Group 16 Elements: Oxygen Family | Inorganic Chemistry PDF Download

GROUP 16 (OXYGEN FAMILY)

(1) ATOMIC & PHYSICAL PROPERTIES

(2) ABUNDANCE IN THE EARTH CRUST: O > S > Se > Te

(3) PHYSICAL STATE

Oxygen is gas while other are solids at room temperature. Oxygen exists as diatomic molecule where as other elements e.g. sulphur exists as shown into the following crown shape (puckered ring structure S₈)

(4) METALLIC ACID NON-METALLIC CHARACTER

Metallic character increases with increase in atomic number.

(5) OXIDATION STATES

• Oxygen shows-2 oxidation state in most of its compounds (being highly electronegative). Exception, in OF₂ & O₂F₂ oxidation state of oxygen is +2 and +1 
• Sulphur shows –2, +2, +4 and +6 oxidation states. Availability of +4 and + 6 oxidation states are due to the presence of empty d-orbitals. 
• Sulphur shows higher oxidation states with compounds of oxygen and fluorine (h ighly electronegative elements( (SO₂, SO₃, SF₄, SF₆) 
• The tendency to show –2 oxidation state decreases down the group since the electronegativity decreases.

(A) Oxygen (O)

If differs from the remaining element of the VI^th group because of the following properties (A) Small Size (B) High electronegativity (C) Non-availability of d-orbitals.

(i) Preparation

1. 2HgO  2Hg + O₂; 

2Ag₂O  4Ag + O₂

2. 2NaNO₃ 2NaNO₂ + O₂; 

2KClO₃ 2KCl + 3O₂ (laboratory method)

3. 4K₂Cr₂O₇  4K₂CrO₄ + 2Cr₃O₃ + 3O₂

2KMnO₄ K₂ MnO₄ + MnO₂ + O₂

(ii) Physical Properties

Colourless, odourless and tasteless gas. It is paramagnetic and exhibits allotropy. Three isotopes of oxygen are  Oxygen does not burn but is a strong supporter of combustion. 

(iii) Chemical Properties

(a) OXIDES 

(a-i) Acidic oxides.

Oxides of Non-metals and metals in highest oxidation state.

They dissolve in water forming oxyacids. e.g. CO_2, SO₂, N₂O₃, P₄O₆, P₄O₁₀, Cl₂O₇, CrO₃, Mn₂O₇ etc.

Ex.: Cl₂O₇ + H₂O → 2HClO₄; 

SO₃ + H₂O → H₂SO₄

(a-ii) Basic oxides (Oxides of metals) 

They either dissolve in water to form alkalies or combine with acids to form salts and water or combine with acidic oxides to form salts.

Li₂O, Na₂O, BeO, MgO, CaO, SrO, BaO, CuO, FeO etc.

Na₂O + H₂O→2NaOH

(a-iii) Natural Oxides (Oxides of Non metals)

The neither combine with acids nor with the bases to form salts e.g. CO, N₂O, NO, H₂O etc.

(a-iv) Amphoteric oxides (Metal oxides)

These can combine with acid as well as bases e.g. ZnO, Al₂O₃, BeO, Sb₂O₃, Cr₂O₃, PbO, PbO₂, etc.

PbO + 2NaOH→Na₂PbO₂ + H₂O;

PbO + H₂SO₄ →PbSO₄ + H₂O

(a-v) Peroxides (Metal oxides)

They react with dil. Acids and form H₂O₂, e.g. Na2O₂, K₂O₂, BaO₂ etc.

Na₂O₂ + H₂SO₄→Na₂SO₄ + H₂O₂

(a-vi) Dioxides (metal oxides)

Like peroxide they contain excess of oxygen but do not yield H₂O₂ with dil. Acids e.g. PbO, MnO₂ etc. They evolve Cl₂ with conc. HCl and O₂ with conc. H₂SO₄.

MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O

(a-vii) Super Oxides (metal oxides)

They contain O₂^– ion, e.g. KO₂, RbO₂ and CsO₂. These oxides react with water forming H₂O₂ and O₂ 2KO₂ + 2H₂O → 2KOH + H₂O₂ + O₂

(a-viii) Sub Oxides

They contain less oxygen than expected from the normal valency of the elements e.g. C₃O₂, N₂O, Pb₂O, Hg₂O etc.

C₃O₂ → O=C=C=C=O

(b) OZONE (O₃)

(i) Preparation

It is prepared by passing silent electric discharge through pure and dry oxygen

O₂ + O →O₃;

ΔH = 245 kJ mol^-1

(ii) Properties 

(a) Oxidising agent: Ozone is a strong oxidizing agent in acidic medium

O₃ + 2H⁺ + 2e_–→ O₂ + H₂O; SRP = + 2.07 V (acidic medium)

O₃ + H₂O + 2e→O₂ + 2OH; SRP = + 1.24 V (alkaline medium)

O₃→O₂ + [O]

E.g.

1. 2KI + H₂O + [O]→2KOH + O₂ + I₂

2. KI + 3O₃→KIO₃ + 3O₂

3. S + 3O₃ + H₂O→H₂SO₄ + 3O₂

4. 2I₂ + 9[O₃]→I₄O₉ + 9O₂

• I₂O₉ yellow solid has the composition I⁺³ (IO^–3)₃. Formation of this compound is a direct evidence in favour of basic nature of I₂ (i.e. its tendency of form cations)

O₃ + H₂O₂→2O₂ + H₂O

5.Oxidising Redusing

agent  agent

TAILING OF MERCURY

Pure mercury is mobile liquid but when brought in contact with O₃ its mobility decreases and it starts sticking to glass surface forming a type of tail due to the dissolution of Hg₂O (mercury suboxide) in Hg.

• O₃ is used as a germicide and disinfectant for sterilizing water and improving the atmosphere of crowded places.

HYDROGEN PEROXIDE (H₂O₂)

(i) Preparation

(1) BaO₂.8H₂O + H₂SO₄(cold)→BaSO₄ ↓ (white) + H₂O₂ + 8H₂O

2) H₂S₂O₈ + H₂O H₂SO₄ + H₂SO₅  BaSO₄ ↓ (white) + H₂O₂(aq)

Comparison (H₂O & H₂O₂)

(i) Its boiling point 144 °C more than water but freezing point ( –4) less than water. Density a nd dielectric constant are higher than H₂O.

(ii) Its aqueous solution is more stable than the anhydrous liquid where it decomposes into water and O₂.

2H₂O₂→2H₂O + O₂

H₂O₂ is not kept in glass containers because traces of alkali metal ions from the glass can catalyzed the explosive decomposition of H₂O₂. Therefore, aqueous solutions is stored in plastic containers and some urea or phosphoric acid or glycerol is added to that solution because these compounds have been found to behave as negative catalyzed for the decomposition of H₂O₂.

(iii) Acidic nature

Behaves as a weak acid according to the following equation

Aqueous solution of H₂O₂ turns blue litmusred which is then bleached by the oxidizing property of H₂O₂.

Na₂CO₃ + H₂O₃→Na₂O₂ + H₂O + CO₂

Ba(OH)₂ + H₂O₂ + 6H₂O→BaO₂.8H₂O↓

A 30% H₂O₂ solution has pH = 4.0

(iv) Oxidizing Agent

2e + 2H⁺ + H₂O₂→2H₂O; SRP = 1.77 v (in acidic medium)

2e + H₂O₂ →2HO^–; SRP = 0.87 v (In alkaline medium)

On the basis of the above potentials, we can say that H 2O₂ is strong oxidizing a gent in acidic medium but kinetically its is found that reactions are faster in basic medium.

Cr₂O7^2– + 2H⁺ + 4H₂O₂→2CrO₅ + 5H₂O

CrO₅ bright blue coloured compound is soluble in ether. 

Uses

(i) In bleaching of delicate materials such as silk, wool, cotton, ivory etc.

Properties of Hydrides

Table: Some Properties of H₂O, H₂S, H₂Se and H₂Te 

The hydrides decreases in stability from H₂O to H₂Te. They become less stable because the bonding orbitals become larger and more diffuse hence overlap with the hydrogen ‘1s orbital’ is less effective.

The H—O—H bond angle in water is 104°28’, in accordance with the VSEPR prediction of slightly less than tetrahedral due to the presence of lone pairs of electrons. Thus the orbitals used for bonding by O are close to sp³ hybrids. In H₂S to H₂Te the bond angles become close to 90°.this suggests that almost pure p orbitals on Se and Te are used for bonding to hydrogen. In a series of similar compounds, the boiling points usually increase as the atoms become larger and heavier. If the boiling points increase, then the volatility decrease. This trend is shown by the boiling points of H₂S. H₂Se & H₂T_e, but the boiling point of water is anomalous. 

Water has an abnormally low volatility because its molecules are associated with each other by means f hydrogen bonds in both the solid and liquid states.

SULPHUR (S)

Sodium Thiosulphate (Na₂S₂O₃.10H₂O): (Hypo)

Preparation

(i)  Na₂SO₃S  Na₂S₂O₃

Na₂ SO₃+ 2SO₂ (excess)+ H₂O→2NaHSO₃+ CO ;

(ii) 2NaHSO₃+ Na₂CO₃→2Na₂S₂O₃+ H₂O +CO₂

Structure of Oxo acids of S

1. Sulphurous acid series

H2SO3 Sulphurous acid		S(+IV)
H2S2O5 pyrosulphurou s acid		S(+V), S(+III)
H2S2O4, dithionous acid		S(+III)

2. Sulphuric acid series

H2SO4 Sulphuric acid		S(+IV)
H2S2O3 thiosulphuric acid		S(+VI), S(–II)
H2S2O7 di or pyrosulphuric acid		S(+VI)

3. Thionic acid series

H2S2O6 dithionic acid		S(+VI)
H2SnO6 polythionic acid (n = 1 – 12)		S(+V) S(0)

4. Peroxoacid series

H2SO5 peroxomonosulphuric acid		S(+VI)
H2S2O8 peroxodisulphuric acid		S(+VI)