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NCERT Class 12 Chemistry Textbook Chapter 7 With Answer PDF Free Download
Chapter 7: The p-Block Elements
Group 15 includes nitrogen, phosphorus, arsenic, antimony, and bismuth. As we go down the group, there is a shift from non-metallic to metallic through metalloids character.
Nitrogen and phosphorus are non-metals, arsenic and antimony metalloids and bismuth is a typical metal.
Molecular nitrogen comprises 78% of the volume of the atmosphere. In the earth’s crust, it occurs as sodium nitrate, NaNO3 (called Chile saltpeter), and potassium nitrate (Indian saltpeter).
It is found in the form of proteins in plants and animals. Phosphorus occurs in minerals of the apatite family, Ca9 (PO4)6 CaX2 (X = F, Cl or OH) (e.g., fluorapatite Ca9 (PO4)6. CaF2) which are the main components of phosphate rocks.
Phosphorus is an essential constituent of animal and plant matter. It is present in bones as well as in living cells. Phosphoproteins are present in milk and eggs.
Arsenic, antimony, and bismuth are found mainly as sulfide minerals.
Sulphuric acid is a very important industrial chemical. A nation’s industrial strength can be judged by the quantity of sulphuric acid it produces and consumes.
It is needed for the manufacture of hundreds of other compounds and also in many industrial processes.
The bulk of sulphuric acid produced is used in the manufacture of fertilizers (e.g., ammonium sulphate, superphosphate). Other uses are in:
(a) petroleum refining (b) manufacture of pigments, paints, and dyestuff intermediates (c) detergent industry (d) metallurgical applications (e.g., cleansing metals before enameling, electroplating, and galvanizing (e) storage batteries (f) in the manufacture of nitrocellulose products and (g) as a laboratory reagent.
Like other elements of the p-block present in the second period of the periodic table, fluorine is anomalous in many properties.
For example, ionization enthalpy, electronegativity, and electrode potentials are all higher for fluorine than expected from the trends set by other halogens.
Also, ionic and covalent radii, m.p. and b.p., enthalpy of bond dissociation, and electron gain enthalpy are quite lower than expected.
The anomalous behavior of fluorine is due to its small size, highest electronegativity, low F-F bond dissociation enthalpy, and non-availability of d orbitals in the valence shell.
Most of the reactions of fluorine are exothermic (due to the small and strong bond formed by it with other elements).
It forms only one oxoacid while other halogens form a number of oxoacids. Hydrogen fluoride is a liquid (b.p. 293 K) due to strong hydrogen bonding. Other hydrogen halides are gases.
| Author | NCERT |
| Language | English |
| No. of Pages | 44 |
| PDF Size | 7.1 MB |
| Category | Chemistry |
| Source/Credits | ncert.nic.in |
NCERT Solutions Class 12 Chemistry Chapter 7 The p-Block Elements
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, N_{2}N2. In N_{2}N2, 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 p\pi−p\pipπ−pπ 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 the group − 15
(i) Reactivity towards hydrogen:
The elements of group 15 react with hydrogen to form hydrides of type EH_{3}EH3, where E = N,P, As, Sb, or Bi. The stability of hydrides decreases on moving down from NH_{3}\;to \;BiH_{3}NH3toBiH3.
(ii)Reactivity towards oxygen:
The elements of group 15 form two types of oxides: E_{2}O_{3}E2O3 and E_{2}O_{5}E2O5, where E = N, P, As, Sb, or Bi. 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: EX_{3}\;and\;EX_{5}EX3andEX5. However, nitrogen does not form NX_{5}NX5 as it lacks the d-orbital. All trihalides (except NX_{3}NX3) are stable.
(iv) Reactivity towards metals:
The group 15 elements react with metals to form binary compounds in which metals exhibit
−3 oxidation states.
Q 4: Why does NH_{3}NH3 form a hydrogen bond but PH_{3}PH3 does not?Answer
When compared to phosphorus nitrogen is highly electronegative. This results in a greater attraction of electrons towards nitrogen in NH_{3}NH3 than towards phosphorus in PH_{3}PH3. Hence, the extent of hydrogen bonding in PH_{3}PH3 is very less as compared to NH_{3}NH3.
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.
NH4Cl (aq ) + NaNO2 →N2(g) + 2H2O(l) + NaCl(aq)
NO, and HNO_{3}HNO3 are produced in small amounts. These are impurities that can be removed on passing nitrogen gas through aqueous sulphuric acid, containing potassium dichromate.
NCERT Class 12 Chemistry Textbook Chapter 7 With Answer PDF Free Download