Q.(a) Describe the preparation of potassium dichromate. OR

(a) Potassium dichromate is manufactured from chromite ore in three stages — oxidative fusion, acidification, then a KCl double-decomposition/crystallisation step. OR (b) Werner's theory explains coordination compounds using two distinct types of valency, of which the secondary (spatially directed) valency accounts for their definite geometric shapes.

(a) Preparation of potassium dichromate from chromite ore ($FeCr_2O_4$):

Step 1 — fusion: powdered chromite ore is fused with sodium/potassium carbonate in free access of air (or with an added oxidant like $KNO_3$), in a reverberatory furnace, oxidising the chromium to give sodium chromate: $4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \xrightarrow{\Delta} 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2$

Step 2 — acidification: the yellow solution of sodium chromate is filtered (to remove insoluble $Fe_2O_3$) and acidified with dilute sulphuric acid, converting the chromate to the more soluble, orange dichromate: $2Na_2CrO_4 + H_2SO_4 \rightarrow Na_2Cr_2O_7 + Na_2SO_4 + H_2O$

Step 3 — conversion to potassium dichromate: the sodium dichromate solution is treated with potassium chloride; since potassium dichromate is considerably less soluble than sodium chloride, it crystallises out on cooling/concentration, while $NaCl$ remains in solution: $Na_2Cr_2O_7 + 2KCl \rightarrow K_2Cr_2O_7\downarrow + 2NaCl$

OR (b) Werner's postulates:

1. Metal ions in coordination compounds show two types of valency: primary valency (ionisable, satisfied by negative/anionic ligands, numerically equal to the oxidation state of the metal), and secondary valency (non-ionisable, satisfied by neutral molecules or negative ions, fixed in number for a given metal — the coordination number). …