MI H2 CHEM P2 Prelim

Class Adm No Candidate Name: This question paper consists of 19 printed pages and 1 blank page. 2015 Preliminary Examination II Pre-university 3 H2 CHEMISTRY 9647/02 Paper 2 Structured Questions 16th Sept 2015 2 hours Candidates answer on the Question paper. Additional materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so Write your name, class and admission number on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. Question 1 2 3 4 5 Total Marks 12 10 24 15 11 72

2 1 Planning (P) Ascorbic acid is a naturally occurring organic compound commonly found in citrus fruits and Vitamin C supplements. It is sometimes called an ‘anti-oxidant’ or a reducing agent. Ascorbic acid is water soluble. Direct iodometric titration can be used to determine the amount of ascorbic acid in samples of juices and Vitamin C supplements. Iodine rapidly oxidises ascorbic acid, C6H8O6, to produce dehydroascorbic acid, C6H6O6. C6H8O6 + I2 → C6H6O6 + 2I– + 2H+ In the direct iodometric titration, the ascorbic acid is titrated against an iodine solution of known concentration using starch as an indicator. Once all the ascorbic acid has been oxidised, the excess iodine is free to react with the starch indicator. A student was given a Vitamin C sample which the ma nufacturer claims to contain 0.25 g of ascorbic acid per tablet. She was tasked to plan an experiment to determine if the manufacturer’s claim was true. The following reagents were provided: FA 1 0.00500 mol dm-3 I2 solution FA 2 starch indicator (a) State the colour at the end-point of the titration. ………………………………………………………………………………………………………… [1] (b) The student was provided with a 250 cm 3 volumetric flask to prepare an ascorbic acid solution using one Vitamin C tablet. The solution was labelled FA 3. Assuming that the manufacturer’s claim is true, calculate the concentration of ascorbic acid in FA 3. [1]

3 [Turn over (c) Given that the average titre volume should be in the range of 10.0 to 30.0 cm 3, determine the minimum volume of FA 3 that should be used in the experiment. Hence, suggest a suitable pipette volume for titration. [3] (d) Write a plan for the direct iodometric titration of ascorbic acid sample FA 3. You may use the reagents and apparatus normally found in a school or college laboratory. Your plan should include:  details for the preparation of FA 3 solution;  the essential details of the titration procedure;  how the results should be tabulated. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. …………………………………………………………………………………………………………….

4 ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………….....[7] [Total: 12]

5 [Turn over 2 Group II elements are known as alkaline earth metals. They have similar chemical properties and are soft silver metals. (a) The ionisation energy of Group II elements varies down the group. (i) Write an equation to represent the first ionisation energy of calcium. …………………………………………………………………………………………………… [1] (ii) State and explain how the first ionisation energy of group II elements varies down the group. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. …………………………………………………………………………………………………… [2] (iii) Write the full electronic configuration of a Ca atom and a Ca2+ ion. Ca………………………………… Ca2+…………………………………… [1] (iv) Explain why the ionic radius of Ca2+ is smaller than the atomic radius of Ca. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. …………………………………………………………………………………………………… [2]

6 (b) Group II nitrates decompose on heating. (i) Write a balanced equation to show the thermal decomposition of calcium nitrate. …………………………………………………………………………………………………… [1] (ii) A 1.0 g sample of powdered calcium nitrate is heated at a temperature, T. The graph below shows how the mass of the sample changes with time. Sketch a curve in the graph above to show how you would expect the mass of a 1.0 g sample of magnesium nitrate would change with time when it is heated at the same temperature, T. Explain your answer. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. …………………………………………………………………………………………………… [3] [Total: 10] mass/g 1.0 Time/min Ca(NO3)2 sample

7 [Turn over 3 (a) Carbonyl compounds can undergo acid or base catalysis to form enols. enol Any carbon atom directly attached to the carbonyl carbon (C=O) is designated as an α-carbon. A ny hydrogen atom directly attached to an α-carbon atom is termed as an α-hydrogen. Enols are alkenes with a hydroxyl group attached to one of the carbon atoms of the double bond (C=C–OH). Enols are isomers of al dehydes or ketones in which one α-hydrogen has been removed and replaced on the oxygen atom of the carbonyl group. (i) Draw the structural formula of the enol that could be formed from the acid catalysis of butanal. [1] (ii) Draw the structural formulae of the two other isomers of butanal, CH3CH2CH2CHO, that gives a positive test with 2,4-dinitrophenylhydrazine. A B [2]

8 (iii) Suggest a chemical test that could be used to distinguish the two isomers in (a)(ii). You should state the reagents and conditions as well as observations for each compound. Test: ………………………………………………………………………………………………… Observations: A: …………………………………………………………………………………………………… B: …………………………………………………………………………………………………[2] (b) The incomplete mechanism for the acid catalysed reaction for propanone to form an enol is shown below. Step 1: Step 2: Compound A (i) Complete the mechanism by drawing in the curly arrows in the mechanism above. [2] (