Class 12 NCERT solutions p-block (Important Questions along with Answers)

Ques 1:- Write a note on Inert pair effect.

Ans: the reluctance of the pair of s-electrons belonging to the valence shell of an element to take part in bond formation is known as Inert pair effect.

     Let us illustrate it by taking the example of group 13 elements. Elements of group 13 elements have three electrons in their valence shell (ns²np¹) and thus exhibit an oxidation state of +3. In addition to this they also exhibit an oxidation state of +1, which becomes more and more stable as we move down the group. This is because the pair of electrons of the s-orbital in the valence shell remain paired and do not participate in bonding. As such +1-oxidation state of thallium is more stable than +3 oxidation state.

Ques 2:- Why ammonia is a good complexing agent?

Ans: Because the nitrogen atom in ammonia has a lone pair of electrons which it can easily lose and act as a ligand. Due to small size of nitrogen atom, the lone pair of electrons is distributed over a smaller area. This increases the availability of electron pair and hence the basic character of NH3. Due to greater availability of lone pair of electrons, NH3 is a good complexing agent also.

Ques 3:- Explain why the first elements of a group differ from other elements of its group.

Ans: the group elements of a group differ considerably from the other members of the group due to the following reasons:

1. Small size
2. High electronegativity
3. Non-availability of d-orbitals for bonding

Ques 4:- Why phosphorus exists as P4 but nitrogen as N2 gas?

Ans: Due to the smaller atomic size of N, pπ-pπ bonding in nitrogen is possible. As such nitrogen exists as triply bonded discrete diatomic molecule and is a gas at room temperature. On the other hand, atomic size of phosphorus is much larger than nitrogen. As such pπ-pπ bonding in phosphorus is not possible and it exist as pyramidal tetratomic molecule (P4) in which each P atom is bonded to three other P atoms by three sigma bonds.

Ques 5:- Why PH3 is a weaker base than NH3?

Ans: Nitrogen atom having a small atomic size, have more electron density and thus the lone pair of electrons is more available for sharing with a proton. On moving and thus the lone pair of electron density goes on decreasing due to increase in atomic size and thus the lone pair of electrons is less available for sharing with a proton. Thus, ammonia is distinctly basic, phosphene is weakly basic, while other hydrides of this group do not show any basic character.

Ques 6:- Why does nitrogen show catenation properties less than phosphorus?

                                             Or

Unlike phosphorus, nitrogen shows little tendency for catenation?

Ans: Nitrogen has lesser tendency for catenation than phosphorus because N-N single bond is weaker than P-P single bond. The N-N single bond is lesser than P-P single bond length because atomic size of N is less than of P. Due to smaller interatomic distance nitrogen is repulsive interactions between lone pairs on two nitrogen atoms is more. This weakens the N-N single bond. On the other hand, due to longer P-P single bond such repulsive interactions between lone pairs are almost absent in phosphorus. This makes P-P single bond stronger than N-N single bond.

Ques 7:- Why white or yellow phosphorus is always kept under water?

Ans: Due to its great reactivity white or yellow phosphorus is always kept under water. In air, white or yellow phosphorus is spontaneously oxidised to P2O3 and P2O5 even at room temperature.

Ques 8:- Why does the reactivity of nitrogen differ from the phosphorus?

                                          Or

Why molecular nitrogen is not particularly reactive?

Ans: Nitrogen exists as a diatomic molecule. Due to the presence of a triple bond between the atoms N-atoms, the bond dissociation energy is large. As a result, nitrogen is inert and unreactive in its elemental state.

         In contrast, white or yellow phosphorus exists as a tetratomic molecule (P4). Since the P-P single bond is much weaker than N triple bond N therefore, phosphorus is much more reactive than nitrogen.

         Another factor for the large reactivity of phosphorus is the presence of angular strain in its molecule. Presence of angular strain in phosphorus makes it highly reactive.

Ques 9:- Why is oxygen gas while sulphur is a solid at room temperature?

Ans: Oxygen forms a stable diatomic molecule. In oxygen molecule, two atoms of oxygen are possible are joined together double bond (O=O). the multiple bonding in the oxygen is possible due to small size of oxygen. Due to smaller molecular mass oxygen is gas at room temperature.

       Sulphur, selenium and tellurium have polyatomic molecules because multiple bonding is not possible in these elements due to large atomic size. For example, sulphur has 8 atoms arranged in the form of a puckered ring per molecule. In this structure, each sulphur atom is linked to two other sulphur atoms through single covalent bonds. Consequently, sulphur has high molecular mass. Because of this, sulphur is a solid at room temperature.

Ques 10:- H2S is a gas while H2O is liquid at room temperature. Why?

Ans: Since atomic size of oxygen is less than sulphur, and the electronegativity of oxygen is more than sulphur, water shows inter-molecular hydrogen bonding but hydrogen sulphide does not. As such a large number of water molecules are associated with each other to form a cluster of molecules. Thus, a large amount of energy is required for their separation. This energy is not available at room temperature. Thus, water is a liquid and hydrogen sulphide are a gas at room temperature.

Ques 11:- Why the compounds of fluorine with oxygen are called fluorides of oxygen and not the oxides of fluorine?

                                               Or

OF2 should be called oxygen difluoride and not fluorine oxide. Why?

Ans: This is because fluorine is more electronegative than oxygen.

Ques 12:- Give two examples to show the anomalous behaviour of fluorine.

Ans: 

1. Most of the reactions of F2 are exothermic. It is due to small and strong bond formation by it with other elements.
2. It forms only one oxoacid, HOF while other halogens form a number of oxoacids.
3. HF is liquid at room temperature due to H-bonding, while other hydrogen halides are gaseous under similar conditions.

The anomalous character of fluorine is due to

• Its small size 
• Highest electronegativity,
• Low F-F bond dissociation enthalpy and
• Non-availability of d-orbitals in valence shell.

Ques 13:- Why are halogens most reactive?

Ans: The high reactivity of halogens is due to the following reasons:

1. Low bond dissociation energies: The halogens have very low bond dissociation energies. Therefore, they can readily dissociate into atoms and react with other substances.
2. High electron affinity values: Halogens have very high electron affinity values and therefore, have very strong tendency to gain an electron.

Thus, halogens are very reactive elements

Ques 14:- Why are halogens coloured?

Ans: Halogens absorb light in the visible region, which result in the excitation of outer electrons. Fluorine having small atomic size absorb higher energy violet radiations and the transmitted light appears pale yellow. On the other hand, iodine with large atomic size absorbs lower energy yellow and green radiations and thus transmitted light appears violet. 

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