Start by assigning oxidation numbers to all the atoms that take part in the reaction.
Notice that oxidation state of sulfur goes from +4 on the reactants' side, to +6 on the products' side, which means that it is being oxidized.
At the same time, the oxidation state of chromium goes from +6 on the reactants' side, to +3 on the products' side, which means that it is being reduced.
The oxidation and reduction half-reactions will look like this
- Oxidation half-reaction
Balance the oxygen and hydrogen atoms by adding water molecules to the side that needs oxygen, and protons,
- Reduction half-reaction
Notice that you have two cxhromium atoms on the reactants' side, and only one on the products' side. Multiply the product by 2 to get
Notice that two chromium atoms lose a total of 6 electrons to go from an oxidation state of +6 to an aoxidation state of +3 per atom.
Once again, balance the oxygen and hydrogens atoms by adding water and protons.
In redox reactions, the number of electrons gained in the reduction half-reaction must be equal to the number of electrons lost in the oxidation half-reaction.
Multiply the oxidation half-reaction by 3 to balance the number of transferred electrons.
Add the two half-reactions together to get
The balanced net ionic equation will be