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