(i) When a solution of brine (NaCl) is electrolysed, Na⁺ and Cl ions are formed.
(ii) Na⁺ ion reacts with OH^- ions of water and forms sodium hydroxide. Hydrogen and chlorine are liberated as gases.
NaCI \(\longrightarrow \) Na⁺ + CI^-1
H₂O \(\longrightarrow \) H⁺ + OH^-
Na⁺ + OH^- \(\longrightarrow \) NaOH
At the cathode,
H⁺ + e^- \(\longrightarrow \) H
H + H\(\longrightarrow \)H₂
At the anode,
Cl^- \(\longrightarrow \) CI + e^-
CI + CI \(\longrightarrow \) Cl₂

Reaction with metals:
(i) Sulphuric acid reacts with metals and gives different product depending on the reactants and reacting condition
(ii) Dilute sulphuric acid reacts with metals like: tin, aluminium, zinc to give corresponding: sulphates.
Zn + H₂SO₄\(\longrightarrow \) ZnSO₄ + H₂ \(\uparrow \)
2AI + 3H₂SO₄ \(\longrightarrow \) Al₂(SO₄)₃+ 3H₂ \(\uparrow \)
(iii) Hot concentrated sulphuric acid reacts with copper and lead to give the respective sulphates as shown below 
Cu + 2H₂SO₄ \(\longrightarrow \) CuSO₄ + 2H₂O + SO₂\(\uparrow \)
Pb + 2H₂SO₄ \(\longrightarrow \) PbSO₄ + 2H₂O + SO₂\(\uparrow \)
(iv) Sulphuric acid doesn't react with noble metals like gold, silver and platinum.

Oxidising property of H₂SO₄: 
Sulphuric acid is an oxidising agent as it produces nascent oxygen as shown below.
\({ H }_{ 2 }{ SO }_{ 4 }\longrightarrow { H }_{ 2 }O+\underset { nascentoxygen }{ { SO }_{ 2 } } +\left( O \right) \)
Sulphuric acid oxidises elements such as carbon, sulphur and phosphorus. It also oxides bromide and iodide to bromine and iodine respectively.
C + 2H₂SO₄ \(\longrightarrow \) 2SO₂ + 2H₂O + CO₂
S + 2H₂SO₄ \(\longrightarrow \) 3SO₂ + 2H₂O
P₄ + 10H₂SO₄ \(\longrightarrow \) 4H₃PO₄ + 10SO₂ + 4H₂O
H₂S + H₂SO₄ \(\longrightarrow \) SO₂ + 2H₂O + S
H₂SO₄ + 2HI \(\longrightarrow \) SO₂ + H₂O + I₂
H₂SO₄ + 2HBr \(\longrightarrow \) 2SO₂ + 2H₂O + Br₂

Manufacture of sulphuric acid by contact process:
The contact process involves the following steps.
(i) Initially sulphur dioxide is produced by burning sulphur or iron pyrites in oxygen/ air.
\(S+{ O }_{ 2 }\longrightarrow { SO }_{ 2 }\)
\({ 4FeS }_{ 2 }+{ 11O }_{ 2 }\longrightarrow { 2Fe }_{ 2 }{ { O }_{ 3 } }+8{ SO }_{ 2 }\)
(ii) Sulphur dioxide formed is oxidised to sulphur trioxide by air in the presence of a catalyst such as V₂O₅ or platinised asbestos.
(iii) The sulphur trioxide is absorbed in concentrated sulphuric acid and produces oleum (H₂S₂O₇). The oleum is converted into sulphuric acid by diluting it with water. 
\(\mathrm{SO}_{3}+\mathrm{H}_{2} \mathrm{SO}_{4} \longrightarrow \mathrm{H}_{2} \mathrm{~S}_{2} \mathrm{O}_{7} \stackrel{\mathrm{H}_{2} \mathrm{O}}{\longrightarrow} 2 \mathrm{H}_{2} \mathrm{SO}_{4}\)
(iv) To maximise the yield the plant is operated at 2 bar pressure and 720 K. The sulphuric acid obtained in this process is over 96 % pure.

Oxidising property : 
Sulphur dioxide, oxidises hydrogen sulphide to sulphur and magnesium to magnesium oxide.
\({ 2H }_{ 2 }S+{ SO }_{ 2 }\longrightarrow 3S+{ 2H }_{ 2 }O\)
\(2Mg+{ SO }_{ 2 }\longrightarrow 2MgO+S\)
Reducing property : 
As it can readily be oxidised, it acts as a reducing agent. It reduces chlorine into hydrochloric acid.
\({ SO }_{ 2 }+2{ H }_{ 2 }O+{ { Cl }_{ 2 }\longrightarrow { H }_{ 2 }{ SO }_{ 4 }+2HCl }\)
It also reduces potassium permanganate and dichromate to Mn²⁺ and Cr³⁺ respectively.
\({ 2KMnO }_{ 4 }+5{ SO }_{ 2 }+2{ H }_{ 2 }O\longrightarrow { K }_{ 2 }{ SO }_{ 4 }+{ 2MnSO }_{ 4 }+2{ H }_{ 2 }{ SO }_{ 4 }\)
\({ K }_{ 2 }{ Cr }_{ 2 }{ O }_{ 7 }+{ 3SO }_{ 2 }+{ H }_{ 2 }{ SO }_{ 4 }\longrightarrow { K }_{ 2 }{ SO }_{ 4 }+{ Cr }_{ 2 }\left( SO_{ 4 } \right) _{ 3 }+{ H }_{ 2 }O\)

(a) NaCl + MnO₂ + 4H₂SO₄ \(\longrightarrow \) MnCl₂+ 4NaHSO₄ + 2H₂O + Cl₂
(b) 6XeF₄ + 12H₂O \(\longrightarrow \) 4Xe + 2XeO₃ + 24HF + 3O₂

(i) Stability of higher oxidation state decreases down the group from S to Te because stability of lower oxidation state increases down the group from S to Te.
(ii) Fluorine and oxygen are most electronegative and very reactive. So they can ionised noble gases and form compounds.

N₂O_S is more acidic than N₂O₃ because N₂O₃ is the anhydride of nitrous acid N₂O₃ dissolves in water to form the unstable acid.
\({ N }_{ 2 }{ O }_{ 3 }+{ H }_{ 2 }O\longrightarrow \underset { unstable }{ { 2HNO }_{ 2 } } \)
On the other hand, N₂O₅ is the anhydride of nitric acid N₂O₅ dissolves in water to form nitric acid
\({ N }_{ 2 }O_{ 5 }+{ H }_{ 2 }O\longrightarrow { { 2HNO }_{ 3 } }\)
N₂O₅ has higher acidic strength than N₂O₃. Acidic nature depends on oxidation number. Higher the oxidation number greater the tendency of gaining electron.
(ii) Thermal stability of hydrides decrease on moving down the group. This is due to the decrease in the bond dissociation enthalpy (H-E) of hydrides on moving down the group where E = O, S, Se, Te.
(iii) Hydration enthalpy is a measure of energy released when attractions are set up between positive or negative ions and water molecules. These attractions are stronger when the ion is smaller. Since F^- ion is smaller than Cl^- ion, F^- ion process higher hydration enthalpy than Cl^- ion.

(I) Since the byproduct of roasting to sulphide ore is SO₂ It turns lime water milky.
Therefore, gas 'B' must be SO₂
(ii) As the gas 'B' is obtained when amorphous solid 'A' burns in air therefore, amorphous solid 'A' must be sulphur S₈
\(\underset { (A) }{ { S }_{ g } } +{ 8O }_{ 2 }\overset { \Delta }{ \longrightarrow } \underset { (B) }{ { 8SO }_{ 2 } } \)
(iii) Gas (B) reduces acidified aqueous KMnO₄ solution and reduces Fe³⁺ to Fe²⁺ salts as shown below:
\(\underset { (yellow) }{ { 2MnO }_{ 4 } } ^{ - }+\underset { (b) }{ { SO }_{ 2 } } +2{ H }_{ 2 }O\longrightarrow { 2Fe }^{ 2+ }+\underset { (Green) }{ { SO }_{ 4 }^{ 2- } } +{ 4H }^{ + }\)
(iv) Thus, solid 'A' is S₈ and gas 'B' is SO₂

(i) KCIO₃ \(\overset { \Delta }{ \underset { { MnO }_{ 2 } }{ \longrightarrow } } \) 2KCI + 3O₂
(ii) 2 ZnS + 3O₂\(\longrightarrow \) 2 ZnO + 2 SO₂
(iii) AI_zO₃(s) + 6 NaOH_(aq) + 3 H₂O_(l) \(\longrightarrow \) 2 Na₃[AI(OH)₆]_(aq)
(iv) 2NaOH + SO₂\(\longrightarrow \)Na₂SO₃ + H₂O
(v) 2KCl + H₂SO₄ \(\longrightarrow \) 2 HCl + K₂SO₄

(i) O₂ molecules are held together by weak Vander Waal's force because of small size and high electronegativity of oxygen.
(ii) In contrast, sulphur shows catenation and forms stronger S-S bonds.
(iii) Due to catenation, sulphur forms octa-atomic S₈ molecules having eight membered puckered ring structure.
(iv) Because of its bigger size the force of attraction holding S₈ molecules are much stronger.
(v) Hence sulphur is a solid at room temperature or in other words, that is why there is a large difference between the boiling point (also melting points) of the two elements.

(a) Rhombic Sulphur (α - Sulphur):
(a) It is yellow in colour.
(b) Its melting point is 385.8K and specific gravity is 2.06
(c) It is stable form of sulphur at room temperature.
(d) It is formed on evaporating the solution of sulphur in CS₂.
(e) It in insoluble in water, readily soluble in CS₂ and dissolves to some extent in benzene, alcohol and ether.
(b) Monoclinic sulphur \(\left( \beta -sulphur \right) \):
(a) Its melting point is 393K and specific gravity is 1.98
(b) It is prepared by melting rhombic sulphur in a dish and cooling, till crust is formed. Two holes are made in crust and remaining liquid is powered out. On removing crust, colourless needle - shaped crystals of β - sulphur is formed.
(c) Monoclinic sulphur is stable above 369K and below 369K α - sulphur is stable.
(d) At 369K both forms are stable and this temperature is called transition temperature.
(e) Both rhombic and monoclinic sulphur have S₈ molecules, these are packed to give different crystal structure S₈ form is puckered and has crown shape.
Several other modifications containing 6-20 sulphur atoms per ring are synthesised 

(f) In Cyclo-S₆ the ng adopts chair form.

(g) At elevated temperatures (～1000K), S₂ is dominant species and is, paramagnetic like O₂

(i) Because of its lightness and noninflammability helium is used to filling balloons for meteorological observations.
(ii) Because of its lightness it is used in inflating aeroplane tyres.
(iii) Helium oxygen mixture is used by deep sea divers in preference to nitrogen oxygen mixtures. This prevents bends when a diver comes to the surface.
(iv) A mixture of oxygen and helium is used in the treatment of asthma.
(v) Liquid helium (b.pt 4.2K) is used as cryogenic agent for carrying out various experiments at low temperatures.
(vi)It is used to produce and sustain powerful super conducting magnets of modern NMR Spectrometers and Magnetic Resonance Imaging system (MRI) for clinical diagnosis.

Each halogen combines with another halogen to form several compounds known as interhalogen compounds.

Methods of Preparation interhalogen compounds:
I₂+ Cl₂(equimolar) \(\longrightarrow \)2ICI (AX)
Cl₂ + 3F₂ (Excess) 2CIF₃ (AX₃)
I₂+ 5F₂ (Excess) \(\longrightarrow \) 21F₅(AX₅)
IF₅ + F₂ (Excess)\(\overset { 573K }{ \longrightarrow } \) IF₇ (AX₇)
Structure of interhalogen compounds.
(i) Type AX: These are covalent compounds in which the larger halogen forms the central atom.
Eg: CIF, BrF
AX sp³ hybridisation-linear linear
(ii) Type AX₃:

(a) Without lone pair of electrons, it is T-shaped.
(b) BipyramidaI structure.
(c) sp³ hybridisation. Eg: CIF₃
(iii) Type AX₅:

(a) Without lone pair, it iF square pyramidal
(b) Octahedral structure
(c) sp³d² hybridisation 
Eg: IF₅
(iv) Type AX₇:

(a) Pentagonal pyramidal.
(b) sp³d³ hybridisation Eg: IF₇