(a)(i) List four characteristic properties of transition metals
(ii) Name two metals that can be extracted from their ore by electrolysis.
(b)(i) Determine the oxidation number of chromium in Cr\(_2\)O\(^{2-}_{7}\)
(ii) State the colour observed on adding a few drops of dilute tetraoxosulphate (VI) acid to the system representedby the following equation: Cr\(_2\)O\(^{2-}_{7(aq)}\) + H\(_2O_{(l)}\) \(\rightleftharpoons\) 2CrO\(^{2-}_{4(aq)}\) + 2H\(^+_{(aq)}\). Explain your answer.
(c)(i) State and explain what would be observed if hydrogen sulphide gas were bubbled into acidified K\(_2\)Cr\(_2\)O\(_7\). Write an equation for the reaction.
(ii) What precaution should be taken to avoid excessive exposure to hydrogen sulphide gas while it is being generated in the laboratory?
Explanation
(a)(i) Calcium and Magnesium.
(b)(i) 2Cr - 14 = -2;
2Cr = -2 + 14 = 12
Cr = \(\frac{12}{2}\) = 6
the oxidation number of Cr = +6
(ii) On addition of few drops of dilutes H\(_2\)SO\(_4\) the equilibrium system, the solution colour changes from yellow to green because Cr\(^{+6}\) is reduced to Cr\(^{3+}\).
(i) The solution changes from yellow colour to green and sulphur is precipitated because H\(_2\)SO\(_4\) is oxidized to elementary sulphur.
\(Cr_2O^{2-}_{7(aq)} + 3H_2S_{(g)} + 8H^+_{(aq)} \to 2Cr^{3+}_{(aq)} + 7H_2O(11 + 3S_{(s)})\)
(ii) The gas should be cautiously generated in the fume cupboard.