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Pb 3 O 4 is a stoichiometric mixture of 2 mol of PbO and 1 mol of PbO 2 . In PbO and PbO 2 the oxidation state of lead is +2 and +4 respectively. PbO 2 thus behaves as an oxidising agent and therefore can oxidise Cl - ion of HCl into chlorine. Also, PbO being basic in nature, therefore the reaction Pb 3 O 4 + 8HCl → 3PbCl 2 + Cl 2 + 4H 2 O can be split into two reactions namely: (a) Acid-base reaction 2PbO + 4HCl → 2PbCl 2 + 2H 2 O (b) Redox reaction +4 -1 +2 0 PbO 2 + 4HCl → PbCl 2 + Cl 2 +2H 2 O Since HNO 3 itself is in oxidant, therefore, the reaction between PbO 2 and HNO 3 is not possible. However, the acid-base reaction occurs between PbO and HNO 3 as: 2PbO + 4HNO 3 → 2Pb(NO 3 ) 2 + 2H 2 O (acid-base reaction). It is the passive nature of PbO 2 against HNO 3 that makes the reaction different from the one that follows with HCl.

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Redox Reactions

Long Answer Type

91.
Why do the following reactions proceed differently?

Pb₃O₄ +8HCl --->3PbCl₂ +Cl₂ +4H₂O
Pb₃O₄ +4HNO₃ --->2Pb(NO₃)₂ +PbO₂ +2H₂O

Pb₃O₄ is a stoichiometric mixture of 2 mol of PbO and 1 mol of PbO₂. In PbO and PbO₂ the oxidation state of lead is +2 and +4 respectively. PbO₂thus behaves as an oxidising agent and therefore can oxidise Cl^- ion of HCl into chlorine. Also, PbO being basic in nature, therefore the reaction
Pb₃O₄ + 8HCl → 3PbCl₂ + Cl₂ + 4H₂O
can be split into two reactions namely:

(a) Acid-base reaction
2PbO + 4HCl → 2PbCl₂ + 2H₂O

(b) Redox reaction
+4 -1 +2 0
PbO₂ + 4HCl → PbCl₂ + Cl₂ +2H₂O

Since HNO₃ itself is in oxidant, therefore, the reaction between PbO₂ and HNO₃ is not possible. However, the acid-base reaction occurs between PbO and HNO₃ as:

2PbO + 4HNO₃ → 2Pb(NO₃)₂ + 2H₂O
(acid-base reaction).

It is the passive nature of PbO₂ against HNO₃ that makes the reaction different from the one that follows with HCl.

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92. While sulphur dioxide and hydrogen peroxide can act as oxidising as well as reducing agent is their reactions, ozone and nitric acid act only an oxidants. Why?

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93.

Consider the reaction:

Why does the same reductant—thiosulphate react differently with iodine and bromine?

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Short Answer Type

94. Why does the following reaction occur?

bold XeO subscript bold 6 superscript bold 4 bold minus end superscript bold left parenthesis bold aq bold right parenthesis bold space bold plus bold 2 bold F to the power of bold minus bold left parenthesis bold aq bold right parenthesis bold space bold plus bold 6 bold H to the power of bold plus bold left parenthesis bold aq bold right parenthesis bold space bold space bold space bold space bold space bold space bold space bold space bold space bold space bold space bold space bold space bold space bold space bold rightwards arrow bold XeO subscript bold 3 bold left parenthesis bold g bold right parenthesis bold space bold plus bold F subscript bold 2 bold left parenthesis bold g bold right parenthesis bold plus bold 3 bold H subscript bold 2 bold O bold left parenthesis bold l bold right parenthesis

conclusion about the compound Na₄XeO₆ (of which XeO₆^4– is a part) can be drawn from the reaction ?

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Long Answer Type

95. Justify giving reactions that among halogens, fluorine is the best oxidant and among hydrohalic compunds, hydroiodic acid is the best reductant.

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96. When a reaction between an oxidising agent and a reducing agent is carried out, a compound of lower oxidation state is formed if the reducing agent is in excess and a compound of higher oxidation state is formed when the oxidising agent is in excess. Justify this statement giving three illustrations.

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97. How do you account for the following observations?

(a) Though alkaline potassium permanganate and acidic potassium permanganate both are used as oxidants; yet in the manufacture of benzoic acid from toluene we use alcoholic potassium permanganate as an oxidant. Why? Write a balanced redox for the equation reaction.

(b) When concentrated sulphuric acid is added to an inorganic mixture containing chloride, we get colourless pungent smelling gas HCl, but if the mixture contains bromide then we get red vapours of bromine. Why?

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98.

Consider the reactions:

$bold left parenthesis bold a bold right parenthesis bold space bold space bold 6 bold CO subscript bold 2 bold left parenthesis bold g bold right parenthesis bold space bold plus bold space bold 6 bold H subscript bold 2 bold O bold left parenthesis bold l bold right parenthesis bold space bold space bold rightwards arrow bold space bold space bold space bold C subscript bold 6 bold H subscript bold 12 bold O subscript bold 6 bold left parenthesis bold aq bold right parenthesis bold space bold plus bold space bold 6 bold O subscript bold 2 bold left parenthesis bold g bold right parenthesis$
Why is it more appropriate to write these reactions as:
$bold left parenthesis bold a bold right parenthesis bold space bold 6 bold CO subscript bold 2 bold left parenthesis bold g bold right parenthesis bold space bold plus bold space bold 12 bold H subscript bold 2 bold O bold left parenthesis bold l bold right parenthesis bold space bold space bold space bold space bold space bold space bold space bold space bold space bold space bold space bold space bold rightwards arrow bold space bold space bold space bold C subscript bold 6 bold H subscript bold 12 bold O subscript bold 6 bold left parenthesis bold aq bold right parenthesis bold space bold plus bold space bold 6 bold H subscript bold 2 bold O bold left parenthesis bold I bold right parenthesis bold space bold plus bold space bold 6 bold O subscript bold 2 bold left parenthesis bold g bold right parenthesis bold left parenthesis bold b bold right parenthesis bold space bold O subscript bold 3 bold left parenthesis bold g bold right parenthesis bold space bold plus bold space bold H subscript bold 2 bold O subscript bold 2 bold left parenthesis bold l bold right parenthesis bold space bold space bold rightwards arrow bold space bold space bold space bold space bold space bold H subscript bold 2 bold O bold left parenthesis bold l bold right parenthesis bold space bold plus bold space bold O subscript bold 2 bold left parenthesis bold g bold right parenthesis bold space bold plus bold space bold O subscript bold 2 bold left parenthesis bold g bold right parenthesis$

Also, suggest a technique to investigate the path of the above (a) and (b) redox reactions.

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99.

What is the oxidation number of sulphur in:
(i)H₂SO₄ (ii) Na₂S₂O₃?

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100.

Determine the oxidation number of:
$left parenthesis straight i right parenthesis space straight S space in space straight S subscript 2 straight O subscript 3 superscript 2 minus end superscript space straight S subscript 2 straight O subscript 3 superscript 2 minus end superscript space space space space space left parenthesis ii right parenthesis space Br space in space BrO subscript 4 superscript minus space space space space space space left parenthesis iii right parenthesis space straight P space in space HPO subscript 4 superscript 2 minus end superscript$