2024 TMJC JC2 H2 Chemistry - Prelim P4 (Suggested Solutions)

Answer all questions in the spaces provided. 1 Investigation of some inorganic and organic reactions In this question, you will be investigating some inorganic and organic reactions by carrying out the stipulated tests. Carefully record your observations in Tables 1.1 and 1.2. Unless otherwise stated, the volumes given below are approximate and should be estimated rather than measured. (a) Organic analysis In this question, you will deduce the structure of an organic compound, FA 1. FA 1 has the molecular formula C4H8O2 with two functional groups present. Do not carry out the tests for which observations have been recorded. Do not use the Bunsen burner for heating in this part of the question. Instead, use the hot water provided. Table 1.1 tests observations (i) Place about 2 cm depth of aqueous sulfuric acid in a test-tube. To this test -tube, add about 1 cm depth of FA 1, followed by 1 drop of aqueous potassium manganate(VII). Warm the mixture in the hot water bath for two minutes.  Purple KMnO4 decolourises / turns colourless. (ii) Add 1 cm depth of aqueous silver nitrate to a test-tube. Then slowly add 1 cm depth of aqueous sodium hydroxide. Add aqueous ammonia slowly, with shaking, until the precipitate just dissolves. To this mixture, add about 1 cm depth of FA 1. Place the test-tube containing the mixture in the water bath for two minutes.  Black / b rown / grey ppt formed  Black / b rown / grey ppt dissolves to give a colourless solution  No silver mirror observed. (iii) Place about 1 cm depth of FA 1 in a test -tube. To this test -tube, add 2,4 -dinitrophenylhydrazine dropwise. Orange ppt. formed (iv) Place about 1 cm depth of FA 1 and add 8 drops of aqueous sodium hydroxide in a test -tube. Then, add aqueous iodine dropwise, until a permanent yellow / orange colour is obtained. Warm the mixture in the hot water bath for two minutes.  Brown / Orange solut ion decolourises.  Pale yellow ppt observed. (v) Place 1 cm depth of FA 1 in a test-tube. To this test-tube, cautiously add a small piece of anhydrous phosphorus pentachloride. Dense white fumes, which turns moist blue litmus paper red. [2] M1 M2 4 to 6 marking points – [M1] and [M2] 2 to 4 marking points – [M1] only Markers’ Comments o Some candidates were unfamiliar with the expected observations for these common reagents used in Organic Qualitative Analysis. o Many candidates omitted the observations that accompanied the preparation of Tollens’ reagent. Candidates also wrongly wrote “no observable change” for the negative Tollens’ reagent test, even though it was not permissible to write so.

2024 H2 Chemistry Paper 4 (Suggested Solutions) 2 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H2 Chemistry (b) (i) Observations from (a)(ii) and (a)(iii) can be used to identify one of the functional groups present in FA 1. Identify the functional group and explain your answer, showing clearly your reasoning. functional group: ketone explanation: [1] (ii) Using observations from either ( a)(iv) or (a)(v), identify the other functional group present in FA 1. Quote evidence from the relevant test to support your conclusion. functional group: alcohol evidence: [1] (iii) Suggest a possible structure of FA 1 that are consistent with all the observations in Table 1.1. OR [M5] [1] M3 M4 M5 Since FA 1 undergoes condensation with 2,4 –DNPH in (a)(iii), FA 1 contains either a ketone or aldehyde. However, since FA 1 does not undergo oxidation with Tollens’ reagent in (a)(ii) and it does not contain an aldehyde. [M3] Since FA 1 undergoes nucleophilic substitution with PCl5 in (a)(v) to give HCl, it contains an alcohol functional group. [M4]  Molecular Formula: C4H8O2  Contains ketone & alcohol functional group  Contains CH3 C O AND / OR CH3 C OH H Markers’ Comments o Some candidates were unfamiliar with the requirements of this question. They did not indicate o type of reaction with the reagent o reference to the test / reagent o reference to the observations o Some candidates also failed to consider the molecular formula while proposing the functional group present and wrongly proposed having carboxylic acid with a ketone. Markers’ Comments o This question was generally well attempted by candidates who deduced the correct functional groups in the earlier part. o Some candidates failed to account for the molecular formula while proposing the structural formula. Common mistakes include omitting hydrogen atoms / not having a structure with correct bonding.

(c) Inorganic analysis FA 2 is an aqueous solution containing Fe3+. FA 3 contains one cation and FA 4 contains one anion listed in the Qualitative Analysis Notes on pages 23 – 24. You should indicate clearly at what stage in a test a change occurs. Test and identify any gases evolved. If there is no observable change, write no observable change. At each stage of any test, you are to record details of the following.  colour changes seen  the formation of any precipitate  the solubility of such precipitate in an excess of the reagent added No additional tests for ions present should be attempted. Table 1.2 tests observations (i) Test the FA 2 solution using Universal Indicator paper.  Universal indicator paper turns o red (pH 1) o orange-red (pH 2) o orange (pH 3) Must include colour and pH of UI paper (ii) To 1 cm depth of FA 2, add 1 cm depth of hydrochloric acid followed by a small spatula of zinc powder. Shake well. Leave the mixture to stand for five minutes.  Effervescence observed.  H2(g) “pops” with a lighted splint / extinguishes a lighted splint with a pop. Must include identity of gas and the confirmatory test for gas  Yellow solution decolourises / fades / becomes lighter in colour. (iii) To 1 cm depth of FA 2, add a few drops of aqueous sodium thiosulfate with shaking. Leave the mixture to stand.  Yellow solution turns purple/ dark violet / brown.  Solution turns back to yellow. [2] M6 M7 4 – 6 marking points – [M6] and [M7] 2 – 3 marking points – [M6] only Markers’ Comments o This question was not well attempted by candidates. Many candidates missed out on the required observation points that they should be familiar with by now. o Some candidates also did not carry out the procedur e, e.g. not following the specified quantities of reagent specified, which resulted in erroneous observations ; not observing again after leaving the mixture to stand.

2024 H2 Chemistry Paper 4 (Suggested Ans) 4 Tampines Meridian Junior College 2024 JC2 Preliminary Examinations H2 Chemistry tests observations (iv) To 1 cm depth of FA 3, add an equal volume of aqueous potassium iodide, then  white / off-white / cream ppt formed  brown / yellow-brown / orange-brown / red-brown solution formed add aqueous sodium thiosulfate dropwise with shaking until no further change is observed.  Brown solution fades / decolourises / turns lighter / turns pale yellow / pale brown etc.  Cream / white / off-white ppt observed. (v) To 1 cm depth of FA 3, add aqueous ammonia slowly, with shaking, until no further change is seen.  (pale) blue ppt formed  ppt soluble in excess NH3 to give a dark blue solution [3] (d) (i) Explain your observations in (c)(i), given that the cation present in FA 2 is Fe3+. [1] (ii) State the type of reaction that is likely to have occurred in (c)(iii). [1] Fe3+ has high charge density and can distort the electron cloud of the surrounding H2O molecules, weakening and breaking the O –H bond , thus releasing H+. The hydrolysis of Fe3+ in water accounts fo