10. Group 2
A section of Chemistry, 9701
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Complete the electronic configurations of the following ions. Cr3+: 1s22s22p6 Mn2+: 1s22s22p6 Both KMnO4 and K2Cr2O7 are used as oxidising agents, usually in acidic solution. Use information from the Data Booklet to explain why their oxidising power increases as the [H+] in the solution increases. What colour changes would you observe when each of these oxidising agents is completely reduced? • KMnO4 from to • K2Cr2O7 from to Manganese(oxide, MnO2, is a dark brown solid, insoluble in water and dilute acids. Passing a stream of SO2through a suspension of MnO2 in water does, however, cause it to dissolve, to give a colourless solution. Use the Data Booklet to suggest an equation for this reaction, and explain what happens to the oxidation states of manganese and of sulfur during the reaction. The pH of the suspension of MnO2 is reduced. Explain what effect, if any, this would have on the extent of this reaction. The main ore of manganese, pyrolusite, is mainly MnO2. A solution of SnCl2 can be used to estimate the percentage of MnO2 in a sample of pyrolusite, using the following method. • A known mass of pyrolusite is warmed with an acidified solution containing a known amount of SnCl2. • The excess Sn2+ions are titrated with a standard solution of KMnO4. In one such experiment, 0.100 g of pyrolusite was warmed with an acidified solution containing 2.00 × 10–3 mol Sn2+. After the reaction was complete, the mixture was titrated with 0.0200 mol dm–3 KMnO4, and required 18.1 cm3 of this solution to reach the end point. The equation for the reaction between Sn2+and MnO4 –is as follows. 2MnO4 – + 5Sn2+ + 16H+ 2Mn2+ + 5Sn4+ + 8H2O Use the Data Booklet to construct an equation for the reaction between MnO2 and Sn2+ ions in acidic solution. Calculate the percentage of MnO2 in this sample of pyrolusite by the following steps. • number of moles of MnO4 – used in the titration • number of moles of Sn2+ this MnO4 – reacted with • number of moles of Sn2+ that reacted with the 0.100 g sample of pyrolusite • number of moles of MnO2 in 0.100 g pyrolusite. Use your equation in . • mass of MnO2 in 0.100 g pyrolusite • percentage of MnO2 in pyrolusite percentage = %
9701_w11_qp_43
THEORY
2011
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