Write ionic equations for the reaction of dichromate ions with
i) hydroxyl ions
ii) $F e^{+ 2}$ ions in Acidic medium
In which one of the above two reactions will the oxidation number of chromium remains unchanged?

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Solution

The reaction of dichromate ions with
hydroxyl ions
When we add hydroxyl ions to the potassium dichromate the orange colour of the solution turns yellow due to the formation of potassium chromate.
$K_{2} C r_{2} O_{7} + O H^{-} \to K_{2} C r O_{4} + H_{2} O$

2. $F e^{+ 2}$ ions in acidic medium
In presence of dilute sulphuric acid potassium dichromate oxidises ferrous sulphate( $F e^{+ 2}$ ) to ferric sulphate( $F e^{+ 3}$ ), potassium dichromate is reduced to chromic sulphate. Orange colour of the solution turns green.
$K_{2} C r_{2} O_{7} + 6 F e S O_{4} + 7 H_{2} S O_{4} \to K_{2} S O_{4} + C r_{2} (S O_{4})_{3} + 3 F e_{2} (S O_{4})_{3} + 7 H_{2} O$
here in second reaction the oxidation number of chromium changes from $+ 6$ to $+ 3$ .
In the first reaction the oxidation number of chromium remains unchanged.

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