WAEC Chemistry 2014 Theory — Question 31

Model answer

(i) The oxidation number of chromium decreases from +6 in K₂Cr₂O₇ to +3 in CrCl₃ — this is reduction. The oxidation number of chlorine increases from −1 in HCl to 0 in Cl₂ — this is oxidation. Since both reduction and oxidation occur, the reaction is a redox reaction. (ii) Oxidation half equation: 2Cl⁻→Cl₂+2e⁻ Reduction half equation: Cr₂O₇²⁻+14H⁺+6e⁻→2Cr³⁺+7H₂O (iii) Overall equation: Cr₂O₇²⁻+14H⁺+6Cl⁻→2Cr³⁺+3Cl₂+7H₂O (or the fully balanced molecular form: K₂Cr₂O₇+14HCl→2KCl+2CrCl₃+3Cl₂+7H₂O)