G 17
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This document contains notes on Group 17 (halogen) elements from a Class 12 chemistry class. It discusses various properties and reactions of halogens like fluorine and chlorine. Key points include: halogens are strong oxidizing agents due to their high electronegativity and ability to readily accept electrons; fluorine forms only one oxoacid (HOF) due to the absence of d-orbitals; oxygen can form hydrogen bonds due to its smaller size compared to chlorine; and fluorine is a stronger oxidizing agent than chlorine despite having a less negative electron gain enthalpy, due to its lower bond dissociation energy and higher hydration enthalpy.
G 17
- 1. JAI GURUDEV CHEMISTRY CLASSES // BY AJEET KUAMR // M.N. 7209985022// CLASS -12 (UNIT-07-p-block elements) Group-17 notes 1. Why are halogens strong oxidising agents? Ans: - Due to low bond energy, high electronegativity, large negative electron enthalpy and high hydration energy, halogen have high electrode potential and a strong tendency to accept electron and hence they are the strong oxidising agent. 2. Explain why fluorine forms only one oxoacid, HOF. Ans: - Due to absence of d orbitals fluorine forms only one oxoacid, HOF. 3. Explain why inspite of nearly the same electronegativity, oxygen forms hydrogen bonding while chlorine does not. Ans: - inspite of nearly the same electronegativity, oxygen forms hydrogen bonding while chlorine does not because oxygen has smaller atomic size than chlorine. 4. Write two uses of ClO2. Ans: - Two uses of ClO2 are :- (i) It acts as bleaching agent. (ii) it is used as germicide 5. Why are halogens colored? Ans: -Halogens absorb part of light in the visible region which causes excitation of outer electrons to higher energy levels. Hence halogens are colored. 6. Write the reactions of F2 and Cl2 with water. Ans : - F2 + H2O 4HF + O2 Cl2 + H2O HCl + HOCl 7. How can you prepare Cl2 from HCl and HCl from Cl2? Write reactions only. Ans : - MnO2 + HCl MnCl2 + Cl2 + 2H2O Cl2 + H2 2HCl 8. 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) NaCl + H2SO4 + MnO2 NaHSO4 + MnCl2 + H2O + Cl2 (ii) Cl2 + NaI NaCl + I2 9. With what neutral molecule is ClO _ isoelectronic? Is that molecule a Lewis base? Ans: - ClF is the isoelectronic with ClO, ClF is Lewis base. 10. Why is ICl more reactive than I2? Ans: - The bond dissociation energy of I-Cl is less than I-I, hence ICl is more reactive than I2. 11. When HCl reacts with finely powdered iron, it forms ferrous chloride and not ferric chloride. Why? Ans: - Its reaction with iron produces H2. Fe + 2HCl FeCl2 + H2 Liberation of hydrogen prevents the formation of ferric chloride. 12. Considering the parameters such as bond dissociation enthalpy, electron gain enthalpy and hydration enthalpy, compare the oxidising power of F2 and Cl2. Ans: - The electrode potential of F2 (+2.87V) is much higher than that of Cl2 (=1.36V), therefore F2 is much stronger oxidising agent than Cl2. Electrode potential depends on the following factors :- (i) Bond dissociation energy of F2 & Cl2 are 159 KJ/mol and 243 KJ/mol (ii) Electrode gain enthalpy for F2 & Cl2 are -333 KJ/mol and -349 KJ/mol (iii) Hydration energy for F2 & Cl2 are -515 KJ/mol & -381 KJ/mol. On account of higher energy in F2, electrode potential of F2 is higher than Cl2; hence the oxidising power of F2 is higher than Cl2. 1. Give two examples to show the anomalous behaviour of fluorine. Ans: - Two examples to show the anomalous behaviour of fluorine are: - (i) Fluorine forms only one oxo acid while other halogens form a number of oxoacids. (ii) Hydrogen fluoride is a liquid (B.P. 293 K) due to strong hydrogen bonding. Other hydrogen halides are gases. 2. Sea is the greatest source of some halogens. Comment. Ans: - Sea water contains chlorides, bromides and iodides of sodium, potassium, magnesium and calcium but mainly sodium chloride (2.5% by mass). The deposits of dried up seas contain these compounds, e.g., sodium chloride and carnallite, KCl.MgCl2.6H2O. Certain forms of marine life contain iodine in their systems; various seaweeds, for example, contain upto 0.5% of iodine and Chile saltpetre contains upto 0.2% of sodium iodate. 3. Fluorine exhibits only 1 oxidation state whereas other halogens exhibit + 1, + 3, + 5 and + 7 oxidation states also. Explain. Ans :- Due to absence of d-orbitals, Fluorine exhibits only 1 oxidation state whereas other halogens exhibit + 1, + 3, + 5 and + 7 oxidation states also. 4. Although electron gain enthalpy of fluorine is less negative as compared to chlorine, fluorine is a stronger oxidising agent than chlorine. Why? Ans: - It is due to (i) low enthalpy of dissociation of F-F bond (ii) high hydration enthalpy of F - 5. Halogens have maximum negative electron gain enthalpy in the respective periods of the periodic table. Why? Ans: - Halogens have the smallest size in their respective periods and therefore high effective nuclear charge. As a consequence, they readily accept one electron to acquire noble gas electronic configuration.