Chemistry Class 12 NCERT Solutions: Chapter 7 The p Block Elements Part 1

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p block elements

P Block Elements

p block elements

Q: 1. Discuss the general characteristics of Group 15 elements with reference to their electronic configuration, oxidation state, atomic size, ionisation enthalpy and electronegativity.

Answer:

General trends in group 15 elements

(i) Electronic configuration: All the elements in group 15 have 5 valence electrons. Their general electronic configuration is .

(ii) Oxidation states: All these elements have 5 valence electrons and require three more electrons to complete their octets. However, gaining electrons is very difficult, as the nucleus will have to attract three more electrons. This can take place only with nitrogen as it is the smallest in size and the distance between the nucleus and the valence shell is relatively small. The remaining elements of this group show a formal oxidation state of in their covalent compounds. In addition to the state, N and P also show and oxidation states.

All the elements present in this group show and oxidation states. However, the stability of oxidation state decreases down a group, whereas the stability of oxidation state increases. This happens because of the inert pair effect.

(iii) Ionization energy and electronegativity

First ionization decreases on moving down a group. This is because of increasing atomic sizes. As we move down a group, electronegativity decreases, owing to an increase in size.

(iv) Atomic size:

On moving down a group, the atomic size increases. This increase in the atomic size is attributed to an increase in the number of shells.

Q: 2. Why does the reactivity of nitrogen differ from phosphorus?

Answer:

Nitrogen is chemically less reactive. This is because of the high stability of its molecule, . In , the two nitrogen atoms form a triple bond. This triple bond has very high bond strength, which is very difficult to break. It is because of nitrogen's small size that it is able to form bonds with itself. This property is not exhibited by atoms such as phosphorus. Thus, phosphorus is more reactive than nitrogen.

Q: 3. Discuss the trends in chemical reactivity of group 15 elements.

Answer

General trends in chemical properties of group-15

(i) Reactivity towards hydrogen:

The elements of group 15 react with hydrogen to form hydrides of type , . The stability of hydrides decreases on moving down from .

(ii) Reactivity towards oxygen:

The elements of group form two types of oxides: and , where , As, , or . The oxide with the element in the higher oxidation state is more acidic than the other. However, the acidic character decreases on moving down a group.

(iii) Reactivity towards halogens:

The group 15 elements react with halogens to form two series of salts: . However, nitrogen does not form as it lacks the d:orbital. All trihalides are stable.

(iv) Reactivity towards metals:

The group elements react with metals to form binary compounds in which metals exhibit oxidation states.

Q: 4. Why does form hydrogen bond but does not?

Answer:

Nitrogen is highly electronegative as compared to phosphorus. This causes a greater attraction of electrons towards nitrogen in than towards phosphorus in . Hence, the extent of hydrogen bonding in is very less as compared to .

Q: 5. How is nitrogen prepared in the laboratory? Write the chemical equations of the reactions involved.

Answer

An aqueous solution of ammonium chloride is treated with sodium nitrite.

and are produced in small amounts. These are impurities that can be removed on passing nitrogen gas through aqueous sulphuric acid, containing potassium dichromate.

Q: 5. How is nitrogen prepared in the laboratory? Write the chemical equations of the reactions involved.

Ans:

An aqueous solution of ammonium chloride is treated with sodium nitrite.

and are produced in small amounts. These are impurities that can be removed on passing nitrogen gas through aqueous sulphuric acid, containing potassium dichromate.

Q: 6. How is ammonia manufactured industrially?

Answer:

Ammonia is prepared on a large-scale by the Haber’s process.

The optimum conditions for manufacturing ammonia are:

(i) Pressure

(ii) Temperature

(iii) Catalyst such as iron oxide with small amounts of and

Q 6 The Optimum Conditions for Manufacturing Ammonia

Q 6 the Optimum Conditions for Manufacturing Ammonia

Q 6 The Optimum Conditions for Manufacturing Ammonia