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) \(\mathrm{O} \cdot \mathrm{N}\), is \(+6\) \\\ (b) \(\mathrm{CCl}_{4}\) & (q) \(\mathrm{O} \cdot \mathrm{N}\). 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 NH4+

The ammonium ion \(\mathrm{NH}_4^+\) is a cation formed when ammonia \(\mathrm{NH}_3\) gains a proton \(\mathrm{H}^+\) through a coordinate covalent bond. Thus, the best match for \(\mathrm{NH}_4^+\) is (s) Coordinate covalent bond.

Step 2: Examine CCl4

Carbon tetrachloride \(\mathrm{CCl}_4\) contains a central carbon atom bonded to four chlorine atoms. The molecule is symmetric, so it has no net dipole moment, resulting in a dipole moment of zero, \(\mu=0.0 \mathrm{D}\). Therefore, the match for \(\mathrm{CCl}_4\) is (r) \(\mu=0.0 \, \mathrm{D}\).

Step 3: Identify CaOCl2

Calcium oxychloride, \(\mathrm{CaOCl}_2\), can be viewed as the combination of calcium chloride \(\mathrm{CaCl}_2\) and calcium hypochlorite \(\mathrm{Ca(ClO)}_2\). In hypochlorite ions, the oxidation number of the oxygen is \(-1\). Thus, the match for \(\mathrm{CaOCl}_2\) is (q) \(\mathrm{O} \, .\, \mathrm{N}\,.\, \text{is} \,-1\).

Step 4: Evaluate K2Cr2O7

Potassium dichromate \(\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7\) has a chromium oxidation state of \(+6\) since the dichromate ion \(\mathrm{Cr}_2\mathrm{O}_7^{2-}\) contains two chromium atoms combined with seven oxygens, while the oxidation state is calculated as \[2x + 7(-2) = -2\]. Solving for x gives +6. Therefore, the best match for \(\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7\) is (p) \(\mathrm{O} \, .\, \mathrm{N}\,.\, \text{is} \, +6\).

Key Concepts

Coordinate Covalent Bond

In chemistry, a coordinate covalent bond, also known as a dative bond, is a particular type of covalent bond in which both electrons in the shared pair come from the same atom. This is different from a normal covalent bond where each atom supplies one electron to the pair sharing.
An excellent example is the formation of the ammonium ion \(\mathrm{NH}_4^+\). In this ion, nitrogen provides a pair of electrons to bond with a hydrogen ion (\(\mathrm{H}^+\)), forming a coordinate bond. It is important in many biological and chemical processes.

• The donating atom often has a lone pair of electrons to share.
• This type of bond is still considered a covalent bond and not ionic.
• Coordinates play crucial roles, especially in complex ions and coordination chemistry.

Coordinate covalent bonds can also affect the molecule's shape and properties, leading to various fascinating chemical behaviors.

Dipole Moment

Dipole moment is a measure of the separation of positive and negative charges in a molecule. It's a vector quantity; this means it has both magnitude and direction. The unit of a dipole moment is Debye (\(\mathrm{D}\)).
When a molecule is symmetric, like carbon tetrachloride (\(\mathrm{CCl}_4\)), the effects of polar bonds cancel each other out, resulting in a net dipole moment of zero (\(\mu=0.0 \, \mathrm{D}\)).

• A molecule can have polar bonds but still be non-polar overall.
• Asymmetrical molecules tend to have a nonzero dipole moment.
• The dipole moment influences boiling points, solubility, and interactions with other molecules.

In conclusion, understanding whether a molecule has a dipole moment helps predict its interactions and behavior.

Oxidation Number Calculation

Oxidation number, commonly known as oxidation state, indicates the degree of oxidation or reduction of an element in a compound. Calculation of oxidation states helps in determining the distribution of electrons among the atoms in a molecule.
For instance, in potassium dichromate (\(\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7\)), the oxidation number of chromium is calculated as +6. Here's how:

• Each oxygen has an oxidation state of \(-2\).
• The sum of oxidation states in a neutral compound is zero, so we solve \((2x + 7(-2) = -2)\) to find \(+6\) for chromium.

This calculation is essential as it helps in understanding and balancing chemical reactions, particularly redox reactions.

Symmetry in Molecules

Symmetry in molecules plays a vital role in determining many of their chemical and physical properties. A symmetric molecule is one that can be divided into two identical halves by one or more symmetry operations like rotation or inversion.
Consider a molecule like carbon tetrachloride (\(\mathrm{CCl}_4\)), which is symmetric because each chlorine atom is equidistant from the central carbon atom, and arranged in a tetrahedral structure. This symmetry contributes to having no net dipole moment.

• Symmetry is related to molecular geometry, hybridization, and bond angles.
• Symmetrical molecules tend to be nonpolar.
• Understanding symmetry helps predict infrared spectral peaks and chemical reactivity.

In chemistry, symmetry is a cornerstone in understanding molecular properties and behavior.