CHAPTER-7

CLASS-XII

THE p- BLOCK ELEMENTS

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Prepared by

Mr. B.SENTHAMIZHSELVAN

PGT CHEMISTRY

JNV MYSORE

KARNATAKA

HYDERABAD REGION

PART -4

Contents

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• Allotropes of Sulphur - Rhombic sulphur and Monoclinic sulphur
• Preparation, Physical and Chemical Properties and uses of :
• Sulphur di-oxide
• Sulphuric acid
• Structure of oxo-acids of sulphur

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PART - 4

SULPHUR

Allotrophic forms of Sulphur

Rhombic Sulphur

( α – Sulphur )

Monoclinic Sulphur

( β – Sulphur )

Rhombic Sulphur ( α – Sulphur / Yellow sulphur )

Rhombic Sulphur is� ~yellow in colour� ~melting point - 385.8 K� ~specific gravity – 2.06� ~formed on evaporating

the solution of Roll

Sulphur in carbon

disulphide � ~insoluble in water and

readily soluble in carbon

disulphide �

�At room temperature, rhombic sulphur is stable and it transforms to monoclinic sulphur when heated above 369K or 96^o C. This temperature is called as transition temperature.�

Monoclinic sulphur ( β – Sulphur )

Monoclinic sulphur

~melting point – 393 K

~specific gravity – 1.98

~soluble in carbon

disulphide

~prepared by melting

rhombic sulphur in a

dish and cooling

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Both Rhombic and Monoclinic Sulphur have S₈ molecules. They both have puckered or typical forms. Several other modifications of sulphur containing 6 – 20 atoms per ring have been prepared. �

QUESTION

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• Which form of sulphur shows paramagnetic behaviour ?
• In vapour state sulphur partly exists as S2 molecule which has two unpaired electrons in the antibonding π * orbitals like O2 and, hence, exhibits paramagnetism.

SULPHUR DIOXIDE

Methods of Preparation:

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• General Method

S (s) + O₂ 🡪 SO₂ (g)

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• Lab Method

SO₃^2- (aq) + 2H⁺ 🡪 H₂0 (l) + SO₂ (g)

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• Industrial Method

4FeS₂ (g) + 11O₂ (g) 🡪 2Fe₂O₃ (s) + 8SO₂ (g)

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PROPERTIES OF SULPHUR DIOXIDE

• Colourless gas

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• Pungent smell

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• Highly soluble in water

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• When passed through water forms sulphurous acid (H₂SO₃)

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• When passed through NaOH

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2NaOH + SO₂ 🡪 Na₂SO₃ + H₂O

Na₂SO₃ + H₂O + SO₂ 🡪 2NaHSO₃

• When made to react with chlorine it gives sulphuryl chloride

SO₂ (g) + Cl₂ (g) 🡪 SO₂Cl₂ (l)

• When reacted with oxygen in presence of vanadium (V) pentoxide forms sulphur dioxide

2SO₂ (g) + O₂ (g) ----🡪 2SO₃ (g)

• It behaves as Reducing agents
• Converts Fe³⁺ 🡪 Fe²⁺

2Fe³⁺ + SO₂ + 2H₂O 🡪 2Fe²⁺ + SO₄^2- + 4H⁺

• Decolourises acidified KMnO4 solution

5SO₂ + 2MnO₄^- + 2H2O 🡪 5SO₄^2- + 4H⁺ + 2Mn²⁺

• It is a test for detection of SO₂ gas.
• Its angular in shape

Resonating structures:

�(i) in refining petroleum and sugar �(ii) in bleaching wool and silk and �(iii) as an anti-chlor, disinfectant and � preservative. �(iv)Sulphuric acid, sodium hydrogen sulphite� and calcium hydrogen sulphite (industrial� chemicals) are manufactured from sulphur � dioxide. �(v) Liquid SO2 is used as a solvent to dissolve a� number of organic and inorganic chemicals.

USES OF SULPHUR DIOXIDE

OXOACIDS OF SULPHUR

SULPHURIC ACID

Preparation of Sulphuric acid –CONTACT PROCESS

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1. Burning of sulphur or sulphide ore in air to generate SO₂.

S + O2 -----🡪 SO2

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2. Conversion of SO₂ to SO₃ by the reaction with oxygen in the presence of a catalyst V₂O₅.

V₂O₅

2SO₂ (g) + O₂ (g) ----🡪 2SO₃ (g) + H

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3. Absorption of SO₃ in H₂SO₄ to give Oleum (H₂S₂O₇)

SO₃ + H₂SO₄ 🡪 H₂S₂O₇ (oleum)

H₂S₂O₇ + H₂O 🡪 H₂SO₄

Flow chart for the formation/manufacture of� sulphuric acid by contact process

Properties of sulphuric acid�

a) Colourless, dense, oily liquid

b) Dissolves in water with large quantity of

heat evolved. This is the reason why

concentrated acid must be added slowly into

water with constant stirring.

c) Chemical activities/chemical reaction/chemical

properties of sulphuric acid are a result of

following characteristics:

• low volatility
• strong acidic character
• strong affinity for water
• ability to act as an oxidizing agent

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� Q. How do you prove that it is a dibasic acid ?�� Ans. This acid forms two series of salts: � Normal sulphates and � acid sulphates (Bisulphates).�� It can also be proved by following two reactions :�� H₂SO₄ (aq) + H₂O (l) 🡪 H₃O⁺ (aq ) + HSO₄^- (aq)�� HSO₄^- (aq) + H₂O (l) 🡪 H₃O⁺ (aq) + SO₄^2- (aq)��(i) Because of its low volatility it is used in manufacture of � more volatile acids from their volatile salts�� 2MX + H₂SO₄ 🡪 2HX + M₂SO₄ (X= F, Cl, NO₃)�� �

CCCCCCCCCCCCCCCCCCC

(ii) Concentrated H₂SO₄ is a strong dehydrating agent � 🡪It is used to dry wet gases provided; they do not

react with it.� 🡪It is used to remove water from organic compound

like charring of carbohydrates.

� C₁₂H₂₂O₁₁ ------🡪 12C + 11H₂O��(iii) Strong oxidizing agent property of hot concentrated

H₂SO₄ is intermediate between phosphoric and

nitric acid.

� Cu + 2H₂SO₄ (conc.) 🡪 CuSO₄ + SO₂ + 2H₂O� C + 2H₂SO₄ (conc. ) 🡪 CO₂ + 2SO₂ + 2H₂0� 3S + 2H₂SO₄ (conc. ) 🡪 3SO₂ + 2H₂O�

� � Sulphuric acid is a very important industrial chemical. � A nation’s industrial strength can be judged by the quantity� of sulphuric acid it produces and consumes. It is needed for� the manufacture of hundreds of other compounds and also � in many industrial processes. � The bulk of sulphuric acid produced is used in the � manufacture of fertilisers (e.g., ammonium sulphate, � superphosphate). � Other uses are in:� (a) petroleum refining � (b) manufacture of pigments, paints and dyestuff intermediates� (c) detergent industry � (d) metallurgical applications (e.g., cleansing metals before� enameling, electroplating and galvanising � (e) storage batteries � (f ) in the manufacture of nitrocellulose products and � (g) as a laboratory reagent.�

USES OF SULPHURIC ACID