(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?
(a)(i) Four characteristic properties of transition metals: they form coloured ions/compounds; they show variable oxidation states; they and their compounds act as catalysts; they form complex ions. (They are also hard, dense metals with high melting points and are often paramagnetic.)
(a)(ii) Two metals extracted by electrolysis: sodium and aluminium (also potassium, calcium, magnesium).
(b)(i) Oxidation number of Cr in Cr2O72-:
\[ 2x + 7(-2) = -2 \Rightarrow 2x = +12 \Rightarrow x = +6 \]
(b)(ii) Adding dilute H2SO4 (increasing H+) to the equilibrium \(\text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O} \rightleftharpoons 2\text{CrO}_4^{2-} + 2\text{H}^+\) shifts the position of equilibrium to the left (Le Chatelier's principle). The colour therefore changes from yellow (chromate) to orange (dichromate).
(c)(i) Bubbling H2S into acidified K2Cr2O7: the orange solution turns green. H2S is a reducing agent and reduces Cr6+ (orange dichromate) to green Cr3+, while sulphur is deposited (a pale yellow precipitate):
\[ \text{Cr}_2\text{O}_7^{2-} + 8\text{H}^+ + 3\text{H}_2\text{S} \to 2\text{Cr}^{3+} + 7\text{H}_2\text{O} + 3\text{S} \]
(c)(ii) Precaution: generate and use the hydrogen sulphide in a fume cupboard (fume chamber) so that the poisonous gas is drawn away and not inhaled.
(a)(i) Four characteristic properties of transition metals: they form coloured ions/compounds; they show variable oxidation states; they and their compounds act as catalysts; they form complex ions. (They are also hard, dense metals with high melting points and are often paramagnetic.)
(a)(ii) Two metals extracted by electrolysis: sodium and aluminium (also potassium, calcium, magnesium).
(b)(i) Oxidation number of Cr in Cr2O72-:
\[ 2x + 7(-2) = -2 \Rightarrow 2x = +12 \Rightarrow x = +6 \]
(b)(ii) Adding dilute H2SO4 (increasing H+) to the equilibrium \(\text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O} \rightleftharpoons 2\text{CrO}_4^{2-} + 2\text{H}^+\) shifts the position of equilibrium to the left (Le Chatelier's principle). The colour therefore changes from yellow (chromate) to orange (dichromate).
(c)(i) Bubbling H2S into acidified K2Cr2O7: the orange solution turns green. H2S is a reducing agent and reduces Cr6+ (orange dichromate) to green Cr3+, while sulphur is deposited (a pale yellow precipitate):
\[ \text{Cr}_2\text{O}_7^{2-} + 8\text{H}^+ + 3\text{H}_2\text{S} \to 2\text{Cr}^{3+} + 7\text{H}_2\text{O} + 3\text{S} \]
(c)(ii) Precaution: generate and use the hydrogen sulphide in a fume cupboard (fume chamber) so that the poisonous gas is drawn away and not inhaled.
