TJC H2 CHEM P2 QN&ANS Prelim

1 9647/02/TJC Prelim 2014 [Turn Over For Examiner’s Use s CANDIDATE NAME CIVICS GROUP / CENTER NUMBER S INDEX NUMBER CHEMISTRY 9647/02 Paper 2 Structured Questions 29th August 2014 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your Civics Group and name on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions. 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. This document consists of 22 printed pages. For Examiner’s Use 1 /12 2 /8 3 /10 4 /16 5 /10 6 /16 Total PRELIMINARY EXAMINATIONS HIGHER 2

2 9647/02/TJC Prelim 2014 [Turn Over For Examiner’s Use 1 Planning (P) Chromatography is used to separate mixtures of substances into their components. The components move at different speeds when they are carried by a liquid or a gas called the “mobile phase” over an adsorbent material, called the “stationary phase”. (a) A quick and convenient method of chromatographic analysis for organic compounds is thin - layer chromatography (TLC). The stationary phase is silica gel which is coated onto sheets of glass, and the sheets are cut into thin strips. A solution of a mixture of two organic compounds to be analysed is spotted near the bottom of the plate. The plate is then stood in a closed jar which is saturated with an appropriate solvent as the mobile phase. The reason for covering the jar is to make sure that the atmosphere in the beaker is saturated with solvent vapour. As the solvent travels up the plate by capillary action, the components of the mixture travel upwards at different rates and thus separate. The solvent is allowed to rise until it almost reaches the top of the plate (indicated by the solvent front), giving a maximum separation of the mixture components. The TLC plate is then removed from the jar for further analysis. Figure 1: TLC analysing a solution of a mixture of two organic compounds (i) A pencil line is drawn near the bottom of the TLC plate. Suggest a reason why any labeling on the TLC plate has to be drawn in pencil and not in ink.  If any labeling were to be done in ink, dyes from the ink would also move and separate as the chromatogram developed. Cover Solvent front Solvent (0.5cm depth) Baseline above the level of the solvent TLC Plate Spots on TLC Plate

3 9647/02/TJC Prelim 2014 [Turn Over For Examiner’s Use Silica gel is a form of silicon dioxide. At the surface of the silica gel, the silicon atoms are attached to –OH groups. The diagram below shows a small part of the silica surface. Figure 2: Silica surface (ii) It is noticed that polar compounds travel a shorter distance on the TLC plate than non- polar compounds. In terms of bonding involved, suggest reasons for the above observation. The surface of the silica gel is very polar due to the numerous –OH groups. Polar compounds can form hydrogen bonds OR strong permanent dipole - permanent dipole (pd-pd) interactions with the surface of the silica gel and thus, is carried much slower in the mobile phase. Choosing of an appropriate solvent system is very crucial in a thin -layer chromatography (TLC) analysis. The distance at which the compound travel up the plate depends on 2 factors: how soluble the compound is in the mobile phase (solvent) and how strongly the compound is attracted to the stationary phase. Student A chose hexane as the solvent while Student B chose methanol as the solvent. The results of their thin-layer chromatography (TLC) plates are as follows: Figure 3: TLC plate of Student A Figure 4: TLC plate of Student B (iii) In terms of bonding involved, suggest reasons why the spots on Student A’s TLC plate are close to the baseline while the spots on Student B’s TLC plate are close to the solvent front.  The spots on Student A’s TLC plate are close to the baseline indicate that the solvent chosen, hexane is not polar enough to separate the mixture and the compounds are bonded more strongly to the stationary phase (silica gel) more than the mobile phase (hexane). Baseline Solvent front

4 9647/02/TJC Prelim 2014 [Turn Over For Examiner’s Use  The spots on Student B’s TLC plate are close to the solvent front indicate that the solvent chosen, methanol, is too polar and thus, the organic compounds are both bonded more strongly to the mobile phase (methanol) than the stationary phase (silica gel). (iv) Given the following list of solvent and with reference to both students’ TLC plates in (iii), suggest the most appropriate solvent which can better separate the mixture above. Explain your answer. List of solvent: water, benzene, dichloromethane  Dichloromethane  The polarity of the solvent is more polar than hexane but less polar than methanol. (Intermediate polarity) [6] (b) You are to determine the dye contents of the shell coating of M& M candies through analysis using thin -layer chromatography (TLC). The colours of the shells of M&M candies contain food colouring dye that are FDA approved organic compounds. Write a plan to identify the possible food colouring dye that are present in the shell coating of a sample of blue M&M candies. You may assume that you are provided with:  A sample of a solution containing the shell-coating colouring of M&M candies  Food colouring dye blue #1  Food colouring dye blue #2  TLC plate  Jar with cover  Appropriate solvent Your plan should include details of:  an outline of how thin-layer chromatography (TLC) is to be carried out  a sketch of the TLC plate at the start of the experiment which you will use to determine the food dyes present in M&M shell-coating  how you would determine the food dyes present in the sample of M&M shell-coating.

5 9647/02/TJC Prelim 2014 [Turn Over For Examiner’s Use  1) Fill the jar with the appropriate solvent about 0.5 cm depth. Cover the jar to ensure that the air in the jar is saturated with solvent vapour. 2) Using a pencil, draw a baseline about 1 cm (must be more than the depth of the solvent in the jar) from the bottom.  3) Label the TLC plate as follows:  4) Using forceps, dip the TLC plate into the jar. Cover the jar and allow the solvent to move up the TLC plate. Remove the TLC plate from the jar when the solvent reaches about the top of the plate. 5) Compare and match the spots of the various food dyes and the components present in the M&M shell-coating travelled on the TLC plate.  6) The shell -coating of the M&M candies would contain food -dyes that move the same distance as the various food-dyes sample. E.g. If M&M has a spot which move the same distance as B#1, then the shell -coating