2019 SAJC H2 Chem Prelim P4 ANS

[Turn Over H2 Chemistry SAJC Prelims Paper 4 Review 1 To standardise a 0.03 mol dm –3 of iron( III) chloride solution and determine the concentration of sodium thiosulfate solution. (a) Planning FA1 is a standard solution containing 0.03 mol dm–3 of FeCl3 solution. You are to design an experiment to show how FA1 is prepared using solid FeCl3. You are provided with : a solid sample of approximately 1.50 g of FeCl3 (Mr = 162.3), 250 cm3 volumetric flask and the usual laboratory apparatus In your plan, you should include details on:  the mass of FeCl3(s) you use with justification,  the apparatus you would use, and  the procedures which you would follow. Do not carry out this plan as FA1 is provided for you to continue with 1(b). To make standard solution, a 250 cm3 volumetric flask is used. Amount of FeCl3 in 250 cm3 = 0.03 x 250 / 1000 = 7.50 × 10–3 mol Mass of FeCl3 = 7.50 × 10–3 x 162.3 = 1.22 g Procedure: 1. Weigh accurately about 1.22 g (2 d.p) of FeCl3 into a dry clean weighing bottle. 2. Dissolve the solid using distilled water (~ 50 cm3) in a 100 cm3 beaker. 3. Transfer the solution after ensuring all the solid has dissolved. Rinse the weighing bottle and beaker with water and transfer all washings into a 250 cm 3 volumetric flask to ensure quantitative transfer 4. Top up the solution with distilled water to the 250 cm3 mark. 5. Stopper and shake well to obtain a homogenous solution. Label this solution FA 1. procedures 1-5. [3]

2 (b) In addition to FA1, you are also provided with the following: FA2 is 0.060 mol dm–3 potassium iodide, KI. FA3 is aqueous sodium thiosulfate, Na2S2O3. When FA1 is combined with FA3, a complex, [Fe(S2O3)2]–, is formed. Reaction 1: Fe3+(aq) + 2S2O32–(aq) ⇌ [Fe(S2O3)2]–(aq) To 1 cm3 of FA3 in a test–tube, add 2–3 drops of FA1 and shake them thoroughly. State the colour of the complex formed. (Pale) purple/ lilac /Pink/ violet. [1] (c) A student aimed to determine the concentration of FA3 using volumetric analysis involving the use of FA1 and FA2. Reaction 2: 2Fe3+(aq) + 2I–(aq) → 2Fe2+(aq) + I2(aq) Reaction 3: I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq) Using the above reactions, the student planned and carried out the following procedure. 1. Fill a 50.00 cm3 burette with FA3 solution. 2. Using a 10 cm3 measuring cylinder, measure 5 cm3 of FA1 and place it in a 100 cm3 conical flask. 3. Using another 10 cm 3 measuring cylinder, measure 5 cm 3 of FA2 into the conical flask. 4. Run FA3 from the burette into the conical flask. Near the endpoint, when the brown solution becomes pale, add 2 drops of starch. 5. Continue adding FA3 slowly, the endpoint is reached when th e solution first becomes colourless. 6. Record the titration results, to an appropriate level of precision, in an appropriate table and repeat the experiment once more. (i) Carry out the student’s procedure twice only to obtain two titration readings. Your titre values need not be consistent. Note: After the end point is reached, you may observe the appearance of the coloured mixture again. There is no need to titrate any further.

3 [Turn Over Record your titration in an appropriate table in the space below. 1 2 Final burette reading/ cm3 14.50 28.60 Initial burette reading/ cm3 0.00 14.50 Volume of FA3/ cm3 14.50 14.10 [2] (ii) Planning A teacher commented that it is appropriate for the student to use 5 cm 3 instead of 25 cm 3 of FA1 for this titration. Justify the teacher’s comment using suitable calculations. Assume the concentration of FA3 is 0.006 mol dm–3. Reaction 2: 2Fe3+(aq) + 2I–(aq) → 2Fe2+(aq) + I2(aq) Reaction 3: I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq) No. of moles of 5 cm3 of FA1 = 5 / 1000 x 0.03 = 1.5 × 10–4 mol No. of moles of I2 = 1.5 × 10–4 / 2 = 7.5 × 10–5 mol No. of moles of S2O32– = 7.5 × 10–5 x 2 = 1.5 × 10–4 mol If the concentration of FA3 is 0.006 mol dm–3, Approximate titre volume = 1.5 × 10–4 / 0.006 x 1000 = 25 cm3 OR If 25 cm3 of FA1 = 25/ 1000 x 0.03 = 7.5 × 10–4 mol No. of moles of S2O32– = 7.5 × 10–4 mol Approximate titre volume = 7.5 × 10–4 / 0.006 x 1000 = 125 cm3 If 25 cm 3 of FA1 is used, the titre value will be 5 times (125 cm 3) and exceeds the capacity of the burette . It results in high inaccuracy as re filling of burette is required. [2]

4 (iii) Planning Justify whether the amount of FA2 that the student used in step 3 of the procedure in 1(c) is sufficient to determine the concentration of FA3. No. of moles of KI required = 1.5 × 10–4 mol Minimum volume of FA2 = 1.5 × 10–4 / 0.06 x 1000 = 2.5 cm3 To ensure excess FA2 (KI), add 5 cm3 of KI (accept 3 – 5 cm3). [1] (iv) The student did the titration several times but could not get consistent titre values of FA3. Based on your observation during titration, suggest a reason why. Reaction 1: Fe3+(aq) + 2S2O32–(aq) ⇌ [Fe(S2O3)2]–(aq) Reaction 2: 2Fe3+(aq) + 2I–(aq) → 2Fe2+(aq) + I2(aq) Reaction 3: I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq) After the dark blue colouration disappear to get the colourless solution, the purple colour reappeared because the position of equilibrium of reaction 1 shift left due to the decrease in [S 2O32–] during the titration. This result in the increase in the [Fe3+] which reacts with excess KI to produce more I2. The presence of I2 in starch is revealed as the reappearance of blue black colouration . Hence, it is difficult to judge the correct end point and titration results is consistent. [1] (v) Suggest another reason why the results obtained are not reliable. The limiting reagent FeC l3 was measured using a 10 cm 3 measuring cylinder which has a low precision. Hence, the amount measured in each titration may vary slightly. OR The volume of FA1 and FA2 used are small, this would results in high percentage error. [1] (vi) The student performed the titration and had a titration value of 18.50 cm3. Given the errors (uncertainties) associated with each reading using a measuring cylinder and burette are 0.1 cm 3 and 0.05 cm 3 respectively, c alculate the maximum total percentage error (uncertainty) from the apparatus in the titration.

5 [Turn Over Percentage uncertainty (error) in using the measuring cylinder = ( 0.1 / 5.0) x 100 % =  2.00 % Each measuring cylinder reading has an uncertainty (error) of  2.00 %. Since two readings using measuring cylinder are made, total percentage error from measuring cylinder is 2.00 x 2 =  4.00 % Percentage uncertainty (error) in using the burette = (0.05 x 2) 18.50 x 100 % =  0.541 % Hence maximum percentage error (uncertainty) from the apparatus in the experiment = ( 4.00) + ( 0.541) =  4.54 % (3 significant figures) [3] [Total: 14 marks] 2 To investigate the kinetics of the reaction between iron(III) ions and iodide ions In this experiment, you are investigating the rate of reaction between iron(III) ions and iodide ions. You are provided with: FA1 is 0.03 mol dm–3 of FeCl3 solution FA2 is 0.060 mol dm–3 potassium iodide, KI FA3 is sodium thiosulfate, Na2S2O3 The reaction is started by mixing a solution of iron(III) chloride with sodium thiosulfate, potassium iodide, and starch. The iodine, I2, produced in reaction 2 reacts immediately with thiosulfate ions, S2O32– in reaction 3. Reaction 1: Fe3+(aq) + 2S2O32–(aq) ⇌ [Fe(S2O3)2]–(aq) Reaction 2: 2Fe3+(aq) + 2I–(aq) → 2Fe2+(aq) + I2(aq) Reaction 3: I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq) When all the thiosulfate have been used, the iodine produced will turn starch indicator blue–black. The rate of the reaction can therefore be determined by finding the time for the blue–black colour to appear.

6 You are advised to read the instructions before starting any practical work and draw a