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प्रश्न
Indicate the steps in the preparation of K2Cr2O7 from chromite ore.
Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?
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उत्तर
Dichromates are generally prepared from chromate, which in turn is obtained by the fusion of chromite ore (FeCr2O4) with sodium or potassium carbonate in free air. The reaction with sodium carbonate occurs as follows:
\[\ce{4FeCr_2O_4 + 8Na2CO3 + 7O_2 ->8Na_2CrO_4 + 2Fe_2O_3 + 8CO2}\]
The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, Na2Cr2O7. 2H2O can be crystallised.
\[\ce{2Na_2CrO_4 + 2H^+ -> Na_2Cr_2O_7 + 2Na^+ + H_2O}\]
Sodium dichromate is more soluble than potassium dichromate. Therefore, the latter is prepared by treating a sodium dichromate solution with potassium chloride.
\[\ce{Na_2Cr_2O_7 + 2KCl -> K_2Cr_2O_7 + 2NaCl}\]
Orange crystals of potassium dichromate crystallise out. The chromates and dichromates are interconvertible in an aqueous solution, depending on the pH of the solution. The oxidation state of chromium in chromate and dichromate is the same.
\[\ce{2CrO^{2-}_4 + 2H^+ -> Cr2O^{2-}_7 + H2O}\]
\[\ce{Cr2O^{2-}_7 + 2OH^- -> 2CrO^2-_4 + H2O}\]
Hence, when the pH is increased, making the solution alkaline, dichromate ions (orange in colour) are converted into chromate ions, and the solution becomes yellow.
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संबंधित प्रश्न
Complete the following equations:
When chromite ore FeCr2O4 is fused with NaOH in presence of air, a yellow-coloured compound (A) is obtained, which on acidification with dilute sulphuric acid gives a compound (B). Compound (B) on reaction with KCl forms an orange coloured crystalline compound (C).
(i) Write the formulae of the compounds (A), (B) and C.
(ii) Write one use of compound (C).
Complete the following equations : 2 Na2CrO4 + 2 H + →
Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?
Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with iron (II) ions? Write the ionic equation for the reaction.
Give an example and suggest a reason for the following feature of the transition metal chemistry:
A transition metal exhibits the highest oxidation state in oxides and fluorides.
Give an example and suggest a reason for the following feature of the transition metal chemistry:
The highest oxidation state is exhibited in oxoanions of a metal.
Write the formula of an oxo-anion of Manganese (Mn) in which it shows the oxidation state equal to its group number.
Complete the following chemical equations :
Cu + H2SO4(conc.) →
Write the chemical equation for the preparation of 4 KMnO from 2. MnO Why does purple colour of acidified permanganate solution decolourise when it oxidises Fe2- to Fe3+ ?
Using IUPAC norms, write the formula of potassium trioxalatochromate(III).
Which of the following is amphoteric oxide?
\[\ce{Mn2O7, CrO3, Cr2O3, CrO, V2O5, V2O4}\]
\[\ce{KMnO4}\] acts as an oxidising agent in alkaline medium. When alkaline \[\ce{KMnO4}\] is treated with \[\ce{KI}\], iodide ion is oxidised to ______.
The product formed upon heating methyl bromide with potassium tert-butoxide is
KMnO4 is coloured due to ______.
What is the effect of pH on dichromate ion solution?
Complete the reaction mentioning all the products formed:
\[\ce{2KMnO4 ->[\Delta]}\]
Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with SO2? Write the ionic equation for the reaction.
