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.