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JEE Advanced 2020 Paper 2 · Chemistry · d- and f-Block Elements and Metallurgy · hard

Chemistry of chromium, manganese and copper is/are correct?

One or more options correct

Which of the following statements about the chemistry of chromium, manganese and copper is/are correct?
  1. A. Acidified K2Cr2O7\mathrm{K_2Cr_2O_7} oxidises H2S\mathrm{H_2S} to sulfur and the solution turns green.
  2. B. In strongly alkaline medium, permanganate is reduced by a mild reductant to manganate, MnO42−\mathrm{MnO_4^{2-}}.
  3. C. Cu2+\mathrm{Cu^{2+}} treated with excess KCN\mathrm{KCN} gives a colourless, diamagnetic complex, [Cu(CN)4]3−\mathrm{[Cu(CN)_4]^{3-}}, with evolution of cyanogen.
  4. D. Cu2+\mathrm{Cu^{2+}} treated with excess KI\mathrm{KI} gives a white precipitate of CuI2\mathrm{CuI_2}.
Show answer and solution

Answer: A, B, C

Option A: Cr2O72−+8H++3H2S→2Cr3++3S+7H2O\mathrm{Cr_2O_7^{2-}+8H^++3H_2S\to2Cr^{3+}+3S+7H_2O}, and the product Cr3+\mathrm{Cr^{3+}} is green, so A is correct. Option B: in strongly alkaline solution MnO4−+e−→MnO42−\mathrm{MnO_4^-+e^-\to MnO_4^{2-}} (green), so B is correct; reduction to MnO2\mathrm{MnO_2} occurs in neutral or weakly alkaline medium. Option C: 2Cu2++4CN−→2CuCN+(CN)2\mathrm{2Cu^{2+}+4CN^-\to2CuCN+(CN)_2}, and excess cyanide dissolves CuCN\mathrm{CuCN} as [Cu(CN)4]3−\mathrm{[Cu(CN)_4]^{3-}}, in which copper is Cu(I)\mathrm{Cu(I)}, d10d^{10}, hence colourless and diamagnetic, so C is correct. Option D: Cu2+\mathrm{Cu^{2+}} with iodide gives 2Cu2++4I−→2CuI↓+I2\mathrm{2Cu^{2+}+4I^-\to2CuI\downarrow+I_2}, so copper(II) iodide is not formed; the precipitate is white CuI\mathrm{CuI} stained brown by iodine, and D is wrong.
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