SAJC_H2_CHEM_P3_QNS Prelim

1 [Turn Over 1 (a) Phosphorus pentachloride, PC l5, is a useful reagent in organic synthesis. It can undergo auto-ionisation in polar solvents according to the following equilibrium: 2PCl5 PCl4 + + PCl6 ‒ Draw the dot-and-cross diagrams for PC l4 + and PC l6 ‒ ions and state the shapes around the central atom for each ion. [3] (b) PCl5 exists in equilibrium with PCl3 and chlorine gas as shown below: PCl5 (g) PCl3 (g) + Cl2 (g) ∆H = +124 kJ mol‒1 At equilibrium, at 200 °C and a total pressure of 5 atm, 40% of PCl5 is dissociated. (i) Write an expression for Kp for the above equilibrium. (ii) Calculate a value for Kp at 200 °C and state its units. (iii) Predict and explain the effect of increasing the temperature on the value of Kp. [5] (c) Aluminium oxide is insoluble in water. However, it is able to dissolve in an aqueous solution of PCl5. (i) Explain why aluminium oxide is insoluble in water. (ii) Explain why aluminium oxide can dissolve in an aqueous solution of PCl5 and write balanced equations for any reactions that occur. [4]

2 [Turn Over 1 (d) 5-hydroxy-2-coumaranone can be synthesised using compound F as the starting material. The structural formulae of 5-hydroxy-2-coumaranone and compound F are shown below. HO O O 5-hydroxy-2-coumaranone Compound F Suggest a synthesis to convert compound F into 5-hydroxy-2-coumaranone in not more than five steps. State all reagents and conditions used and all intermediates formed. [4] (e) G and H have the following structure: Describe a chemical test which would distinguish between F and G and a different chemical test to distinguish between F and H. [4] [Total: 20]

3 [Turn Over 2. (a) Hydroxyapatite, Ca 5(PO4)3OH, is a group of phosphate minerals which can be found in the teeth and bones of the human body. Thus, it is commonly used as a filler to replace amputated bone or as a coating to promote bone ingrowth into prosthetic implants. (i) Write an equation to illustrate what is meant by the term enthalpy change of formation of hydroxyapatite. (ii) Using the following information and with the aid of the Data Booklet , construct a Born Haber cycle to calculate the enthalpy change of lattice energy of hydroxyapatite. Value / kJ mol -1 Hformation Ca5(PO4)3OH (s) -12 969 Hformation OH- (g) -230 Hformation PO4 3- (g) -1913 Hatomisation Ca (s) +178.2 (iii) Hydroxyapatite thermally decomposes to produce calcium phosphate, calcium oxide and steam. Write a balanced equation to show the thermal decomposition of hydroxyapatite. Henc e, calculate the mass of calcium oxide that could be obtained by de