Electronic configurations

Give proper explanations for the variation of  (i) Electronic configurations

          (ii) Oxidation states in the halogen family.

 Ans:- (i) Electronic configurations:

These elements belong to p-block elements since the differentiating electron enters the p-orbital of outer most orbit. The elements of this group have seven electrons in the outermost shell and have the general electronic configuration ns2np5

Element Atomic number Electronic configuration
F 9 [He] 2s22ps
CI 17 [Ne] 3s23p5
Br 35 [Ar] 3d104s24p5
I 53 [Kr] 4d105s25P5
At 85 [Xe] 5d104f146s26p5

 

(ii) Oxidation states in the halogen family:

Fluorine exhibits only the oxidation state of -1 in all its compounds. The other halogens ( CI, Br, I) exhibit – 1, + 1, + 3, + 5 and + 7 oxidation states in their compounds. Fluorine always exhibit -1 oxidation state in all its compounds since it is the most electronegative element. Fluorine cannot exhibit higher oxidation states due to the absence of empty d-orbitals in the valence shell.Chlorine, bromine and iodine exhibit higher oxidation states (+ 3, + 5 and + 7) in different excited states due to the presence of empty d-orbitals in their valence shell as shown below.

State of halogen atom Electronic configuration Oxidation states
Ground state ns2np5nd° -1 or +1
First excited state ns2np4nd1 +3
Second excited state ns2np3nd2 +5
Third excited state ns1p3d3 +7

 

Explain the gradation of the following properties in the halogens.

(i) Physical states            (ii) IP values               (iii) Electron affinity

Ans :

(i) Physical state:

Fluorine is yellow coloured gas, chlorine is a greenish yellow gas, bromine is reddish-brown liquid and iodine is a dark violet solid.The change in the physical state is due to the increase in the vander waal’s forces of attraction between the molecules, which increases with the atomic size.

(ii) IP Values:

The ionisation potential values of halogens are very high. This indicates that they have very little tendency to lose electrons. However, down the group, the ionisation potential values decreases. This is due to gradual increase in atomic size.

(iii) Electron affinity:

              Halogens have maximum electron affinities in their respective periods. This is due to the fact that the atoms of these elements have only one electron less than the stable noble gas configuration. Therefore, they have maximum tendency to accept an electron. Ingeneral, electron affinity decreases from top to bottom in a group due to increase in atomic size.

 CI>F>Br>I

Electron affinity among halogens varies as

Fluorine has unexpectedly low electron affinity than chlorine. This is due to very small size of the fluorine atom. As a result, there are strong inter electronic repulsions in the relatively small 2p subshell of fluorine and thus, the incoming electron does not feel much attraction. Therefore, its electron affinity is less.

Write an essay on the chemical reactivity of halogens.

Ans:  Halogens are highly reactive elements.

The order of reactivity can be seen in their reactions with water or alkalies or hydrogen or metals act as:

Fluorine > Chlorine> Bromine > Iodine

Reaction with water:

Their solubility in water decreases from fluorine to iodine.

Fluorine decomposes water very vigorously and forms oxygen and ozone.

3F2+3H2O            6HF+O3

2F2+2H2O           4HF+O2

Chlorine dissolves in water and gives chlorine water.

Cl2 + H2O        HCl + HOCI

Bromine is not freely soluble in water. It dissolves in water and gives HBr and HOBr.

Br2+H2O         HBr + HOBr

I2 , neither react with water at ordinary temperature nor dissolves in water.

Reaction with hydrogen:

Halogens react with hydrogen forming hydrogen halides(HX).

H2 + X2            2HX (X – Halogen )

H2 + F2            2HF(takes place even in the dark)(highly energetic)

H2 + Cl2   2HCl

H2 + Br2   2HBr

H2 + I2    2HI

Reaction with metals:

Metals react with halogens forming metal halides( MXn ).

2M + nX2         2MXn

Fluorine shows the highest activity by reacting even with noble metals such as Au, Pt, etc.

Reaction with alkalies:

Fluorine reacts with dilute NaOH and gives oxygen difluoride( OF2)

2F2 + 2NaOH (dilute)        2NaF + H2O + OF2

 Fluorine reacts with concentrated NaOH and gives oxygen

2F2 +4NaOH (conc)          4NaF + 2H2O + O2(  )

Other halogens (X2 = Cl2, Br2,I2) react with alkalies and give their hypohalites(NaOX) or halates (NaXO3) in addition to the halide.

  1. Give an account of the oxidising capacities of halogens.

            Ans : Halogens are good oxidising agents. Their oxidising capacity decreases from fluorine to iodine. Fluorine is the strongest oxidising agent eventhough it has low electron affinity than chlorine because of the following two reasons.

  1. i) Low bond dissociation energy in F2
  2. ii) High heat of hydration due to small size of Fion

The decreasing trend of oxidising capacity from fluorine to iodine is explained on the basis of

  1. a) Net enthalpy change in the reaction, X2 (solid) X(aq)

The magnitude of the enthalpy change in the reaction when halogen changes to a hydrated ion can beestimated by the application of following Born-Habertype of energy cycle.

Greater the magnitude of negative AH , greater is the oxidising capacity of the halogen.

  1. B) Standard Electrode Potentials:

The standard electrode potentials for the different halogens are given below.

F: +2.87V;CI: f 1.36V; Br: +1.065V;I j +0.536V

As the standard electrode potentials decreases from fluorine to iodine, oxidising capacity decreases.The relative oxidising capacities of halogens are illustrated by their reaction conditions with ionic halides.Fluorine displaces all other halogens from their ionic compounds.

Chlorine oxidises bromides and iodides to Br2 and I2 respectively.

Bromine oxidises iodides to I2.

Iodine cannot oxidise any ionic halide.

  1. Describe Whytlaw-Gray method for the preparation of fluorine

            Ans : Fluorine is prepared by the electrolysis of fused potassium hydrogen fluoride( KHF2).

The electrode reactions are.

  • Electrolysis takes place in a copper coil which acts as cathode.
  • Graphite rod dipped in electrolyte acts as anode.
  • Anode is surrounded by a copper diaphragm perforated at the bottom.
  • Diaphragm prevents the mixing of H2 and F7.
  • H2 gas liberates at cathode and F2 gas liberates at anode.
  • The impurity HF present in flourine gas is removed by passing F? gas through U-tube             containing NaF. NaF + HF        NaHF2
  • The gaskets used in the cell are coated with teflon to prevent corrosion of the parts.
  1. Write the chemical properties of F, with relevant equations.

           Ans :  i) Reaction with water.

F2 reacts with water and gives ozonized oxygen.

2F2 +2H2O            4HF+O2 : 3F2 +3H2O           6HF+O3

  1. ii) Reaction with alkalies:
  2. When F2 reacts with cold, dilute NaOH gives sodium fluoride and oxygen difluoride (OF2)

2NaOH + 2F2           2NaF + OF2 +H2O

  1. b) When F, reacts with hot, concentrated NaOH gives sodium fluoride and oxygen. 4NaOH + 2F2 4NaF+O2 +2H2O

iii) Reaction with other halides:

Fluorine oxidises all other halide ions to the corresponding halogens.

F2 + 2KCl          2KF+Cl2

  1. iv) Reaction with inert gases:

Heavier inert gases like Kr and Xe form compounds with fluo –ne

Xe + 3F2                XeF6

  1. v) Reaction with  KHSO4  (Potassium hydrogen sulphate)

Fluorine oxidises potassium hydrogen sulphate to potassium Dersulphate( K2S2O8)

F2 +2KHS04          K2S2O8 + 2HF

  1. vi) Reaction with H2.S

Fluorine oxidises H2.S to SF6

H2S + 4F2          2HF + SF6

vii) Reaction with non-metals

Except oxygen and nitrogen, other non-metals directly combine with fluorine and give binary compounds.

S + 3F2           SF6

viii) Reaction with metals:

   All the metals(including noble metals like Au, Pt, etc) form metal fluorides.

Cu + F2           CuF2

07        What is the principle of preparing CI. in the laboratory? Describe Nelson’s method for its manufacture.

Ans  :  

  • Perforated ‘U’ shaped steel tube is used as cathode
  • ‘U’ tube is lined inside with asbestos
  • Brine solution which acts as electrolyte is taken inside the ‘U” tube
  • a carbon rod is dipped in the Brine solution which acts as Anode.
  • The ‘IT shaped tube is suspended in outer tank.
  • Out lets for Cl2 and H2 gases are provided in the ‘IT tube

Reaction:

When electricity is passed the following reactions takes place

Ionization of NaCI                                                   : 2NuCt(aq)           2Na+(aq) +2Cl(aq)

Formation of chlorine at anode                                : 2Cl(aq)          Cl2+2e

Reduction at Cathode                                               : 2H2O+2e          2OH(aq) +H2(aq)

Formation of NaOH                                                 : 2Na(aq) + 2OH(aq)         2NaOH(aq)

  1. i) Ionization of Na+
  2. ii) CI ions reaches carbon anode undergoes oxidation to give chlorine gas.

iii)    Reduction of water at cathode releases H2 gas and OH ions

  1. iv) Na+ and OH inos combine to give NaOH
  2. v) Steam is always sent from the bottom of tank to keep the solution hot and perforations opened.

1) Perforated steel U – tube & cathode                     2) Asbestos lining           3) Brine solution

4) NaOH solution collected                                     5) Cl2 outlet                     6) H, outlet

7) Steam inlet                                                           8) Brine inlet                   9) Graphite a

  1. Give the reactions of CI, with following:

(i) Alkalies                       (ii) Metals                     (iii} Ethanol

Ans  : (i) Reaction with alkalies:

  1. a) When chlorine react with cold, dilute NaOH gives sodium chloride and sodium hypochlorite(NaOCI).

CI2 + 2NaOH         NaCl + NaOCI + H20

  1. b) When chlorine reacts with hot, concentrated NaOH gives sodium chloride and sodium chlorate (NaCIO3).

3C12 + 6NaOH         5NaGl + NaCIO3 + 3H3O

 (ii) Reaction with metals:

Active metals combine with chlorine to give respective metal chlorides.

Ex: 2Na + CI2           2NaCl

If a metal form more than one chloride, that chloride of the metal in which the metal is in its highest, stable oxidation state, is produced by direct union of the elements.

Ex: 2Fe+3Cl2         2FeCI3

(iii) Reaction with ethanol:

Chlorine oxidises ethanol to acetaldehyde

CH3CH2OH + Cl2         CH3 CHO + 2HCl

Chlorine converts acetaldehyde toTrichloro acetaldehyde

CH 3CHO+3Cl2           Cl2 CCHO+3HCI

  1. Giving equations, write any four chemical porperties of CI2

Ans  i) Reaction with NaOH

  1. a) Chlorine reacts with cold, dilute NaOH and gives sodium chloride and sodium                        hypochlorite(NaOCI).

Cl2 + 2NaOH        NaCl + NaOCI + H2O

  1. b) Chlorine reacts with hot, concentrated NaOH and gives sodium chloride and

sodium chlorate (NaCIO,)

3CI2 + 6NaOH           5NaCI + NaCIO3 + 3H2O

  1. ii) Reaction with ammonia
  2. a) With excess of Cl2nitrogen trichloride (NC13) and HCI are formed.

NH3 + 3CI2 (excess)        NCI3 +3HC1

  1. b) With excess of NH3, ammonium chloride and nitrogen are formed.

8NH3(excess) + 3CI2          6NH4CI + N2

iii) Reaction with metals:

Active metals combine with chlorine to give respective metal chlorides.

Ex: 2Na + Cl2        2NaCI

If a metal form more than one chloride, that chloride of the metal in which the metal is in its highest, stable oxidation state, is produced by direct union of the elements.

Ex: 2Fe+3Cl2          2FeCl3

  1. iv) Reaction with water:

Chlorine reacts with water forming HCI and HCIO. HCIO is not so stable and decomposes giving nascent oxgyen which is responsible for oxidising and bleaching properties of chlorine.

Cl+H2O HCI + HCIO (Hypochlorous acid)

HCIO           HCI+ (O)

Coloured matter + (O)             Colourless matter

  1. 1 Write the structures of all the oxyacids of CI2 .

Ans :

Name Formula Oxidation state of chlorine Basicity
Hypochlorous acid HCIO +1 1
Chlorous acid HCIO2 +3 1
Chloric acid HCIO3 +5 1
Perchloric acid HCI04 +7 1

 

Structures of oxyacids of chlorine:

1 Hypochlorous acid : HCIO: The chlorine atom in this acid undergoes sp   hybridisation. The conjugate base of

Hypochlorous acid is Hypochlorite ion (C10)

2- Chlorous acid: HCIO2:

The chlorine atom in this acid undergoes sp3 hybridisation. The chlorine atom has 2 bonds, one bond ( p – d  )and two lone pairs.

The conjugate base of chlorous acid is chlorite ion ( CIO3 ) which is angular in shape with a bond angle of 1110

  1. Chloric acid: HCIO3

The chlorine atom in this acid undergoes sp3   hybridisation. The chlorine atom has 3 (bonds, 2bonds (both are p-d) and one lone pair.

The conjugate base of chloric acid is chlorate ion ( C107) which is pyramidal in shape with a bond angle of 106 .

  1. Perchloric acid: HCIO4

The chlorine atom in this acid undergoes sp3 hybridisation. The chlorine atom has 4 bonds, three   bonds ( all d  – p) and no lone pairs.

The conjugate base of perchloric acid is perchlorate ion ( CI04) which is tetrahedral in shape with a bond angle of 109.50

  1. The acid strength of different oxyacids of chlorine increases with an increase in the oxidation state of the chlorine.

HOCI < HCIO2 < HCIO3 < HCIO4

——-acid strength increase———à

  1. What do you know about bleaching powder ? Write a note on it.

Ans ; a)  Bleaching powder is also known as “chloride of lime”. It is the mixed salt of CaCl2 and Ca(OCI)2.

  1. b) Its chemical name is calcium chlorohypochlorite.
  2. c) The structure of bleaching powder is
  3. d) In bleaching powder the oxidation states of chlorine are -1 and +1
  4. e) Bleaching powder is prepared by the action of chlorine on dry, slaked lime.

  1. f) It is manufactured by Hassenclever plant and Bachmann’s plant.
  1. g) Bleaching powder is a yellowish white powder with a strong smell of chlorine. Most of it dissolves in water leaving small amount of insoluble part(lime).
  2. h) When bleaching powder is treated with excess of a dilute sulphuric acid or CO2, Cl2 is evolved, called available chlorine

The quality of a sample of bleaching powder is determined by the amount of available chlonne given by the sample. Agood sample of bleaching powder contains 35-38% of available chlorine. Bleaching powder is used as an oxidisingagent. It is used as disinfectant and germicide. It is used in the sterilization of water. It is used as a bleaching agent for cotton, wood and paper pulr

  1. Write all the chemical properties of bleachingpowder.Given equations. How is it useful to man.

   Asn  :   1.  On long standing, bleaching powder undergoes auto oxiadation and changes into chloride and chlorate.

6CaOCl2          5CaCl2 +Ca(ClO3)2

  1. Reaction with water

With cold water, bleaching powdlPpvfs%iloride and hypochlorite ion.

CaOCI2         Ca2+ + CI­ + CIO

In hot water, it undergoes auto oxidation and gives chloride and chlorate ions.

  1. Reaction with insufficient amount of dilute acids:

When small amounts of dilute acid is added, bleaching ppvwter liberates oxygen.

2CaOCl2 + H2S04         CaCl2 + CaS04 + 2HC1 + 02

  1. Reaction with excess of dilute acids:

On treating bleaching powder with excess of dilute acid, chlorine is liberated. This liberated chlorine is known as “available chlorine”.

CaOCl2 + H2S04           CaS04 + H20 + Cl2

  1. Effect of a catalyst:

Bleaching powder decomposes to give 02 in the presence of a catalyst Cobalt chloride

  1. Oxidising property:

Bleaching powder oxidises lead salts to lead dioxide and ethanol to acetaldehyde.

Uses:

It is used in tr#sterilization of water, as bleaching agent, as an oxidising agent and in the preparation of chloroform

  1. How is bleaching powder prepared industrially ? Give any of its four chemical properties with equations. Industrial preparation of bleaching powdej|Bachmann’s plant):

Ans :  1)  Bachmann’s plant consists of a vertical iron tower provided with a hopper at the top      and inlets for chlorine and hot air slightly above the base.

2)  The tower is. fitted with a number of horizontal shelves at regular heights. Each shelf   is fitted with a rotating rake.

3)   Dry slaked lime is introduced into the tower through the hopper at the top.

4)   The slaked lime moves downwards with the help of the rotating rakes and comes i n   contact with the current of chlorine rising upwards.

5)    Slaked lime reacts with chlorine and gets converted into bleaching powder which                   is collected in the container placed at the bottom.

6)   The hot air drives away unreacted chlorine   Ca(OH)2 + Cl2 —> CaOCl2 + H20

1) Hot air inlet       2) Cl2 Inlet        3) Hopper       4) Outlet for gases   5) Horizontal shelves

6) Rotating rakes   7) Receiver    8) Bleaching powder Chemical properties of bleaching powder

  1. Reaction with water:

With cold water, bleaching powder gives chloride and hypochlorite ion.

  1. In hot water, it undergoes auto oxidation and gives chloride and chlorate ions.

Reaction with insufficient amount of dilute acids:

When small amounts of dilute acid is added, bleaching powder liberates oxygen.

  1. Reaction with excess of dilute acids:

On treating bleaching powder with excess of dilute acid, chlorine is liberated. This            liberated chlorine is known as”available chlorine”

  1. Effect of a catalyst:

Bleaching powder decomposes to give O2, in the presence of a catalyst COCl2

14 What are interhalogen compounds ? Do ail these Interhalogen compounds have halogens in the same hybridized state ? Write the structures of all the interhalogen compounds.

Ans : Halogens mutually combine together to form a set of compounds of general formula AXnwhere both A and X are halogen atoms and n = 1, 3, 5 or 7. These compounds are called interhalogen compounds.

In all interhalogen compounds of the type AXn halogens are in the different hybridized state.

  • Interhalogen compounds of the type AX are consists of the two halogen atoms linked together through a single covalent bond.

Ex: Ci – F

  • Interhaiogen compounds of the type AX, possess a T-shaped structure and are formed by sp3d hybridisation of the gentral atom A in its first excited state (ns2np4nd’) .

Ex: ClF3      

  • Interhaiogen compounds of the type AX, have square pyramidal structure and are formed by sp’d hybridisation of the central atom A in its second excited state (ns2np3nd2)

Ex: BrF5  

  • The only known interhaiogen compound of the type AX7, i.e., IF7 is formed by sp3d3 hybridisation of the central iodine atom in its third excited state (ns,np3nd3) The molecule has a pentagonal bipyramidal structure.

SHORT ANSWER QUESTIONS

15     Explain the gradation in the properties of halogens with reference to

(i) Electronegativity         (ii) Oxidation states     (iii) Electron affinity

Ans :  (i) Electronegativity:

Electronegativity of halogens decreases down the group due to increase in size.

Fluorine has the highest electronegativity (4.0) on Pauling scale

(ii) Oxidation states:

All the halogens shows -1 oxidation state.

Fluorine has unexpectedly low electron affinity than chlorine. This is due to very small size of the fluorine atom. As a result, there are strong interelectronic repulsions in the relatively small 2p subshell of fluorine and thus, the incoming electron does not feel much attraction. Therefore, its electron affinity is less.

Chlorine, bromine and iodine exhibit higher oxidation states ( + 3, + 5, + 7 ) in different excited states due to the presence of empty d- orbitals in their valence shell

(iii) Electron affinity:

Electron affinity of halogens decreases down the group due to increase in size.

Explain the oxidation states shown by halogens.

Ans : Fluorine exhibits only the oxidation state of-1 in all its compounds. The other halogens (CI. Br. I) exhibit-1, +1, + 3, + 5 and + 7 oxidation states in their compounds. Fluorine always exhibit -1 oxidation state in all its compounds since it is the most electronegative element. Fluorine cannot exhibit higher oxidation states due to the absence of empty d-orbitals in the valence shell. Chlorine, bromine and iodine exhibit higher oxidation states (+ 3, + 5 and + 7) in different excited states due to the presence of empty d-orbitals in their valence shell as shown below.

State of halogen atom

 

Electronic configuration Oxidation states
Ground state ns2np5nd° -1 or+1
First excited state ns2np4nd1 +3
Second excited state ns2np3nd2 +5
Third excited state ns’np3nd3 +7

What is bond energy? Explain the bond energies of X2

Ans : Bond energy: It is defined as the amount of energy required to break one mole of gaseous diatomic molecule into its atoms.
Bond energies of halogens (X2)

With the increase of size, the extent of the overlap of the atomic orbitals decreases. Hence bond energy values inX2 decreases. This trend is clearly observed from chlorine to iodine.
The order of bond energy values among halogens is

Bond energy value of fluorine is abnormally low. The reasons for this is explained as follows

(i) Mulliken explanation: The posibiiity of multiple bonding involving d- orbitals in the halogens (Cl2, Br2) accountsfor the increased bond energy of these molecules. Such type of multiple bonding is not possible in fluorine due to lack of d-orbitals in the valence shell.
(ii) Coulson’s explanation: The low bond energy value in fluorine is due to appreciable internuclear repulsions and large lone pair – lone pair electronic repulsions in fluorine( due to small sizes of fluorine atoms)

What is the order of electron affinities of halogens? Explain.

Ans : Halogens have maximum electron affinities in their respective periods. This is due to the fact that the atoms of theseelements have only one electron less than the stable noble gas configuration. Therefore, they have maximum tendencyto accept an addition electron.

Ingeneral, electron affinity decreases from top to bottom in a group due to increase in atomic size.

Electron affinity among halogens varies as:

Fluorine has unexpectedly low electron affinity than chlorine. This is due to very small size of the fluorine atom. As a result, there are strong interelectronic repulsions in the relatively small 2p subshell of fluorine and thus, the incoming electron does not feel much attraction. Therefore, its electron affinity is less.