Problem 162 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following \begin{tabular}{ll} \hline Column-I & Column-II \\ \hline (a) \(\mathrm{NH}_{4}^{+}\) & (p) O.N. is \(+6\) \\ (b) \(\mathrm{CCl}_{4}\) & (q) O.N. is \(-1\) \\ (c) \(\mathrm{CaOCl}_{2}\) & (r) \(\mu=0.0 \mathrm{D}\) \\ (d) \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) & (s) Coordinate covalent bond \\ & (t) O.N. is \(+1\) \end{tabular}

Step-by-Step Solution

Answer

(a) - (s), (b) - (r), (c) - (q), (d) - (p)

Step 1: Analyze Ammonium Ion (NH_4^+)

Ammonium ion, \(NH_4^+\), possesses a central nitrogen atom with four hydrogen atoms bonded to it. Nitrogen typically forms three bonds, gaining a proton in ammonium form, resulting in a positive charge. This involves a co-ordinate covalent bond where a lone pair from nitrogen is donated to the proton. Therefore, the correct match is \(s\), as it involves coordinate covalent bonding.

Step 2: Study Carbon Tetrachloride (CCl_4)

Carbon tetrachloride, \(CCl_4\), is a tetrahedral molecule with carbon at its center and four chlorine atoms symmetrically surrounding it. Due to its symmetrical shape, the dipole moment (\(\mu\)) is zero. Thus, the correct match is \(r\) as the dipole moment, \(\mu = 0.0 \, D\).

Step 3: Evaluate Calcium Oxychloride (CaOCl_2)

In calcium oxychloride, \(CaOCl_2\), we analyze the oxidation numbers. The \(Cl\) in \(OCl^-\) has an oxidation number of \(-1\) to balance the +2 from calcium, and +1 or -1 in bleach-like mixtures. Thus, chlorine's oxidation number in this context isn't straightforwardly \(-1\), but is generally considered acting as bleach. Therefore, it is likely interacting in a bleaching reaction, corresponding to \(q\).

Step 4: Investigate Potassium Dichromate (K_2Cr_2O_7)

In potassium dichromate, \(K_2Cr_2O_7\), there are two chromium atoms, which need to balance the \(7 \, O^{2-}\) ions and \(2 \, K^+\). Each chromium typically exhibits an oxidation state of \(+6\) in this compound. Consequently, the correct match is \(p\) with chromium's oxidation number being \(+6\).

Key Concepts

Coordinate Covalent Bond

A coordinate covalent bond, also known as a dative bond, is a type of chemical bond where both electrons in the shared pair come from the same atom. This is different from a typical covalent bond, where each atom contributes one electron to the bond.
In the case of the ammonium ion (\(NH_4^+\)), a coordinate covalent bond forms when a nitrogen atom, which has a lone pair of electrons, donates these electrons to bond with a proton (H\(^+\)).
This results in the proton being bonded to the nitrogen, completing the nitrogen's valence shell with four bonds. The coordinate bond formed is indistinguishable from other covalent bonds once the connection is made, but it is important to know that the electron pair originally came from one atom.
Understanding its role in complex molecules helps students grasp why certain atoms can exceed their normal bonding capacity in polyatomic ions.

Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges in a molecule. It is a vector quantity, possessing both magnitude and direction, typically measured in Debye units (D).
In molecules like carbon tetrachloride (\(CCl_4\)), the dipole moment is zero because of its symmetrical tetrahedral shape. The molecule's symmetry allows the dipole moments of the bonds to cancel each other out, resulting in a non-polar molecule.
For students, it is crucial to understand that the geometry of a molecule greatly influences its dipole moment.

• Non-symmetrical molecules tend to have a non-zero dipole moment, making them polar.
• Symmetrical molecules, on the other hand, often have zero dipole moment, making them non-polar.

The concept of dipole moments is vital in predicting the intermolecular forces and behaviors in chemical reactions.

Oxidation Number

The oxidation number (O.N.), sometimes referred to as oxidation state, is a number that represents how many electrons an atom gains or loses when it forms compounds.
In compounds like potassium dichromate (\(K_2Cr_2O_7\)), the chromium has an oxidation number of \(+6\). This is because the chromium atoms must balance the charges contributed by the oxygen and potassium atoms.
For example, each oxygen atom in \(K_2Cr_2O_7\) typically has an oxidation number of \(-2\), totaling \(-14\) for seven oxygens, while two potassium atoms contribute \(+2\). The two chromium atoms must then account for the remaining positive charges, requiring each to have an oxidation number of \(+6\).
Understanding oxidation numbers is essential for students to balance chemical equations and understand redox reactions, as they offer insight into electron transfer processes within molecules.

• Oxidation indicates a loss of electrons, increasing the oxidation number.
• Reduction involves a gain of electrons, decreasing the oxidation number.

Remember, these concepts help in structuring your comprehension of how elements react in various chemical environments.