NJC Prelim CHEM P3

NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 3 Free response 9647/03 Mon 14 Sept 2015 2 hour READ THESE INSTRUCTIONS FIRST Answer any four questions. A Data Booklet is provided. The use of an approved scientific calculated is expected, where appropriate. You are reminded of the need for good English and clear presentation in your answers. At the end of the examination, fasten all your work securely behind the cover page. The number of marks is given in brackets [ ] at the end of each question or part question. This paper consists of 10 printed pages.

NJC Preliminary Exam 9647/03/15 [Turn over 2 Answer any four questions. 1 A student analysed a can of beer to verify its alcohol content. The alcohol content is the percentage by volume of pure ethanol in the beer at room temperature. The alcohol content printed on the can is 4.2 %. The analysis is carried out by titrating 10.0 cm 3 of the beer with 0.20 mol dm −3 acidified potassium manganate( VII). It is found that 17.80 cm 3 of potassium manganate( VII) is required to reach the end-point. (a) (i) Write an overall equation for the reaction between potassium manganate( VII) and ethanol. (ii) Using the titration result, calculate the number of moles of potassium manganate(VII) reacted and hence calculate the mass of ethanol in 10.0 cm 3 of the beer sample. (iii) Given that the density of ethanol is 0.789 g cm−3, determine the alcohol content of this beer sample. (iv) Give a reason why the alcohol content of the beer calculated is different from that printed on the can. (v) Explain how you would determine the end-point of this titration. (vi) Ethanol is highly soluble in water. Explain why the solubility of alcohols in water decreases with increasing carbon chain lengths. [9] (b) In the brewing of beer, yeast is added to break down the starch from malt or barley into glucose, C 6H12O6, which is further converted to ethanol and carbon dioxide. The structure of glucose is as shown: (i) Write a balanced equation for the conversion of glucose to ethanol. (ii) Describe a simple chemical test to distinguish between glucose and ethanol. Give an equation for the reaction. [4]

NJC Preliminary Exam 9647/03/15 [Turn over 3 (c) Ethanol can be used to synthesise ether. Due to the low reactivity of ether, it is a suitable solvent for many chemicals reactions. One method to synthesise ether, RCH 2OCH2R’, is through the Williamson ether synthesis as shown in the equation below. RCH2O– + R’CH2Cl → RCH2OCH2R’ + Cl – (i) Describe the mechanism for the above reaction, including curly arrows showing the movement of electrons and all charges. (ii) Suggest a synthetic route for the formation of diethyl ether, CH 3CH2OCH2CH3, using ethene as the only organic reactant. [7] [Total: 20]

NJC Preliminary Exam 9647/03/15 [Turn over 4 2 The earthquake that hit Nepal in April 2015 was the worst natural disaster to strike Nepal since 1934. Hundreds of thousands of people were made homeless. During an earthquake, the heat generated by the friction of the fault in the Earth’s crust may be large enough to decompose the mineral rocks present in the fault, releasing chemicals like sulfur dioxide, SO2, and aluminium oxide, Al2O3. One method for the removal of SO 2 is to absorb it in a slurry of magnesium oxide, to produce magnesium sulfite, MgSO3. (a) (i) Name the type of reaction between magnesium oxide and sulfur dioxide and write a balanced equation for the reaction. (ii) Based on your knowledge of chemic al periodicity, suggest whether A l2O3 can also be used to remove sulfur dioxide. [2] (b) (i) State and explain the pH of the solutions formed when MgO, A l2O3 and SO 2 are separately added to water. (ii) When an excess of aqueous sodium hydroxide was added to a solid mixture of MgSO3 and A l2O3 and filtered, a white residue and colourless filtrate were obtained. Write an equation to show the formation of the species in the filtrate. [4] (c) Aluminium is the third most abundant element found as minerals in the Earth’s crust. Its compounds are of importance in the synthesis of many organic compounds. (i) Compounds A and B below can be synthesised using two different aluminium-containing compounds in Steps 1 and 2. CH2CH(CH3)CH2OH Step 1 A Step 2 CH2C(CH3)=CH2 Br B Give the reagents and conditions for Steps 1 and 2. (ii) Describe the mechanism in Step 1 , including curly arrows showing the movement of electrons and all charges. (iii) Trimethylaluminium, A l(CH3)3, is an important precursor for the Shrock carbene, (C 5H5)2Ti=CH2. The Shrock carbene can then be used to convert carbonyl compounds to alkenes in one step: (C5H5)2Ti=CH2 + O R R' (C5H5)2Ti=O CH2 R R' + Draw the structure of the carbonyl compound that would give compound B when reacted with Shrock carbene. [6]

NJC Preliminary Exam 9647/03/15 [Turn over 5 (d) Aluminium chloride and magnesium chloride are separately added to two beakers, each containing 20 cm 3 of water. The pH of the solution containing aluminium chloride is 2.65. (i) Given that the pK a of A l3+(aq) is 5.01, write the K a expression of A l3+(aq) and calculate the mass of AlCl3 used. (ii) Explain how the pKa of Mg2+(aq) would compare to that of Al3+(aq). (iii) A student suggested that an aqueous solution containing Mg 2+ and A l3+ ions can be separated by adding an appropriate concentration of aqueous sodium phosphate, Na3PO4. Use the data given in the table below to calculate the concentration of sodium phosphate necessary for an effective separation of a solution containing 0.10 mol dm −3 Mg2+ and 0.50 mol dm −3 Al3+ ions, and briefly describe how the separation is to be carried out. salt Ksp (at 25oC) Mg3(PO4)2 1.3 × 10−16 AlPO4 6.3 × 10−19 [8] [Total: 20]

NJC Preliminary Exam 9647/03/15 [Turn over 6 3 Use the following data on Group II elements to answer (a) – (c) of this question. Eo [M2+(aq)/M(s)] / V metallic radius / nm ionic radius / nm ionic speed / mm s–1 Mg –2.38 0.160 0.065 0.549 Ca –2.87 0.197 0.099 0.616 Sr –2.89 0.215 0.113 0.616 Ba –2.90 0.217 0.135 0.658 (a) Describe and explain the relative reactivities of Mg and Ba with water. [2] (b) (i) Write a balanced equation for the thermal decomposition of strontium nitrate. (ii) Using data provided in the table, describe and explain how the thermal stabilities of the nitrates of the Group II metals vary from magnesium to barium. [3] (c) Ionic speeds show how fast aqueous ions move under standard conditions. Explain why the ionic speeds of calcium and strontium ions are identical even though their ionic radii are different. [2] (d) The presence of lone pair of electrons on the O of alkoxides (RO –) enables them to act as either bases or nucleophiles in organic reactions. 2-Bromopropane reacts with NaOCH 3 in methanol according to the following equation: CC H 3 CH3 Br H + CH3O CC CH3 H H H + Br + CH3OH (i) Name the