Write the net ionic equation for the reaction of potassium dichromate $(V I), K_{2} C r_{2} O_{7}$ with sodium sulphite, $N a_{2} S O_{3},$ in an acid solution to give chromium $(I I I)$ ion and sulphate ion.

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Solution

$S t e p 1 : S k e l e t a l i o n i c e q u a t i o n$

The skeletal ionic equation is:

$C r_{2} O_{7}^{2 -} (a q) + S O_{3}^{2 -} (a q) \to C r^{3 +} (a q) + S O_{4}^{2 -} (a q)$

$S t e p 2 : A s s i g n i n g o x i d a t i o n n u m b e r s f o r C r a n d S$

$+ 6$ $2 -$ $+ 4$ $2 -$ $+ 6$ $2 -$
$C r_{2} O_{7}^{2 -} (a q) + S O_{3}^{2 -} (a q) \to C r^{3 +} (a q) + S O_{4}^{2 -} (a q)$

This indicates that the dichromate ion is the oxidant, and the sulphite ion is the reductant

$Step 3: Calculate the increase and decrease of oxidation number$

Calculate the increase and decrease of oxidation number, and make them equal. From step 2 we can notice that there is change in oxidation state of chromium changes from $+ 6$ to $+ 3.$ There is decrease of $+ 3$ in oxidation state of chromium on right hand side of the equation. Oxidation state of sulphur changes from $+ 4$ to $+ 6.$

There is an increase of $+ 2$ in the oxidation state of sulphur on right hand side. To make the increase and decrease of oxidation state equal, place numeral $2$ before chromium ion on right hand side and numeral $3$ before sulphate ion on right hand side and balance chromium and sulphur atoms on both sides of the equation. Thus we get;

$+ 6$ $2 -$ $+ 4$ $2 -$ $+ 6$ $2 -$
$C r_{2} O_{7}^{2 -} (a q) + 3 S O_{3}^{2 -} (a q) \to 2 C r^{3 +} (a q) + 3 S O_{4}^{2 -} (a q)$

$Step 4: Make ionic charges equal on both sides of the reaction$

As the reaction occurs in the acidic medium, and further the ionic charges are not equal on both the sides, add $8 H^{+}$ on the left to make ionic charges equal.

$C r_{2} O_{7}^{2 -} (a q) + 3 S O_{3}^{2 -} (a q) + 8 H^{+} \to 2 C r^{3 +} (a q) + 3 S O_{4}^{2 -} (a q)$

$Step 5: Count the hydrogen atoms and add appropriate number of water molecules$

Finally, count the hydrogen atoms, and add appropriate numbers of water molecules $(i . e ., 4 H_{2} O)$ on the right to achieve balanced redox change.

$C r_{2} O_{7}^{2 -} (a q) + 3 S O_{3}^{2 -} (a q) + 8 H^{+} \to 2 C r^{3 +} (a q) + 3 S O_{4}^{2 -} (a q) + 4 H_{2} O (l)$

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Oxides and Oxoanions

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Write the net ionic equation for the reaction of potassium dichromate(VI), K2Cr2O7 with sodium sulphite, Na2SO3, in an acid solution to give chromium (III) ion and sulphate ion.

Write the net ionic equation for the reaction of potassium dichromate $(V I), K_{2} C r_{2} O_{7}$ with sodium sulphite, $N a_{2} S O_{3},$ in an acid solution to give chromium $(I I I)$ ion and sulphate ion.