Question

K₂Cr₂O₇ on heating with aqueous NaOH gives
(a) CrO₄²⁻ (b) Cr(OH)₃
(c) Cr₂O₇²⁻ (d) Cr(OH)₂

Answer 1

Explanation:

K₂Cr₂O₇ on heating with aqueous NaOH gives

(a) CrO₄²⁻

(b) Cr(OH)₃

(c) Cr₂O₇²⁻

(d) Cr(OH)₂

Answer 2

CrO₄²⁻ is the formed ion.

Explanation:

• Potassium dichromate is the most oxidised form of chromium.

• The chromium in the dichromate ion has the oxidation number of +7.

• The reaction that takes place when potassium dichromate reacts with aqueous sodium hydroxide is,

• K2Cr2O7 + 2NaOH→ K2CrO4 + Na2CrO4 + H2O.

• So, dichromate ion is clearly seen to be reduced to chromate ion.

• The visual changes include change of colour of the solution from bright orange to yellow.

• The oxidation number of the chromium in chromate is +6.

• Similarly, hydrogen is oxidised to water to balance the reaction.

For more information about dichromate reduction,

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Calculate the equivalent mass of potassium dichromate. The reduction half-reaction in acid medium is,

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2 Cr³⁺ + 7H₂O

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When Pottasium Dichromate is mixed with Hydrogen Peroxide, Dichromate is reduced (as shown in answer key to test). However since H2O2 has a greater reduction potential than dichromate, should H2O2 undergo disproportionation rather than dichromate reducing