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 decomposing 9 g of hydroxyapatite. (iv) Al2PO4(OH)3 is a recently discovered phosphate mineral found in high pressure metamorphic rocks. Suggest, using the Data Booklet, why it has been found that A l2PO4(OH)3 decomposes at a lower temperature than hydroxyapatite. [8]

4 [Turn Over 2 (b) An organophosphate is the general name for esters of phosphoric acid. Acephate is an organophosphate which is used in many insecticides and herbicides. Acephates are acutely toxic where recent studies suggest a possible link to adverse effects in the neurobehavioral development of foetuses and children. (i) Experts have recommended that the concentration of acephate in fruits should not exceed 7 ppm (1 ppm = 1 mg of acephate in 1 kg of fruits). Calculate the maximum mass in grams of acephate that a fruit weighing 50 g can contain before it is deemed hazardous. (ii) Alkyl phosphates are phosphate esters that are formed when 1 mole of phosphoric acid combines stepwise with 3 moles of an alcohol as shown in the reaction below. State the type of reaction in the formation of the phosphate esters.

5 [Turn Over 2 (b) (iii) Alkyl phosphates play a central role in biochemistry. Phosphate ester linkages compose the backbone of the nucleic acid, RNA, which performs multiple vital roles such as genetic expression. Ribose, an alcohol, forms part of the RNA backbone. Using the segment of the RNA molecule above and (b)(ii), suggest the structural formula of the alcohol which reacted with phosphoric acid. (iv) Name three functional groups in the segment of the RNA molecule. [6]

6 [Turn Over 2 (c) An aldose is an alcohol that contains only one aldehyde group per molecule. The Killiani-Fischer synthesis lengthens an aldose carbon chain by adding one carbon to the aldehyde end of the aldose. An example of the synthesis of tetrose from triose is illustrated below. (i) Outline the mechanism of the reaction in Step I. (ii) Tetrose is reacted with NaBH4 to form compound X. Tetrose is reacted with acidified potassium dichromate to form compound Y. Using the structural formulae of X and Y , write balanced equations to explain each of the above reactions. [6] [Total: 20 marks]

7 [Turn Over 3 (a) Electrophoresis is a technique used to separate a mixture of amino acids. Often, this process is carried out using a gel which is soaked in a buffer solution to ensure pH is kept at a relatively constant value. A small well is etched on the gel and the solution containing the mixture of amino acids is dropped carefully in the well. An electric field is then applied across the gel. The table below shows three amino acids to be separated by electrophoresis and their isoelectric points. Amino acid Formula of side chain (R in RCH(NH 2)CO2H) Isoelectric point Cysteine (Cys) ‒CH2SH 5.07 Lysine (Lys) ‒(CH2)4NH2 9.74 Serine (Ser) ‒CH2OH 5.58 (i) Explain what is meant by a buffer solution. (ii) Draw the predominant species of each amino acid when the pH of the buffer solution is 7.20. (iii) Illustrate two ways in which vinegar might interact with a protein that contains the three amino acids in the table above to bring about denaturation. You may find that including sketches or diagrams will help you in your answer. [8] (b) Keratin is an important protein which is a major component in hair. Keratin contains large amounts of cysteine which gives str ength to our hair. Perming hair involves creating permanent waves in hair. This process includes forming new disulfide bonds via oxidation. Hydrogen peroxide is often used as the oxidising agent. (i) Briefly explain how the presence of cysteine in keratin strengthens hair. (ii) Using the Data Booklet, write the half-equations involved in the formation of new disulfide bonds using hydrogen peroxide. [3]

8 [Turn Over 3 (c) Hydrogen peroxide can also act as a reducing agent. It is used to react with sodium hypochlorite, NaClO, to produce oxygen gas. The resulting solution is then reacted with aqueous silver nitrate and a white precipitate is seen. The white precipitate disappears on addition of dilute aqueous ammonia. (i) Using the Data Booklet, write the overall equation that produces oxygen gas. (ii) Identify the white precipitate and explain its subsequent disappearance with the aid of relevant equations. (iii) Sodium hypochlorite can be prepared using chlorine in the laboratory. Suggest a reason why this preparation should not involve heating. (iv) A 250 cm 3 solution contains 0.05 mol dm ‒3 silver nitrate and 0.04 mol dm ‒3 copper(I