Edexcel Jun 2012 Paper 5 Q15

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SECTION C Answer ALL the questions. Write your answers in the spaces provided. 15 Some unusual oxidation states You will probably have seen manganese in its common oxidation states during your study of chemistry. Manganese(II) sulfate occurs as very pale pink crystals in the hydrated form. Manganese(IV) oxide is a black powder which is often used as a catalyst. Potassium manganate(VII) occurs as very dark purple crystals and forms a purple aqueous solution, which is a powerful oxidizing agent. You are less likely to have seen compounds containing the other oxidation states of manganese, which are manganese(VI), manganese(V), manganese(III) and manganese(I). However, compounds containing each of these four oxidation states can be prepared. Manganese(VI) Manganese(VI), in MnO4 reacting manganate(VII) ions with manganese(IV) oxide in alkali. 2, can be prepared in a reverse disproportionation reaction, by 2MnO4 Equation 1 The reaction is not thermodynamically favourable under standard conditions. However, the (aq) + MnO2(s) + 4OH DT 0Q24 2(aq) + 2H2O(l) E 9 Ecell value can be made positive by increasing the concentration of hydroxide ions so that green manganate(VI) ions form. cell = 0.03V Manganese(V) Manganese(V) can be formed by adding a little potassium manganate(VII) to very concentrated (12 mol dm3) aqueous sodium hydroxide. The solution slowly becomes blue as manganate(V) ions, MnO3 (aq), form. The ionic half-equations are: Equation 2 Equation 3 MnO4 (aq) + H2O(l) + 2e0Q23 (aq) + 2OH(aq) 4OH DT +2O(l) + O2(g) + 4e Manganese(III) A deep red solution containing manganese(III) ions is formed by the oxidation of manganese(II) hydroxide by potassium manganate(VII) in acid solution. The ionic equation for the reaction is: MnO4 (aq) + 4Mn(OH)2(s) + 16H+ DT 0Q3+(aq) + 12H2O(l) Equation 4 Manganese(I) Manganese(I) ions are not stable in aqueous solution, but do form stable complex 4, to ions. They can be made by reducing hexacyanomanganate(II) ions, Mn(CN)6 hexacyanomanganate(I) ions, Mn(CN)6 5. 16 *P39309A01620*<br /> (a) (i) Give the formula of manganese(IV) oxide. (1)(ii) How do catalysts speed up reactions? (2)(iii) Explain how transition metal ions can act as homogeneous catalysts. (2)(b) (i) Suggest why the preparation of manganate(VI) ions, MnO4 2, in equation 1, may be described as a reverse disproportionation reaction by considering the relevant oxidation states. (2)*P39309A01720* 17 Turn over<br /> (ii) The two half-equations which are combined to form equation 1 are MnO4 MnO4 (aq) + e U MnO4 2(aq) E 9 = +0.56 V 2(aq) + 2H2O(l) + 2e U MnO2(s) + 4OH(aq) E 9 = +0.59 V Explain, by reference to these half-equations, why increasing the concentration of hydroxide ions alters the electrode potential to make the preparation of manganate(VI) ions more likely. (3)(c) Use equations 2 and 3 to answer the following questions. (i) Identify the gas formed in the preparation of manganate(V) ions. (1)(ii) By appropriately combining these two equations, write the ionic equation for the formation of manganate(V) ions from manganate(VII) ions. (2) (2) (iii) Identify the main hazard and state how you would minimize the associated risk in this preparation of manganate(V) ions.18 *P39309A01820*<br /> (d) Identify the reagents you would use to make manganese(II) hydroxide for the preparation of manganese(III) ions. (2)(e) (i) Draw a dot and cross diagram to show the electron arrangement in the cyanide ion, CN. (ii) Explain how the cyanide ion acts as a ligand. (2) (2)(iii) Suggest the name of the shape of the hexacyanomanganate(I) ion. (1)(Total for Question 15 = 22 marks) TOTAL FOR SECTION C = 22 MARKS TOTAL FOR PAPER = 90 MARKS *P39309A01920* 19<br />

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