Problem 9 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which contains both polar and non-polar bonds? (a) \(\mathrm{NH}_{4} \mathrm{Cl}\) (b) \(\mathrm{HCN}\) (c) \(\mathrm{H}_{2} \mathrm{O}_{2}\) (d) \(\mathrm{CH}_{4}\)

Step-by-Step Solution

Answer

(b) HCN, as it has both polar (C-N) and non-polar (H-C) bonds.

Step 1: Identifying Polar and Non-polar Bonds

First, recall the definitions: a polar bond is a covalent bond between two atoms where the electrons forming the bond are unequally distributed, leading to a dipole moment. A non-polar bond is where the electrons are equally shared between the two atoms. Our task now is to identify these bond types in each of the compounds listed.

Step 2: Analyze NH4Cl

- **Bonds Present**: The compound \( ext{NH}_4 ext{Cl}\) is made up of \( ext{NH}_4^+\) ion and \( ext{Cl}^-\) ion.- **Bond Types**: \( ext{NH}_4^+\) contains \( ext{N-H}\) polar bonds due to the difference in electronegativity between N and H. In \( ext{NH}_4^+ ext{ and } ext{Cl}^-\), the attraction is ionic, not a covalent bond. Hence, it doesn't count as polar or non-polar covalent bonds.

Step 3: Analyze HCN

- **Bonds Present**: \( ext{HCN} \) consists of \( ext{H-C} \) and \( ext{C-N} \) bonds.- **Bond Types**: The \( ext{C-N} \) bond is polar due to the difference in electronegativities, while the \( ext{H-C} \) bond is non-polar because carbon and hydrogen have similar electronegativities.

Step 4: Analyze H2O2

- **Bonds Present**: \( ext{H}_2 ext{O}_2 \) has \( ext{O-O} \), \( ext{O-H} \) bonds.- **Bond Types**: In \( ext{H}_2 ext{O}_2 \), the \( ext{O-H} \) bonds are polar, while the \( ext{O-O} \) bond is non-polar due to identical electronegativities of the oxygen atoms.

Step 5: Analyze CH4

- **Bonds Present**: \( ext{CH}_4 \) consists of four \( ext{C-H} \) bonds.- **Bond Types**: Here, all bonds \( ext{C-H} \) are non-polar due to the small difference in electronegativity between carbon and hydrogen.

Step 6: Conclusion

The compound \( ext{HCN} \) contains both polar and non-polar bonds: polar \( ext{C-N} \) and non-polar \( ext{H-C} \) bonds. None of the other compounds satisfy having both bond types.

Key Concepts

Polar Bonds

Polar bonds occur in covalent bonds where the electrons are shared unequally between two different types of atoms. This happens because one of the atoms has a higher electronegativity, or ability to attract electrons, than the other. As a result, the atom with the higher electronegativity becomes slightly negative, while the other atom becomes slightly positive, creating a dipole moment.
Examples of polar bonds include connections between hydrogen and oxygen or nitrogen, such as those found in water (\(\mathrm{H}_2\mathrm{O} \)).As in the exercise, the \(\mathrm{C-N} \) bond in \(\mathrm{HCN} \) is polar because nitrogen is more electronegative than carbon. This inequality in electron distribution is essential for the polar nature of such bonds.

Non-polar Bonds

Non-polar bonds, on the other hand, are covalent bonds where the electrons are shared equally between the two atoms. This balance happens because the atoms involved have similar or identical electronegativities, meaning neither atom attracts the shared electrons more strongly than the other.
Bonds like \(\mathrm{H-H} \) found in a hydrogen molecule are classic examples of non-polar bonds, as both atoms have the same electronegativity.

• The \(\mathrm{O-O} \) bond in \(\mathrm{H}_2\mathrm{O}_2 \) is non-polar, as both oxygen atoms have the same electronegativity, leading to equal sharing of electrons.
• Similarly, the \(\mathrm{C-H} \) bond, such as in methane (\(\mathrm{CH}_4 \)), is often considered non-polar due to the relatively small difference in electronegativity between C and H.

Maintaining this equal distribution is key to understanding non-polar covalent bonds.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons within a chemical bond. The concept was introduced by Linus Pauling, and it plays a critical role in determining the polarity of a bond.
Atoms with high electronegativity, such as fluorine, oxygen, and nitrogen, have a strong ability to attract electrons, leading to polar bonds when paired with less electronegative atoms.

• In compounds like \(\mathrm{HCN} \), the significant electronegativity difference between carbon (lower) and nitrogen (higher) causes the \(\mathrm{C-N} \) bond to be polar.
• Conversely, when two atoms have similar or close electronegativities, such as C and H, the bond tends to be non-polar.

Understanding electronegativity is crucial for predicting bond polarity and molecular interactions.

Covalent Bonds

Covalent bonds are the chemical links formed by the sharing of electron pairs between atoms. These bonds are fundamental to the structure of many molecules, like the ones students encounter in chemistry exercises.
There are two main types of covalent bonds that are often taught:

• Polar covalent bonds, where electrons are shared unequally due to different electronegativities.
• Non-polar covalent bonds, where electrons are shared equally.

In the context of the exercise, analyzing whether bonds are polar or non-polar in molecules like \(\mathrm{HCN} \), \(\mathrm{H}_2\mathrm{O}_2 \), and \(\mathrm{CH}_4 \) helps students understand the behavior and properties of these substances. Ultimately, mastering the concept of covalent bonding is foundational for deeper studies in chemistry, and it aids in predicting molecular shapes, interactions, and reactions.