Question

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

Answer 1

Step-1: Write the skeleton equation for the given reaction.

Step-2: Find out the elements which undergo a change in oxidation number (O.N.) O.N. decreases by 3 per Cr atom

Here, O.N. of Cr decreases from +6 in Cr₂O₇^2- to +3 in Cr³⁺ while that of S increases from +4 SO₃^2- to +6 in SO₄^2-

Step-3: Balance increase / decrease in O.N. since the total increase in O.N. is 2 and decrease is 6, therefore, multiply SO₃^2- on L.H.S. and SO₄^2- on R.H.S. of Eq. (ii) by 3. Combining steps 2 and 3, we have,

Step-4: Balance O atoms by adding H₂O molecules.

Step-5: Balance H atoms by adding H⁺

Thus, Eq. (v) represents the correct balanced equation.