Potassium dichromate reacts with aqueous iron(II) sulfate in acidic solution…

CHEMISTRY · Multi-part · A Level · Chemistry · Oxidation, reduction and redox reactions · Oxidation, reduction and redox reactions

Problem

Potassium dichromate reacts with aqueous iron(II) sulfate in acidic solution according to the overall reaction K2Cr2O7(aq)+FeSO4(aq)+H3O+(aq)→Cr3+(aq)+Fe3+(aq)+K+(aq)+SO42−(aq)+H2O(l).\text{K}_2\text{Cr}_2\text{O}_7(aq) + \text{FeSO}_4(aq) + \text{H}_3\text{O}^+(aq) \rightarrow \text{Cr}^{3+}(aq) + \text{Fe}^{3+}(aq) + \text{K}^+(aq) + \text{SO}_4^{2-}(aq) + \text{H}_2\text{O}(l). The equation is not balanced. Use the information in the equation to analyze the redox chemistry of the reaction. (a) Assign the oxidation number for every element in each of the following species as they appear in the reaction mixture: Cr2O72−\text{Cr}_2\text{O}_7^{2-}, Fe2+\text{Fe}^{2+}, Fe3+\text{Fe}^{3+}, and Cr3+\text{Cr}^{3+}. Show your reasoning for the oxidation numbers of chromium in both species. (b) Identify which element undergoes oxidation and which element undergoes reduction in the reaction. Justify your choices with reference to the oxidation numbers determined in part (a). (c) Explain, in terms of oxidation–reduction principles, why an acidic environment is required for potassium dichromate to oxidize iron(II) ions effectively.

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