P-Block Groups 15-18

Question 1

Occurence of Halogens

Answer 1

• fluorspar CaF2
• cryolite Na3AlF6
• fluoroapatite 3Ca3(PO4)2.CaF2)
• carnallite KCl.MgCl2.6H2O
• Chile salt petre (NaNO3+NaIO3)

Question 2

Atomic Radii of Group 17

Answer 2

Increases down the group

Question 3

Ionization Enthalpy of G17

Answer 3

Decreases down the group

Question 4

Electron gain enthalpy of G17

Answer 4

Cl>F>Br>I

Question 5

Electronegativity

Answer 5

Decreases down the group

Question 6

Melting point/ Boiling point/ Density of G17

Answer 6

Increases down the group

Question 7

Bond-Dissociation Enthalpy of G17

Answer 7

Cl2>Br2>F2>I2
(due to repulsion of electron pairs in F2)

Question 8

Physical states of halogens

Answer 8

F2, Cl2-gas
Br2-Liqiud
I2-Solid

Question 9

Color of halogens

Answer 9

• F-yellow
• Cl-Greenish yellow
• Br-Red
• I-Violet

Question 10

Solubility of halogens

Answer 10

F and Cl are soluble in water but Br and I are soluble in organic solvents

Question 11

Oxidation state of F

Answer 11

-1

Question 12

Reactivity of halogens

Answer 12

Decreases down the group

Question 13

Oxidising property of halogens

Answer 13

F>Cl>Br>I
A higher halogen can oxidise a lower halogen
F2 + 2X– –> 2F– + X2 (X = Cl, Br or I)
Cl2 + 2X– —> 2Cl– + X2 (X = Br or I)
Br2 + 2I– —> 2Br– + I2

Question 14

Reaction of halogens with H2O

Answer 14

F: F oxidises H2O to O2
F2 +H2O—> F- +O2+H+

Br, Cl: react with water to form corresponding hydrohalic and hypohalous acids.Undergo disproportionation.

I: reaction is not spontaneous. On the other hand
I-+ H+ +O2 —> I2 +H2O (Opposite of reaction with F)

Question 15

Reaction of halogens with H2

Answer 15

Affinity for hydrogen decreases from fluorine. Hydrogen halides dissolve in water to form hydrohalic acids.

Acidic strength, thermal stability: HF< HCl < HBr < HI
Boiling point: HF>HI>HBr>HCl
Melting point: HI>HF>HBr>HCl

Question 16

Reaction of halogens with O2

Answer 16

F: Forms only two unstable oxygen fluorides; O2F2 and OF2(more stable)

Stability of oxides formed by halogens, I > Cl > Br.
Bromine oxides are the most unstable after fluorine and are good oxidising agents.

Question 17

What is used to remove Pu from used nuclear fuel?

Answer 17

O2F2

It converts Pu to PuF6

Question 18

Uses of ClO2

Answer 18

ClO2 is used as a bleaching O2 agent for paper pulp and textiles and in water treatment.

Question 19

What is used in the estimation of CO?

Answer 19

I2O5

Question 20

Preparation of Cl2

Answer 20

MnO2+HCl —-> Cl2+ MnCl2 +H2O
MnO2 +H2SO4+NaCl—> Cl2 +MnCl2+NaHSO4 +H2O [H2SO4 +NaCl is same as HCl]
KMnO4 +HCl —> Cl2 +MnCl2 +KCl +H2O [KMnO4 oxidises Cl- to Cl2]

Electrolysis of NaCl [Industrial method]

Question 21

Reaction of Cl2 with compounds
containing hydrogen

Answer 21

H2 + Cl2 —-> HCl
H2S + Cl2 —-> HCl+ S
C10H16 + 8Cl—-> 16HCl +10C

Question 22

Deacon’s Process

Answer 22

HCl +O2 —–> Cl2 +H2O
Catalyst: CuCl2
Industrial process for manufacture of Cl2

Question 23

Reaction of Cl2 with alkalis

Answer 23

Cold and dilute:
mixture of chloride and hypochlorite
NaOH + Cl2 —> NaCl + NaOCl + H2O

Hot and concentrated:
chloride and chlorate
NaOH + Cl2 —> NaCl + NaClO3 + H2O

Question 24

Bleaching Powder

Answer 24

Cl2 reacts with dry slaked lime to give bleaching powder.
Ca(OH)2 + Cl2 —> Ca(OCl)2 + CaCl2 +H2O
The composition of bleaching powder is Ca(OCl)2.CaCl2.Ca(OH)2.2H2O.

Question 25

Chlorine water

Answer 25

Chlorine water on standing loses its yellow colour due to the formation of HCl and HOCl. Hypochlorous acid (HOCl) so formed, gives nascent oxygen which is responsible for oxidising and bleaching properties of chlorine.Bleaching effect of chlorine is permanent. Cl2 + H2O ---> 2HCl + O Coloured substance + O ---> Colourless substance

Question 26

Cl2 + H2O oxidises I2 to

Answer 26

IO3-

Question 27

Uses of Cl

Answer 27

* for bleaching woodpulp * in the extraction of gold and platinum * the manufacture of dyes, drugs and organic compounds such as CCl4, CHCl3, DDT, refrigerants * in sterilising drinking water * preparation of poisonous gases such as phosgene (COCl2), tear gas (CCl3NO2), mustard gas (ClCH2CH2SCH2CH2Cl).

Question 28

Poisonous gases from Cl2

Answer 28

* phosgene (COCl2) * tear gas (CCl3NO2) * mustard gas (ClCH2CH2)2S

Question 29

Preparation of HCl

Answer 29

heating sodium chloride with concentrated sulphuric acid. NaCl +H2SO4 ----> HCl +NaHSO4

Question 30

Reaction of HCl with NH3

Answer 30

When NH3 is taken in excess: HCl +NH3 ----> NH4Cl When HCl is taken in excess: HCl +NH3 ----> NCl3

Question 31

Aqua Regia

Answer 31

When three parts of concentrated HCl and one part of concentrated HNO3 are mixed, aqua regia is formed. HNO3 +HCl ---> NOCl +H2O +Cl (nascent chlorine) Au and Pt are soluble in aqua regia because of the formation of soluble complexes [Au(Cl)4] and [Pt(Cl)6]. NO gas is released in this process

Question 32

Oxyacids of Halogen

Answer 32

Hypohalous acid: O. S.=+1 Halous acid: O.S.=+3 Halic acid: O.S.=+5 Perhalic acid: O.S.=+7

Question 33

Oxyacid of F

Answer 33

Hypofluorous acid **HOF**

Question 34

Oxyacid of Cl

Answer 34

Hypochlorous acid HOCl Chlorous acid HClO2 Chloric acid HClO3 Perchloric acid HClO4

Question 35

Oxyacids of Br, I

Answer 35

Hypobromous / Hypoiodous acid : HOBr /HOI Bromic /Iodic acid : HBrO3 /HIO3 Perbhromic / Periodic acid :HBrO4 /HIO4

Question 36

Interhalogen Compounds

Answer 36

The interhalogen compounds can be prepared by the direct combination or by the action of halogen on lower interhalogen compounds. These are all covalent molecules and are diamagnetic in nature. They are volatile solids or liquids at 298 K except ClF which is a gas. Interhalogen compounds are more reactive than halogens (except fluorine). This is because X–X' bond in interhalogens is weaker than X–X bond in halogens ClF3 and BrF3 are used for the production of UF6 in the enrichment of 235U. U + ClF3 ----> UF6+ ClF

Question 37

Occurence of Group 18 elements

Answer 37

* Their atmospheric abundance in dry air is ~ 1% by volume of which argon is the major constituent. * Helium and sometimes neon are found in minerals of radioactive origin e.g., **pitchblende, monazite, cleveite.** * The main commercial source of helium is natural gas * Radon is obtained as a decay product of 226Ra (Radium)

Question 38

Electron Gain Enthalpy of G18

Answer 38

They have large positive values of electron gain enthalpy. Ne>Ar=Kr>XE>Rn>He

Question 39

Compounds of G18

Answer 39

Xe+PtF6 – by mixing PtF6 and xenon; red in color; similar to O2+PtF6 KrF2 RnF2

Question 40

Xe-F compounds

Answer 40

XeF2, XeF4 and XeF6 are colourless crystalline solids and. They are powerful fluorinating agents. Xenon fluorides (XeF6) react with fluoride ion acceptors to form cationic species and fluoride ion donors (XeF2, XeF4) to form fluoroanions. When Xe is in excess: Xe + F2 ---> XeF2 1:5 ratio Xe + F2---> XeF4 1:20 ratio Xe + F2---> XeF6 They are readily hydrolysed even by traces of water. XeF2+ H2O --->Xe + HF + O2

Question 41

Xe-O Compounds

Answer 41

Hydrolysis of XeF4 and XeF6 with water gives Xe03. XeF4 +H2O--->Xe + Xe03 +HF +O2 XeF6 +H2O--->XeO3 + HF Partial hydrolysis of XeF6 gives oxyfluorides, XeOF4 and XeO2F2. XeF6 + H2O--->XeOF4 + HF XeF6 + H2O---> XeO2F2 + HF **XeO3 is a colourless explosive solid XeOF4 is a colourless volatile liquid**

Question 42

Uses of G18 Elements

Answer 42

**He:** * used in filling balloons for meteorological observations. * It is also used in gas-cooled nuclear reactors; finds use as cryogenic agent * It is used to produce and sustain powerful superconducting magnets which form an essential part of modern NMR spectrometers and Magnetic Resonance Imaging (MRI) systems for clinical diagnosis. * It is used as a diluent for oxygen in modern diving apparatus because of its very low solubility in blood. **Ne:** * Neon is used in discharge tubes and fluorescent bulbs for advertisement display purposes. * Neon bulbs are used in botanical gardens and in green houses. **Ar:** * Argon is used mainly to provide an inert atmosphere in high temperature metallurgical processes * for filling electric bulbs. * used in the laboratory for handling substances that are air-sensitive.