Bihar Board - Class 12 Chemistry - Chapter 7: p-Block Elements Short Answer Question

The short question answers for Chapter 7: p-Block Elements of Class 12 Chemistry are provided here in English. These questions are prepared based on the NCERT curriculum for Class 12. They will help students understand the chapter easily. These short question answers will be useful not only for the Class 12 board exams but also for entrance exams like NEET. The questions and answers for Bihar Board Class 12 Chemistry, Chapter 7: p-Block Elements, have been prepared by expert teachers at Vidyakul.

                             Short QuestionAnswer 

Q. 1. How can you separate alumina from silica ? Give equations, if any.

Ans⇒ The powdered alumina is fused with carbon and heated to 1800C  in a current of nitrogen. Aluminum forms its nitride while silica is reduced to silicon which volatiles off.

Al2O3+N2+3C   2AlN+3CO (g)  

SiO2+2C Si + 2CO

Q. 2. How is nitrogen prepared in the laboratory ? Write the chemical equations of the reactions involved.

Ans⇒ In the laboratory, nitrogen is prepared by treating an aqueous solution of ammonium chloride with sodium nitrite.

NH4Cl (aq)+NaNO2 (aq) M2 (g)+2H2O (I)+NaCl (aq) 

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Q. 3. Boiling point of  NH3 is higher than  PH3.

Ans⇒ This is because there is intramolecular hydrogen bonding present in NH3 (Due to small size and high electronegativity of N) and not in PH3 (due to large size and less electronegativity of P).

Q. 4. HF is weaker than HI in acetic strength.

Ans⇒ Hydrogen iodide (HI) is more stronger acid than hydrogen fluoride because iodine (I) is more electronegative than Fluorine (F) and hence it can donate H+ more easily than Fluorine (F).

Q. 5. H3PO3 is a di-protic acid.

Ans⇒ Molecular structure of H3PO3  is as following:

A diprotic acid is an acid that contains within its molecular structure two hydroxyl group (O-H) from that H3PO3 , it is clear that H3PO3  the hydroxyl group is bounded with P . Hence H3PO3  is diprotic acid.

Q. 6. Aniline dissolves in HCl .

Ans⇒ Aniline is somewhat basic. It is not as basic as ammonia, since protonation destroys the interaction between the ring and the lone pair on the nitrogen atom but even still it is sufficiently basic (PKb9.4) to dissolve in HCl .

Q. 7. PCl5 is known but NCl5 is not known.

Ans⇒ Nitrogen can’t extend its valence from 3 to 5 due to absence of d-orbital while phosphorous shown penta valances as d-orbital present in its valence cell.

Q. 8. H2O is liquid but H2S is gas.

Ans⇒ In vapor state (100 K) . Sulphur partly exists, as S2 molecules like O2  has two unpaired electrons in the antibonding  molecular orbital and exhibits paramagnetism.

Q. 9. Noble gasses are monatomic in nature.

Ans⇒Noble gasses are monatomic in nature. It is because noble gas make oxidation.

Q. 10. Assign an appropriate reason for each of the following statements :
(i) More metal fluorides are ionic in nature than metal chlorides.
(ii) Solid phosphorus pentachloride exhibits some ionic character.

Ans⇒ (i) 'F' is more electronegative than 'Cl'Therefore, fluorides are more ionic metal chlorides.
(ii) PCl5 exhibits some ionic character because it exists as [PCl4+] [PCl6-] in solid state.

Q. 11. Why are pentahalides more covalent than trihalides ?

Ans⇒ Higher the positive oxidation of the central atom, [ +5 and +3 in pentahalides and trihalides respectively) more will be its polarizing power which, in turn, increases the covalent character of bond formed between the central atom and the other atom.

Q. 12. Why is BiH3 the strongest reducing agent amongst all the hydrides of Group 15 elements ?

Ans⇒ BiH3 is the strongest reducing agent amongst all the hydrides of Group 15 , because it is the least stable out of all hydrides of Group 15 elements.

Q. 13. Why is N2 less reactive at room temperature ?

Ans⇒ The bond enthalpy of triple bond in N N is very high due to p-p overlap. Hence N2 is less reactive at room temperature. It reacts only at high temperatures.

Q. 14. Why is nitrous acid oxidant as well as reductant?

Ans⇒ The oxidation number of N is nitrous acid (H-O-N=O) is +3 which lies in between its lowest oxidation of -3 and maximum oxidation state of +5. Since Ox. no. of +3 in HNO2 can both be decreased as well as increased. There it acts both as an oxidant and a reductant.

Q. 15. SiF62- is known but SiCl62- is not. Why is it so ?

Ans⇒ (i) Due to smaller size of F as compared to Cl , steric repulsions will be less in SiF62- than in SiCl62- .
(ii) Because of smaller Si-F bond as compared to Si-Cl bond the interaction of Flone pair of electrons with Si is stronger than that of Cl ion pairs.

Q. 16. Write the chemical equation to show that chlorine gas can be prepared from bleaching powder.

Ans⇒ Bleaching powder reacts with dilute acids like H2SO4 , HCl etc. liberate Cl2 gas.

CaOCl2+H2SO4 CaSO4+H20+Cl2  (bleaching powder)

Q. 17. Oxygen is diatomic and gaseous in nature, explain.

Ans⇒ Due to small size and high electronegativity, oxygen forms a p-p  double bond and hence exists as diatomic molecules. Since these diatomic molecules are held together by weak V and der Waals forces of attraction which can be easily broken by collisions of the molecule at room temperature, therefore I2 is a gas.

Q. 18. Give reasons for the least reactivity of nitrogen molecules.

Ans⇒ Due to the presence of a triple bond between the two nitrogen atoms (N=N), the bond dissociation energy of N2 is very high. Hence N2 is least reactive.

Q. 19. Why does argon not form diatomic molecules like oxygen and nitrogen ?

Ans⇒ All the orbitals which are occupied by electrons are completely filled in argon and hence it has no tendency to gain, lose, or share electrons with other atoms of Ar to form diatomic molecules. O and , on the other hand, have unpaired electrons and hence share with other O and N atom respectively to form O2 and N2 molecules.

Q. 20. How is pyrophosphoric acid related to orthophosphoric acid ? Write its structure.

Ans⇒ Pyrophosphoric acid (H4P2O7) is the anhydride of orthophosphoric acid and is obtained by removal of a molecule of H2O from two molecules of H3PO4. The structure of pyrophosphoric acid (H4P2O7) is

Q. 21. Why is white phosphorus kept under water? Name the three series of salts formed when orthophosphoric acid reacts with sodium hydroxide.

Ans⇒. (i) Ignition temperature of white phosphorus is very low (303 K) . Therefore, on exposure to air, it spontaneously catches fire forming P4O10.
(ii) NaH2PO4 , Na2HPO4 and Na3PO4.

Q. 22. Noble gasses are mostly inert. Assign reasons.

Ans⇒Noble gasses are chemically inert because of the following reasons.
(i) They have completely filled ns2np6 electronic configuration in their valence shell [except He=IS2]
(ii) They have high ionization enthalpies.
(iii) Electron affinities of noble gasses are almost zero.

Q. 23. Why are interhalogens more reactive than halogens ?

Ans⇒ This is because the bond in the interhalogen [A-X] is weaker than X-X bond in the halogens. This is due to less effective overlapping or orbitals of dissimilar atoms than those between similar atoms.

Q.24. Why HF is stored in wax coated glass bottles ?

Ans⇒ This is because HF does not attack wax, but reacts with glass. It dissolves SiO2 present in glass forming hydrofluorosilicic acid.

SiO2+6 HF H2SiF6+2H2O

Q. 25. Why does NH3 form hydrogen bond but PH3 Doesn't it ?