Group 15 elements(nitrogen group) NCERT QUESTIONS (REASONING)

  NCERT QUESTIONS (REASONING) 

 Q.1 Why BiH3 is the strongest reducing agent amongst all the hydrides of Group 15 elements ?  

Ans. In hydrides of nitrogen family on moving down the group M–H bond length increases so bond strength decreases hence tendency to release hydrogen increases and reducing nature increases. Increasing order of reducing nature is NH3 < PH3 < AsH3 < SbH3 < BiH3 

 

Q.2 Write the reaction of thermal decomposition of sodium azide.  

Ans. Thermal decomposition of sodium azide gives dinitrogen gas. 2NaN3 --------->2Na + 3N2  

Q.3 Why N2 is less reactive at room temperature?  

Ans. N2 is less reactive at room temperature because of the high bond enthalpy of NºN bond. 

Q.4 Why does R3P = O exist but R3N = O does not (R = alkyl group)?  

Ans. Due to presence of vacant d orbital phosphorous can form five covalent bond, while nitrogen restricts its covalency to four due to absence of vacant d orbitals.  

Q.5 Why does nitrogen show catenation properties less than phosphorus?  

Ans. N–N bond is weaker than the single P–P bond due to high interelectronic respulsion of non-bonding electrons as a result the catenation tendency is weaker in nitrogen.  

Q.6 Mention the conditions required to maximise the yield of ammonia.  

Ans. In accordance with Le Chatelier’s principle, high pressure would favour the formation of ammonia. The optimum conditions for the production of ammonia are a pressure of 200 × 105 Pa (about 200 atm), a temperature of ~ 700 K and the use of a catalyst such as iron oxide with small amounts of K2O and Al2O3 to increase the rate of attainment of equilibrium.  

Q.7 How does ammonia react with a solution of Cu2+?  

Ans. The presence of a lone pair of electrons on the nitrogen atom of the ammonia molecule makes it a Lewis base. It donates the electron pair and forms linkage with metal ions and the formation of such complex compounds finds applications in detection of Cu2+ .  

 Cu2+ (aq) +  4NH3 (aq)        ------------> [Cu (NH3)4]2+ (aq) 

(blue)                                                        (deep blue) 

Q.8 Illustrate how copper metal can give different products on reaction with HNO3.  

Ans. 3Cu + 8 HNO3 (dilute) ----------> 3Cu (NO3) 2 + 2NO + 4H2O 

            Cu + 4HNO3 (conc.)  --------->    Cu (NO3) 2 + 2NO2 + 2H2O  

Q.9 Why is nitrogen di-oxide paramagnetic in gaseous state but the solid obtained on cooling it is diamagnetic.  

Ans. NO2 contains odd number of electrons in its valence shell. On cooling it forms dimer and converted to stable N2O4 which is a colourless solid and diamagnetic in nature.  

Q.10 Why NH3 gas cannot be dried by passing over P2O5, CaCl2 and H2SO4?  

Ans. CaCl2 + 8NH3 --------> CaCl2 .8NH3  

         P2O5 + 6NH3 + 3H2O ------> 2(NH4) 3 PO4  

         H2SO4 +2NH3--------> (NH4) 2 SO4 

          So it is dried by passing over quick lime  

           (CaO). CaO + H2O -------> Ca (OH)2  

P-Block elements (important question answer related to Group -17

 Group  - 17.( Fluorine, Chlorine, Bromine, Iodine and Astatine) 

  A) Important reactions:  

1. 2F₂ + 2H₂O → 4HF + O₂ 

 2. Cl₂ + H₂O→ HCl + HOCl  

3. 4HCl + O₂ → 2Cl₂+2H₂O  

4. H₂S + Cl₂ →2HCl + S  

5. In excess of NH₃: ‐ 

     8NH₃ + 3Cl₂ → 6NH₄Cl + N₂  

6. In excess of Cl2: ‐ 

    NH₃ + 3Cl₂ → NCl₃ + 3HCl  

7. In cold and dilute NaOH: ‐

    2NaOH + Cl₂ → NaCl + NaOCl + H₂O 

 8. In hot and conc. NaOH:  

    6NaOH + 3Cl₂ →5NaCl + NaClO₃ + 3H₂O

  

9. SO₂ + 2H₂O + Cl₂ →H₂SO₄ +2HCl  

10. I₂ + 6H₂O + 5Cl₂ → 2HIO₃+10HCl  

11. Cl₂ + H₂O → 2HCl + O               

Coloured substance + O → Colourless substance (permanent bleach)  

B) Reasoning questions  

1. Halogens have maximum negative electron gain enthalpy (ΔegH)  

Ans. Because they have smallest size in their respective periods

2. F has less electron gain enthalpy than that of Cl but fluorine is stronger oxidizing agent than chlorine.

Ans. F is stronger oxidizing agent due to its low bond dissociation enthalpy and high hydration enthalpy.

3. F exhibits only ‐1 oxidation state, other halogen shows +1, +3, +5, +7 oxidation states or F does not exhibit any positive oxidation state. Explain.  

Ans. F is most electronegative element and due to absence of d-orbitals it cannot expand its octet so it does not exhibit positive oxidation state.  

4. Iron reacts with HCl gives Fe (II) chloride and not Fe (III) chloride. Why? Ans. Fe + 2 HCl →FeCl2 + H2 H2 liberated prevents the oxidation of FeCl2 to FeCl3  

5. Why bond dissociation enthalpy of F2 is less than Cl2? 

 Ans. Due to very small size of F there is lone pair repulsion in F2 so it has low bond dissociation enthalpy.  

6. Fluorine does not undergo disproportionation. Explain. 

 Ans. Disproportionation means simultaneous oxidation‐reduction. F being the most electronegative element undergoes only reduction but not oxidation.  

7. Bleaching by Cl2 is permanent but by SO2 is temporary. Why?  

    Ans. Cl2 bleaches by oxidation while SO2 does it by reduction. The reduced product gets oxidized again in air and the colour returns. 

 8. HF has lower acid strength than HI. Explain.  

    Ans. Due to larger size of I, the H‐I bond is weaker than H-F bond so HI is stronger acid.  

9. I₂ is more soluble in KI than in water. Why?  

   Ans. I2 forms complex with KI i.e. KI + I2 → 2KI3  

10. Why HClO is stronger acid than HIO?

Ans. ClO– is more stable than IO– because Cl is more electronegative, so HClO is stronger acid than HIO.  

11. Why HClO4 is stronger acid than HClO3?  

Ans. ClO4– is more stable than ClO3– due to more no. of resonating structures of ClO4-.  

 

12 OF2 should be called fluoride of oxygen and not oxide of F, explain.  

Ans. Because F is more electronegative than O  

 

13. Interhalogens are more reactive than halogens or ICl is more reactive than I2? 

 Ans. They are polar and have weaker X‐X’ bond. 

 

14. HF is stored in wax coated glass bottle   

Ans. Because HF reacts with alkali present in glass.  

15 MF is more ionic than MCl (M is alkali metal) 

 Ans. Because F– is smaller than Cl– and hence it is less polarizable.  

16. Cl2+ KI → brown colour, but excess Cl2 turns it colourless  

Ans. Cl2 is stronger oxidizing agent than I2. It first oxidizes KI into I2 which imparts brown colour.  

Cl2+ KI →I2 + 2KCl 

 In excess of Cl2, I2 is further oxidized to HIO3 which is colourless. 

 I2 + 6H2O + 5Cl2 → 2HIO3 + 10HCl  

17. Why HClO4 is stronger acid than H2SO4? Ans. Oxidation state of Cl in HClO4 is +7. Oxidation state of         S in H2SO4 is +6. Greater is the oxidation state of central atom, more is the acidic strength.  

18. Give the reason for bleaching action of Cl2.  

Ans. Bleaching action is due to oxidation. Cl2 + H2O →2HCl + O Coloured substance + O → Colourless                 substance  

19 Name two poisonous gases which can be prepared from chlorine gas.  

Ans. phosgene (COCl2), tear gas (CCl3NO2)  

20. Write two uses of ClO2.  

Ans. ClO2 is used i) As a bleaching agent for paper pulp and textiles ii) In water treatment.  

21. Why are halogens coloured?  

 Ans. due to absorption of radiations in visible region which results in the excitation of outer electrons to higher energy level. By absorbing different quanta of radiation, they display different colours. 

 

22. How can you prepare

 (i) Cl2 from HCl and (ii) HCl from Cl2? Write reactions only  

Ans. (i) MnO2 + 4HCl →MnCl2 + Cl2 + 2H2O 

       (ii) H2 + Cl2 → 2HCl  

 

23. With what neutral molecule is ClO– isoelectronic?  

Ans. OF2 ( other example is ClF. It is a Lewis base) 

 

24. Write balanced equations for the following:  

(i) NaCl is heated with sulphuric acid in the presence of MnO2.  

(ii) Chlorine gas is passed into a solution of NaI in water. 

 Ans. (i) 4NaCl +MnO2 + 4H2SO4 →MnCl2 + 4NaHSO4 + 2H2O+Cl2  

       (ii) I2 + 6H2O + 5Cl2 → 2HIO3 + 10HCl  

25. Write the reactions of F2 and Cl2 with water.  

Ans. 1. 2F2 + 2H2O →4HF + O2 2. Cl2 + H2O →HCl + HOCl  

26. Why are halogens strong oxidizing agents?  

Ans because of 

 i) ready acceptance of an electron

 ii) Small atomic sizes

 iii) Highly electronegative nature

 iv) Highly negative value of E0 red are the reasons for the strong oxidizing nature of halogens.  

27. Explain why fluorine forms only one Oxo acid, HOF.  

Ans. Due to high electro negativity and small size, fluorine forms only one Oxo acid, HOF.

28. Explain why in spite of nearly the same electronegativity, oxygen forms hydrogen bonding while chlorine does not.  

Ans. Because electron density on oxygen atom is much higher than that of Cl atom.  

29. ClF3 exists but FCl3 does not. Or F never acts as central atom in the inter halogen compounds.

Ans. Because F is more electronegative than Cl.  

30. F2 is the most reactive of all the four common halogens, explain. 

Ans. Because of low F‐F bond dissociation enthalpy.  

31. HOI is a weaker acid than HOCl. 

Ans. Because Cl is more electronegative than I. 

C. Arrange the following as the property indicated against them.  

1. F2, Cl2, Br2, I2 (increasing order of Boiling Point) 

Ans. F2< Cl2< Br2< I2

2. F2, Cl2, Br2, I2 (increasing order of bond dissociation enthalpy.)  

Ans. I2< F2< Br2< Cl2  

3. F, Cl, Br, I (increasing order of electron gain enthalpy)  

Ans. I< Br< F < Cl 

4. HF, HCl, HBr, HI (increasing order of acidic strength)  

Ans. HF < HCl MCl> MBr >MI  

6. HClO, HClO2, HClO3, HClO4 (increasing order of acidic strength) 

Ans HClO < HClO2< HClO3< HClO4 

7. HF, HCl, HBr, HI (decreasing order of B.P.)  

Ans. HF> HBr >HI>HCl