To write a balanced chemical equation for the given redox reaction, we need to know the half reactions of the substances involved. Redox reaction means oxidation-reduction reaction. Oxidation is when one loses electrons making it more positive while reduction is gaining electrons which makes it more negative. Iron here is being oxidized and will have the half reaction:
Fe2+(aq) ---> Fe3+(aq) + e+
S2O2−6 is being reduced in the process and will have a half reaction of:
S2O2-6 + 4H+ + 2e- ----> 2H2SO3
Adding the two half reactions would yield to the balanced chemical equation. We do as follows:
2 (Fe2+ ---> Fe3+ + e+)
S2O2-6 + 4H+ + 2e- ----> 2H2SO3
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2Fe2+ + S2O2-6 + 4H+ ----> 2Fe3+ + 2H2SO3 ( balanced oxidation - reduction reaction )