Question

Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH of potassium dichromate?

Answer 1

Hint: The potassium dichromate is prepared in the three steps using the iron chromite ore. There will be formation of sodium dichromate too. Identify the steps followed to attain the product potassium dichromate. The effect of pH can be defined in the terms of dissociation of potassium dichromate in the equilibrium.

Complete step by step answer:
First, let us discuss the preparation of potassium dichromate. It consists of three steps as mentioned.
Thus, the first step involves the reaction of chromite ore in excess of air (oxygen) with the sodium carbonate, or we can also use potassium carbonate instead of sodium carbonate.
The chemical reaction is,
4FeCr$_2$O$_4$ + 8Na$_2$CO$_3$ + 7O$_2$ $\rightarrow$ 8Na$_2$CrO$_4$ + 2Fe$_2$O$_3$ + 8CO$_2$
So, we can see that in the first step, the product obtained is sodium chromate that is yellow in colour with the by-products carbon-dioxide, and ferric oxide.

Now, the second step includes the reaction of sodium chromate with the acidified solution of sulphuric acid. The chemical reaction is,
2Na$_2$CrO$_4$ + conc.H$_2$SO$_4$ $\rightarrow$ Na$_2$Cr$_2$O$_7$ + Na$_2$SO$_4$ + H$_2$O
Here, the product formed is sodium dichromate, sodium sulphate, and water.

Talking about the third step, it includes the reaction of sodium dichromate, and potassium chloride. The chemical reaction is,
Na$_2$Cr$_2$O$_7$ + 2KCl $\rightarrow$ K$_2$Cr$_2$O$_7$ + 2NaCl

Now, we can see that the product obtained is potassium dichromate along-with sodium chloride. The precipitate of sodium chloride is obtained from the hot solution and is removed using filtration. The mother liquor left is potassium dichromate.

Next we will discuss the effect of increasing pH on a solution of potassium dichromate. As we know, dichromate ions with the chromate ions exist in equilibrium state, and if we increase the pH of the solution it is convertible.
The reaction at pH 4 between chromate ions and dichromate ions is given below-
\[\begin{matrix}
   C{{r}_{2}}{{O}_{7}}^{2-} \\
   \text{Dichromate ion (Orange red)} \\
\end{matrix}+{{H}_{2}}O\overset{{}}{\leftrightharpoons}\begin{matrix}
   2Cr{{O}_{4}}^{2-} \\
   \text{Chromate ion (Yellow)} \\
\end{matrix}+2{{H}^{+}}\]

In the end, we can conclude the change in pH leads to the formation of potassium chromate, and the formation of potassium dichromate is mentioned above.

Note: Don’t get confused while explaining the effect of pH on potassium dichromate solution. As mentioned it can be internally converted; it means that the increase in pH will change the colour of orange crystals of potassium dichromate to the yellow crystals of potassium chromate. This is in accordance with Le chatelier’s principle