S₂O₈^2–(aq) + Cr(s) → Cr³⁺(aq) + SO₄^2–(aq)

acidic environment

[S₂O₈^2–(aq) + 2 e^– → 2 SO₄^2–(aq)] × 3

[Cr(s) → Cr³⁺(aq) + 3 e^–] × 2

sum and eliminate electrons

3 S₂O₈^2–(aq) + 2 Cr(s) → 2 Cr³⁺(aq) + 6 SO₄^2–(aq)