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Chemistry MCQs for NEET — Practice Questions with Answers

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In the reaction,

Cr₂ $O_{7}^{2 -}$ +14H⁺ + 6I^- $\to$ 2Cr³⁺ + 7H₂O + 3I₂,

Which element is reduced?

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Explanation

(d) C $r_{2}^{6 +}$ + 6e $\to$ 2Cr³⁺

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In the reaction,

2Na₂S₂O₃+ I₂ $\to$ Na₂S₄O₆ + 2NaI,

the oxidation state of sulphur is:

decreased

increased

unchanged

none of these

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Explanation

(b) 2 $S_{2}^{2 +}$ $\to$ $S_{4}^{5 / 2}$ +2e

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In the aluminothermic process, aluminium acts as:

an oxidising agent

a flux

a reducing agent

a solder

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Explanation

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Which of the following change represents a disproportionation reaction(s)?

Cl₂ + 2OH^- $\to$ ClO^- + Cl^- + H₂O

Cu₂O(s) + 2H⁺ $\to$ Cu²⁺ + Cu(s) + H₂O

2HCuCl₂ $\to_{water}^{Dilution with}$ Cu + Cu²⁺ + 4 Cl^- + 2H⁺

All of the above

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Explanation

(d) The same species in each reaction is oxidised and reduced well to give disproportionation reaction.

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During a redox change, the oxidant K₂Cr₂O₇ is always reduced to:

Cr⁵⁺

Cr⁴⁺

Cr³⁺

Cr²⁺

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Explanation

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The reaction,

3ClO^-(aq) $\to$ Cl $O_{3}^{-}$ (aq) + 2Cl^-(aq)

is an example of:

oxidation reaction

reduction reaction

disproportionation reaction

decomposition reaction

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Explanation

(c) Cl atom is oxidised (Cl¹⁺ $\to$ Cl⁵⁺ +4e) as well as Cl atom is reduced (Cl¹⁺ + 2e $\to$ Cl^-) Such reactions are called auto redox or disproportionation reactions.

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Oxidation state of +1 for phosphorus is found in:

H₃PO₃

H₃PO₄

H₃PO₂

H₄P₂O₇

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Explanation

$∴$ a = +1

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Which is not a redox change?

CaCO₃ $\to$ CaO + CO₂

2H₂ + O₂ $\to$ 2H₂O

Na + H₂O $\to$ NaOH + 1/2 H₂

MnCl₃ $\to$ MnCl₂ + 1/2 Cl₂

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Explanation

(a) No change in oxidation no. in any of the species.

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If H₂S is passed through an acidified K₂Cr₂O₇ solution, the colour of the solution:

will remain unchanged

will change to deep red

will change to dark green

will change to dark brown

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Explanation

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The oxidation number and covalency of sulphur in the sulphur molecule (S₈) are respectively:

0 and 2

+6 and 8

0 and 8

+6 and 2

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Explanation

(a) Oxidation number in elemental form is zero. Co-valency is two because of S-S-S-S-chain.

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