Question

What are the steps in the preparation of: (i) ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ from chromite ore. (ii) ${\mathrm{KMnO}}_4$ from pyrolusite ore.

Answer 1

Preparation of ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ from chromite ore:

Potassium dichromate can be prepared from chromite ore in three steps.

• Step 1: Conversion of chromite ore into sodium chromate:

$4{\mathrm{FeCr}}_2{\mathrm{O}}_4\left(\mathrm{s}\right)+8{\mathrm{Na}}_2{\mathrm{CO}}_3\left(\mathrm{s}\right)+7{\mathrm{O}}_2\left(\mathrm{g}\right)\to 8{\mathrm{Na}}_2{\mathrm{CrO}}_4\left(\mathrm{aq}\right)+2{\mathrm{Fe}}_2{\mathrm{O}}_3\left(\mathrm{aq}\right)+8{\mathrm{CO}}_2\left(\mathrm{g}\right)$

• Step 2: Conversion of Sodium chromate into sodium dichromate:

$2{\mathrm{Na}}_2{\mathrm{CrO}}_4\left(\mathrm{aq}\right)+2{\mathrm{H}}^{+}\left(\mathrm{aq}\right)\to {\mathrm{Na}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\left(\mathrm{aq}\right)+2{\mathrm{Na}}^{+}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

• Step 3: Conversion of Sodium dichromate into potassium dichromate:

${\mathrm{Na}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\left(\mathrm{aq}\right)+2\mathrm{KCl}\left(\mathrm{s}\right)\to {\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\left(\mathrm{aq}\right)+2\mathrm{NaCl}\left(\mathrm{aq}\right)$

(ii) Preparation of ${\mathrm{KMnO}}_4$ from pyrolusite ore:

The chemical equation for the preparation of ${\mathrm{KMnO}}_4$ from pyrolusite ore is depicted below.

Step 1: Conversion of pyrolusite ore into Potassium manganate

• $2{\mathrm{MnO}}_2\left(\mathrm{aq}\right)+4\mathrm{KOH}\left(\mathrm{aq}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2{\mathrm{K}}_2{\mathrm{MnO}}_4\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

Step 2: Potassium manganate is dissolved into acid to obtain Potassium permanganate

• $3{\mathrm{MnO}}_4^{2-}\left(\mathrm{aq}\right)+4{\mathrm{H}}^{+}\left(\mathrm{aq}\right)\to 2{\mathrm{MnO}}_4^{-}\left(\mathrm{aq}\right)+{\mathrm{MnO}}_2\left(\mathrm{s}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$