Group 16

Question 1

Group 16 elements

Answer 1

Oxygen - Sulphur - Selenium - Tellurium - Polonium

Question 2

What are the group 16 elements also called? Why?

Answer 2

Chalcogens
Chalcogen in Greek means brass (whose main constituent is copper). It is used to show that most copper minerals contain oxygen or Sulphur

Question 3

Occurrence of oxygen

Answer 3

1. Oxygen is the most abundant element on Earth
2. It forms 46.6% of the Earth’s crust
3. It also constitutes 20.5% of dry air

Question 4

Occurrence of sulphur

Answer 4

1. Combined sulphur occurs mainly as sulphate
2. Or even as sulphides - galena, zinc blende
3. It is a part of organic materials like hair, wool, proteins, eggs
4. It is found in traces as H2S in volcanoes

Question 5

Gypsum

Answer 5

CaSO4.2H2O

Question 6

Epsom salt

Answer 6

MgSO4.7H2O

Question 7

Baryte

Answer 7

BaSO4

Question 8

Copper pyrites

Answer 8

CuFeS2

Question 9

Occurrence of Se, Te and Po

Answer 9

1. Se and Te occurs as selenides and tellurides in sulphide ores
2. Po is a decay product of uranium and thorium

Question 10

Electronic configuration

Answer 10

ns2np4

Question 11

Discuss atomic radii trend

Answer 11

1. Atomic size increases as we go down the group due to subsequent increase increase in the number of shells
2. Oxygen atom is exceptionally small though

Question 12

Discuss ionization enthalpy trend

Answer 12

1. Decreases down the group due to increase in size

2. Should be more than Group 15 but is actually less because group 15 has stable half filled configuration

Question 13

Discuss electron gain enthalpy

Answer 13

1. Oxygen has less negative value because of its compact nature but from sulphur to the end of the group, value keeps decreasing
   O > S < Se < Te < Po

Question 14

Discuss electronegativity trend

Answer 14

Oxygen has highest electronegativity after fluorine

The electronegativity decreases down the group with increasing size i.e., metallic character increases down the group

Question 15

Is Po radioactive? What is its half life

Answer 15

Po is a radioactive short lived element.

It has a half life of 13.8 days

Question 16

Physical properties of group 15 elements

Answer 16

1. They all show allotropy
2. Their MP, BP increases with increasing size
3. O, S are non-metals
4. Se, Te are metalloids
5. Po is a non-metal

Question 17

Why is there a large difference in BP/MP of oxygen and sulphur

Answer 17

Because of their atomicities

Oxygen exists as a diatomic molecule while sulphur exists as a polyatomic molecule (S8)

Question 18

Discuss oxidation state variation

Answer 18

1. O tends to show -2 because of its high electronegativity except with OF2 where OS is +2
2. Stability of -2 decreases down the group.
3. +4 and +6 is more common in the other elements
4. S, Se and Te tend to show +4 with oxygen and +6 with fluorine
5. Stability of +4 increases down the group while +6 decreases due to inert pair effect
6. Bonding in +4, +6 are covalent

Question 19

Anomalous behavior of oxygen

Answer 19

1. Because of its small size and high electronegativity, it forms strong hydrogen bonds in H2O which is not seen in H2S
2. Has a maximum covalence of four due to absence of d orbitals but in practice, covalence rarely exceeds two

Question 20

Discuss hydrides formed

Answer 20

1. Of the form H2E
2. Their acidic character increases down the group due to decrease in BDE of H-E bond
3. Thermal stability also decreases down the group
4. Except H2O, they are all good reducing agents and this reducing character increases down the group

Question 21

Discuss properties of EO2

Answer 21

1. Their reducing character decreases down the group
2. SO2 is a reducing agent while TeO2 is an oxidizing agent
3. O3 and SO2 are gases while SeO2 is a solid
4. They are all acidic in nature

Question 22

Discuss properties of EO3

Answer 22

1. Not formed by oxygen

2. SO3, SeO3 and TeO3 are all acidic in nature

Question 23

What kind of halides are formed? How does stability vary

Answer 23

1. EX2, EX4 and EX6 halides are formed
2. The stability varies as:
   F- > Cl- > Br- > I-

Question 24

Discuss properties of EX6

Answer 24

1. Only hexafluorides are stable
2. They are octahedral
3. They are gaseous in nature
4. SF6 is particularly stable for steric reasons

Question 25

Discuss properties of Ex4

Answer 25

1. SF4 is a gas, SeF4 is a liquid and TeF4 is a solid 2. They are all trigonal bipyramidal with one lone pair i.e., seesaw shape 3. sp3d hybridization

Question 26

Discuss properties of EX2

Answer 26

1. Except with oxygen, dichlorides and dibromides are formed | 2. sp3 hybridization and tetrahedral shape

Question 27

Discuss properties of EX

Answer 27

1. Monohalides dimerize 2. Examples - S2Cl2, S2F2 3. They disproportionate to give EX4 + E

Question 28

How is O2 prepared

Answer 28

1. By heating oxygen containing salts like chlorates, permanganates, etc. 2KClO3 = 2KCL + 3O2 2. By thermal decomposition of oxides of metals low in the electrochemical series and higher oxides of some metals 2PbO2 = 2PbO + O2 3. Hydrogen Peroxide readily decomposes to give water and O2 2H2O2 = 2H2O + O2 4. On a large scale, it is prepared by electrolysis of water - hydrogen at the cathode and oxygen at the anode 5. Industrially, CO2 and water is removed from air. This air is then liquified and fractionally distilled to give N2 and O2

Question 29

Physical properties of dioxygen

Answer 29

1. Colorless, odorless gas 2. Soluble in water - solubility is 3.08 cm3 which is just sufficient for marine life 3. It liquifies at 90 K and freezes at 55 K 4. Paramagnetic despite having even number of electrons

Question 30

Discuss isotopes of dioxygen

Answer 30

O(16), O(17) and O(18) | O(18) is radioactive

Question 31

Chemical properties of dioxygen

Answer 31

1. It reacts with all metals and non-metals except the noble gases and inert metals like Au, Ag, Pt. These combination reactions are highly exothermic which helps sustain the reaction but a good amount of energy has to also be supplied because BDE of O=O is quite high 4Al + 3O2 = 2Al2O3 2. Some compounds are also catalytically oxidized 2SO2 + O2 (presence of V2O5) = 2SO3

Question 32

Uses of dioxygen

Answer 32

1. Respiration 2. Oxyacetylene welding 3. Thermite process 4. Extraction of metals 5. Rocket propellant 6. Combustion

Question 33

How are oxides classified

Answer 33

1. Simple (MgO, Al2O3) and mixed (Pb3O4, Fe3O4) Mixed oxides decompose to give one or more simple oxides Fe3O4 = FeO + Fe2O3

Question 34

How are simple oxides classified

Answer 34

1. Based on their acidic, basic or amphoteric nature

Question 35

Comment on acidic oxides

Answer 35

1. Oxides which on dissolving in water produce an acid are called acidic oxides CO2 + H2O = H2CO3 (carbonic acid) 2. All non-metal oxides are acidic 3. Some metal oxides where the metal has high OS are also basic (Mn2O7, CrO3, V2O5)

Question 36

Comment on basic oxides

Answer 36

1. Oxides which on dissolving in water produce a base are called basic oxides CaO + H2O = Ca(OH)2 2. All metal oxides are basic

Question 37

Comment on amphoteric oxides

Answer 37

Oxides that show acidic and basic character are called amphoteric oxides Al2O3

Question 38

Comment on neutral oxides

Answer 38

Oxides that are neither acids or bases are called neutral | CO, NO, N2O