AJC_Prelims H2 Chem Paper 3_Question paper

3 Answer any four questions. 1 (a) One property of manganese that typifies it as a transition element is its ability to exhibit variable oxidation states. What features of manganese allow it to do so? [2] (b) Manganese commonly occurs as the ore py rolusite, which is mainly manganese(IV ) oxide. This is the most usual starting material for the production of compounds of manganese in other oxidation states. Manganese(IV) oxide undergoes a 2-step reaction to produce compounds Y and Z. MnO2 MnO4 2- oxidation in alkaline medium hot water / CO2 gas III brown black solid Y in a purple solution Z (i) The brown black solid Y contains 63.8% by mass of manganese and 36.2% by mass oxygen. Using these information, determine the empirical formula of Y. (ii) Suggest the identity of Z and state the type of reaction that occurs at step II. Construct a balanced equation for the reaction. (iii) With reference to your answer in (b)(ii), explain the role of bubbling carbon dioxide gas into the hot solution of MnO4 2-. [7] (c) Half a million tonnes of manganese( IV) oxide are used each year in dry cell batteries. A particular dry cell, consisting of a sodium anode and a graphite cathode immersed in a polymer electrolyte, has been developed to detect gases like fluorine and chlorine. The graphite cathode is coated with solid manganese( IV) oxide which will be converted to MnO(OH) solid and hydroxide ions during discharge. (i) Write the ion-electron equations for the reaction which occurs at the anode and cathode respectively. (ii) The voltage of this sodium-carbon dry cell is found to be 1.0 V. With reference to the Data Booklet, estimate approximately the E{ of the manganese half cell, explaining how you arrive at your answer and stating any assumptions you make. [4] AJC/H2 Chemistry/Prelims2008 [Turn over

4 (d) Manganese(IV) oxide catalyses the decomposition of hydrogen peroxide. 2 H2O2 2 H2O + O2 The volume of oxygen gas liberated from a sample of hydrogen peroxide contaminated with some manganese(IV ) oxide was measured every minute. The results are tabulated below. Time / min 0 1 2 3 4 5 6 7 Total volume of O2 / cm3 0 23.0 36.5 46.0 51.0 54.5 57.0 58.5 (i) Use a graphical method to deduce the order of reaction with respect to hydrogen peroxide. Explain your answers as fully as you can. (ii) Calculate the rate constant for the reaction, stating its units. (iii) What effect will the presence of the manganese( IV) oxide catalyst have on the rate constant for this reaction? Explain your answer. [7] [Total: 20] AJC/H2 Chemistry/Prelims2008