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Redox Reactions
Balancing Redox Reactions – Ion-Electron Method
1 question from 1 paper, each with its answer.
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- years asked, 2017–2026
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- of Redox Reactions
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Concepts tested
- dichromate–sulphite balancing
All Questions
2023 1 question
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On balancing the given redox reaction, $aCr_2O_7^{2-}+bSO_3^{2-}(aq)+cH^+(aq)\rightarrow2aCr^{3+}(aq)+bSO_4^{2-}(aq)+\frac{c}{2}H_2O(l)$ the coefficients a, b and c are found to be, respectively:
Combining half-reactions gives $Cr_2O_7^{2-}+3SO_3^{2-}+8H^+\rightarrow2Cr^{3+}+3SO_4^{2-}+4H_2O$.
Reduction half reaction: $Cr_2O_7^{2-}+14H^++6e^-\rightarrow2Cr^{3+}+7H_2O$ ... (i) Oxidation half reaction: $[SO_3^{2-}+H_2O\rightarrow SO_4^{2-}+2H^++2e^-]\times3$ ... (ii) Oxygen is balanced by adding water and hydrogen is balanced by adding $H^+$ and the charge is balanced by electrons. Add (eq. (i)) + (3 × eq. (ii)): $Cr_2O_7^{2-}+3SO_3^{2-}+8H^+\rightarrow2Cr^{3+}+3SO_4^{2-}+4H_2O$ a = 1, b = 3, c = 8