VJC Prelim P2_ANS

 VJC 2015 9647/02/PRELIM/15 [Turn over 1 Victoria Junior College 2015 H2 Chemistry Prelim Exam 9647/2 Suggested Answers 1 Planning You are provided with 5 unlabelled bottles containing pure natural acids from the extracts of roasted coffee. Each bottle contains one of the following natural acids:  lactic acid,  maleic acid,  oxalacetic acid,  pyruvic acid,  quinic acid, All the acids above are soluble in water. You are also provided with any other common laboratory reagents and apparatus. (a) All natural acids contain one or more c arboxylic acid functional groups. Other than the carboxylic acid functional group, what other functional groups are also present in these natural acids? lactic acid: secondary alcohol maleic acid: alkene oxalacetic acid: ketone pyruvic acid: ketone quinic acid: Secondary alcohol and tertiary alcohol [2] (b) Two of these natural acids are colourless liquids and the rest are white crystalline solids. Using relevant chemical knowledge, identify the two liquids. Explain your choices. The two liquids are lactic acid and pyruvic acid. Explanation: These two compounds have a relatively smaller Mr and thus weaker dispersion forces between their respective molecules than the other 3 compounds (due to smaller number of electrons present). [2] OH CO2H HO HO HO CH3COCO2H HO2CCOCH2CO2H CH3CH(OH)CO2H HO2CCH=CHCO2H

 VJC 2015 9647/02/PRELIM/15 [Turn over 2 (c) Suggest a reagent that could be used to carry out a test -tube ex periment to distinguish the two liquids. Reagent: Brady’s reagent [or 2,4-dinitrophenylhydrazine] Describe what would be observed for each compound in the experiment. Observation: Pyruvic acid will give an orange ppt. but lactic acid will not. [2] (d) Outline a logical sequence of chemical tests that would enable you to identify the remaining 3 solids. You should aim for a minimum number of reactions. Your plan should include  a positive test to confirm the identity of each compound;  detailed procedure (including quantities of chemicals and conditions used);  expected observations for each compound in each test. Step 1: Prepare aqueous so lutions of each of the 3 solid samples by dissolving 1-cm depth of solid in about 5 cm depth of deionised water in each test-tube. Step 2: To 1 -cm depth of each of the 3 samples in separate test -tubes, add equal volume of Brady’s reagent. Oxalacetic acid will give an orange ppt. while the other 2 samples will not. Step 3: To 1 -cm depth of each of the 2 remaining samples in separate test-tubes, add a few drops of Br2(aq). Maleic acid will decolourise reddish -brown Br 2 while the remaining sample will not. Step 4: To 1-cm depth of the last sample in a test-tube, add a few drops of acidified KMnO4(aq). Heat the mixture in a hot water -bath