Chemistry NCERT Exemplar Solutions Class 11th Chapter Four

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Ans: (i) N₂ → N₂⁺ + e^-

The electronic configuration of N₂ is:

σ1s² σ*1s² σ2s² σ*2s² π2p²_x = π2p²_y σ2p²_z

So, its bond order will be 3

The electronic configuration of N₂⁺ is:

σ1s² σ*1s² σ2s² σ*2s² π2p²_x = π2p²_y σ2p¹_z

Its bond order will be 2.5

Hence the bond order decreases in this reaction N₂ → N₂⁺ + e^-

(ii) O₂→ O₂⁺ + e-

The electronic configuration of O₂ is:

σ1s² σ∗1s² σ2s² σ∗2s² s2p_z² π2p²_y π2p²_x π*2p¹_y π*2p¹_x

Its bond order will be 2

The electronic configuration of O₂⁺ will be:

σ1s² σ∗1s² σ2s² σ∗2s² s2p_z², π2p²_y , π2p²_x, π*2p¹_x

Its bond order will be 2.5 .

Hence, the bond order in this reaction increases O₂→ O₂⁺ + e-

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Ans: The general sequence of the energy level of the molecular orbital is σ1s < σ*1s < σ2s< σ*2s < π2px = π2py < σ2pz

N₂    =   σ1s² σ*1s² σ2s² σ*2s² π2p²_x = π2p²_y σ2p²_z

N₂⁺  =   σ1s² σ*1s² σ2s² σ*2s² π2p²_x = π2p²_y σ2p¹_z

N₂^-   =   σ1s² σ*1s² σ2s² σ*2s² π2p²_x= π2p²_y σ2p²_z σ2p²_x

N₂²⁺ =   σ1s² σ*1s² σ2s² σ*2s² π2p²_x = π2p²_y

The formula for finding the bond order (B. O) for any molecular species is:

Bond order = $\frac{1}{2}$ ( N_b- N_a)

Hence, the bond orders of the given molecular species are:

N₂= $\frac{1}{2}$ ( 10-4)= 3

N₂⁺= $\frac{1}{2}$ ( 9-4)= 2.5

N₂^-= $\frac{1}{2}$ ( 10-5)= 2.5

N₂²⁺= $\frac{1}{2}$ (8-4)= 2

The order for the stability of the given molecular species thus will be:

N_2.> N₂^-> N₂⁺> N₂²⁺

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Ans: Although the hybridisation of the central atom oxygen in both the molecules is 3 sp but dimethyl ether will have a higher bond angle than water molecule.

Due to the presence of two bulky methyl groups in dimethyl ether, the repulsive forces will be greater in them than the two hydrogens in water molecules. In dimethyl ether the -CH₃ is a group attached to three hydrogen atom through s bonds. Thus, the C- H bond pairs increase the electron density on the carbon atom which results in lone pair-bond pair repulsions. Due to this lone pair-bond pair repulsions, the bond angle of dimethyl ether is greater than that of water molecules.

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Ans: (a) Which of the two compounds will have intermolecular hydrogen bonding and which compound is expected to show intramolecular hydrogen bonding.

Ans: The intramolecular hydrogen bonding is shown by compound (I) and the intermolecular hydrogen bonding is shown by compound (II) . In compound (I) the NO₂ and OH group are close together in comparison to that in compound (II) . So, that is why compound (I) shows intramolecular hydrogen bonding. The intermolecular hydrogen bonding in compound (II) is shown as:

(b) The melting point of a compound depends on, among other things, the extent of hydrogen bonding. On this basis explain which of the above two compounds will show higher melting point.

Ans: Due to the formation of intermolecular hydrogen bonds, compound (II) will have a higher melting point. Thus, due to hydrogen bond formation more molecules are joined together.

(c) Solubility of compounds in water depends on power to form hydrogen bonds with water. Which of the above compounds will form hydrogen bond with water easily and be more soluble in it?

Ans: Compound (I) will be less soluble in water due to intramolecular hydrogen bonding; it will not be able to form hydrogen bonds with water. Whereas, compound (II) can form hydrogen bonds with water more easily and is thus more soluble in water.

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Ans: The electronic configurations of O₂⁺ and O₂ ^- according to molecular orbital theory is:

O₂⁺: σ1s² σ∗1s² σ2s² σ∗2s² s2p_z², π2p²_y , π2p²_x, π*2p¹_x

O₂^- : σ1s² σ∗1s² σ2s² σ∗2s² s2p_z², π2p²_y , π2p²_x, π*2p²_y π*2p¹_x

The bond order of O₂⁺

Bond order = $\frac{1}{2}$ ( N_b- N_a)

Bond order = $\frac{1}{2}$ (10-5)=2.5

The bond order of O₂^-

$\frac{1}{2}$ (10-7)= $\frac{3}{2}$ = 1.5

As the bond order of O₂⁺ is higher, it is more stable than O₂ ^-, because higher the bond order more stable is the bond. Both the molecular species have the presence of unpaired electrons. So, they both are paramagnetic in nature.

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Ans: In the Lewis structure of PCl₃, the phosphorus atom is surrounded by three bond pairs (chlorine atoms) and one lone pair. These four electron pairs are arranged in a tetrahedral geometry around the central phosphorus atom. Due to the presence of lone pair of electron on the phosphorus atom, PCl₃ will have a distorted tetrahedral geometry. Thus, it will form a pyramidal shape and is non-linear in structure.

In H₂S, the central sulphur atom is surrounded by two bond pairs and two lone pairs of electrons. It can be said that these four electron pairs are arranged in a tetrahedral geometry. Due to the presence of two lone pairs on the central sulphur atom, the lone pair-bond pair repulsion happens and due to this the molecule H₂S has a V-shaped geometry and is non-linear in structure.