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 [Turn over

AJC/H2 Chemistry/Prelims2008 [Turn over 5 2 (a) The lattice energy of calcium chloride is −2255 kJ mol–1. (i) Use the following values of enthalpy change of hydration to construct a Born-Haber energy cycle and use it to calculate the enthalpy change of solution, ΔHsol for calcium chloride. ΔHhyd / kJ mol–1 Ca2+ –1650 Cl– –381 (ii) How would you expect the temperature to change when calcium chloride is dissolved in water? (iii) By quoting suitable data from the Data Booklet , suggest, with reasons, how the enthalpy change of hydration of Ca 2+ might compare to that of Cu2+. [6] (b) Copper(II) ion forms many complexes. When copper( II) ion coordinates with polydentate ligands, the complex formed has greater stability than one which is formed with comparable monodentate ligands. This is known as the chelate effect. Consider the following complex formation reactions: [Cu(H2O)6]2+(aq) + en(aq) [Cu(en)(H2O)4]2+(aq) + 2H2O(l)I II [Cu(H2O)6]2+(aq) + 2NH3(aq) [Cu(NH3)2(H2O)4]2+(aq) + 2H2O(l) where ethylenediamine (en) = CH2 CH2 H2N NH2 The standard enthalpy changes of reaction, ∆H r {, and the standard entropy changes of reaction, ∆Sr {, for the ligand exchange reactions I and II are as follows. reaction ΔHr { / kJ mol–1 ΔSr { / J K–1 mol–1 I – 54.0 +23.0 II – 46.0 – 8.4 (i) How do the similar values of ∆Hr { for reaction I and II relate to the changes that occur at the molecular level? (ii) Explain why ∆Sr { for reaction I is so different from reaction II. (iii) Calculate the standard free energy changes, ∆Gr {, for reaction I and II and hence explain why ethylenediamine complex exhibits the chelate effect. (iv) “The chelate effect is an entropy effect.” With reference to the Gibbs free energy equation and your answers in (b)(i) , (ii) and (iii), comment qualitatively on the statement. [8]

6 AJC/H2 Chemistry/Prelims2008 [Turn over (c) Another polydentate ligand which can form complexes with transition metal ions is the deprotonated anion of compound X. HS−C(CN)=C(CN)−SH compound X Compound X occurs in two isomeric forms. (i) Draw the displayed formulae of the two isomers of X. (ii) State the type of hybridisation present in the two differently bonded carbon atoms in X? The deprotonated anionic X, however, occurs in only one form when acting as a polydentate ligand. (iii) Suggest a structural formula of the deprotonated anionic X. (iv) Which isomeric form of the deprotonated anionic X acts as a polydentate ligand? Explain why the other isomer cannot be used to form complexes. (v) What type of bond is formed between the anionic ligand X and the transition metal ions in complexes? [6] [Total: 20]

AJC/H2 Chemistry/Prelims2008 [Turn over 7 3 (a) Describe the reactions, if any, of the chlorides of sodium to silicon with water, relating any differences to their bonding. Give equations for any reactions and sketch a graph to show the pH values of the resulting solutions. [5] (b) When aqueous sodium hydroxide is added to an aqueous solution of Al Cl 3, a white precipitate is formed which dissolves in excess sodium hydroxide. (i) Write an equation for the formation of the white precipitate. (ii) State what you would observe when the above procedure is repeated using aqueous MgCl2. Explain the difference(s) in the observations. [3] (c) Lithium aluminium hydride, LiA lH4, is prepared from the reaction between AlCl3 and lithium hydride, LiH. AlCl3 + 4LiH LiAlH4 + 3LiCl LiAlH 4 and sodium borohydride, NaBH 4, are commonly used to reduce the carbonyl group of an aldehyde or a ketone, but not alkenes. Both compounds carry out the reduction by transferring a hydride ion, H -, to the carbonyl carbon. However, LiAl H4 is a more powerful reducing agent than NaBH4 as it can also reduce carboxylic acids and esters. In the following conversion, LiAlH 4 is used to reduce a ketone and carboxylic acid. (i) By considering the bonding in LiAlH 4 and NaBH4, suggest a reason why LiAlH4 is a more powerful reducing agent. (ii) What is the role of the H- ion in the reduction process? Suggest a reason why LiA lH4 and NaBH4 cannot reduce alkenes. (iii) Write the structural formulae of A, B and C. (iv) State the reagents and conditions needed for reactions II to IV. [8] (d) An ester is reduced by LiAlH4 to two alcoh