solubility and reactivity differences among various compounds.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons in a chemical bond. The concept was introduced by Linus Pauling, and it plays a critical role in determining the type of bond that forms between atoms. Generally, on the Pauling scale, fluorine has the highest electronegativity (3.98), while elements like cesium and francium have very low electronegativities (0.7-0.79).

When two atoms with different electronegativities form a bond, the electron density shifts towards the more electronegative atom. This causes the formation of a polar covalent bond. If the difference in electronegativity is very high, an ionic bond will form instead of a covalent bond.

The relative electronegativity of atoms can predict the behavior and properties of the molecules they form, such as polarity, bond strength, and reactivity.

Molecular Geometry

Molecular geometry is the three-dimensional arrangement of atoms within a molecule. It is based on the repulsion between electron pairs (bonding and non-bonding) around a central atom. VSEPR (Valence Shell Electron Pair Repulsion) theory is commonly used to predict molecular geometry.

The geometry of a molecule influences its physical and chemical properties, such as polarity and reactivity. For example, the trigonal planar geometry of boron trifluoride (BF3) results in a symmetrical shape, making it a nonpolar molecule with a zero dipole moment. In contrast, ammonia (NH3) has a trigonal pyramidal geometry due to a lone pair on nitrogen, which results in a polar molecule with a significant dipole moment.

Understanding molecular geometry is essential for predicting how molecules will interact in different environments and with different substances.

Conductivity of Solutions

Conductivity of solutions refers to a solution's ability to conduct electricity, which depends on the presence of charged particles (ions) and their mobility. Pure water is a poor conductor of electricity because it has very few ions. However, when a substance like hydrochloric acid (HCl) is dissolved in water, it ionizes to form H+ and Cl- ions. These ions can move freely and carry electric current, making the solution a good conductor.

In contrast, anhydrous HCl does not have free ions to conduct electricity, making it a bad conductor. The conductivity of a solution is often used to understand the strength of electrolytes in the solution. Strong electrolytes completely dissociate into ions (like HCl in water), while weak electrolytes only partially dissociate.

This concept is important in fields such as chemistry, biology, and environmental science, where the electric properties of solutions can affect processes and reactions.