Copy of VS 2024 Sec 4 Chem Prelim Paper 3 Answers

1 Victoria School Secondary 4 Preliminary Examination 2024 Suggested Answer to Chemistry Paper 3 Qn Skill Answer Mark 1(a) PDO MMO Results table: experiment volume of P /cm3 volume of Q /cm3 initial temperature of P /°C highest temperature of mixture /°C temperature rise /°C 1 40.00 10 31.5 34.0 2.5 - 3 2 35.00 15 31.5 35.5 3.5 - 4.5 3 30.00 20 31.5 37.0 5.5 - 6.5 4 25.00 25 31.5 39.0 7 – 8 5 20.00 30 31.5 38.0 5.5 - 6 6 15.00 35 31.0 37.0 4.5 – 5.5 7 10.00 40 31.0 35.0 3 - 4 Results: Temperature readings to be recorded to nearest 0.5 °C Ignore if student wrote °C or ‘+’ in the table Reject if student writes ‘-‘ All temperature rise calculated correctly All temperature readings within range of teacher’s value. All results display the correct trend (increase from 1-4, decrease from 5-7, with 5 no more than 4) 1 1 1 1 1(b) PDO Scale chosen is appropriate and is not accurate (uniform scale chosen to use more than half of each axis); no awkward scale 2 Lines of best fit (equal points on both sides of the graph and any anomalous points ignored) BOD if best fit lines do not cross above highest point *first line must pass through origin. Only 1 anomalous point allowed. Axes (include labels with units) All points plotted correctly (allow error to ½ a square) 1 1 1 1

2 1(c) ACE Correct volume of Q read from graph with units shown (24.5 cm3 ± 0.50). Correct temperature rise read from graph with units shown (7.8 ± 0.10) Working on the graph (Penalize if working is missing) 1 1 1(d)(i) ACE 2NaOH + H2SO4  Na2SO4 + 2H2O 1 1(d)(ii) ACE No of mol of NaOH = 24.5/1000 x 1.23 = 0.03014 mol = 0.0301 mol 2 mol NaOH ≡ 1 mol H 2SO4 No of mol of acid in P = 0.03014/2 mol = 0.01507 mol = 0.0151 mol UNDERLINED VALUES TO BE TAKEN FROM STUDENT’S GRAPH 1 1(d)(iii) ACE Volume of sulfuric acid = 50 – 24.5 = 25.5 cm3 1 1(d)(iv) ACE PDO Concentration of sulfuric acid = 0.01507 / (25.5)/1000 = 0.591 mol/dm3 appropriate units in final answers in ((d)(iii) (cm 3), (d)(iv) (mol/dm3) 1 1 1(e) ACE PDO ΔH = 50 x 1.05 x 4.2 x 7.8 = -1719.9 J Correct sign (-) and appropriate significant figures (1 d.p.) in final answers in (e) 1 1 1(f) ACE I agree with student A but disagree with student B. (must state but no marks) Both a weak acid and a monobasic acid would result in a lower concentration of H+ ions and hence a lower maximum temperature. However, the (final) volume of a weak dibasic acid would be the same as that of the sulfuric acid. 1 1

3

4 Qn Skill Answer Mark 2a PDO MMO Results table: record initial mass, mass after 2 minutes and final mass, with correct headings and units in a table initial mass of conical flask and acid / g mass of conical flask and contents after 2 minutes / g 180.06 181.82 REJECT: WEIGHT Results: Mass record to 2 d.p. or 3 d.p. (Depending on Lab) 1 1 2b MMO test observations To 1 cm depth of P in a test tube, add aqueous ammonia until no further change occurs. Light blue ppt formed Blue ppt dissolves in excess (aqueous ammonia) to form a dark blue solution To 2 cm depth of P in a test tube, add aqueous sodium hydroxide until no further change is observed. Heat and test any gas evolved. Light blue ppt formed Light blue ppt insoluble in excess (sodium hydroxide) Light blue ppt turns black BOD: Black solid formed. 1 1 1

5 (Colourless gas evolved) (Gas is pungent) Gas turns moist blue litmus paper red Gas is ammonia (Marks awarded in Test 2 OR Test 4) To 1 cm depth of P in a test tube, add aqueous silver chloride. No observable changes (1 mark for Test 3 AND Test 5) To 2 cm depth of Q in a test tube, add aqueous sodium hydroxide until no further change is observed. Add a piece of aluminum foil. Heat and test any gas evolved. Blue/pale blue ppt formed Blue ppt insoluble in excess sodium hydroxide (Marks awarded in Test 2 OR Test 4) Blue ppt turns black / Black solid formed [BOD] (Marks awarded in Test 2 OR Test 4) (Effervescence of Colourless gas) (Gas is pungent) Gas turns moist blue litmus paper red Gas is ammonia (Marks awarded in Test 2 OR Test 4) 1 1

6 To 1 cm depth of Q in a test tube, add aqueous barium nitrate. No observable changes (1 mark for Test 3 AND Test 5) To a 1 cm depth of Q, add aqueous potassium iodide until no further change is observed. Leave the mixture to stand. white/cream/yellowish- brown/brown ppt formed OR blue solution turned brown white ppt settles to the bottom 1 2(c)(i) ACE Tick: Copper(II) 1 2(c)(ii) ACE Cation: ammonium/NH4+ Moist red litmus paper turns blue, ammonia gas produced. BOD: Test 2/ Produced ammonia gas after heating with aqueous sodium hydroxide Anion: Nitrate/NO3- Moist red litmus paper turns blue, ammonia gas produced. BOD: Test 4/ Produced ammonia gas after heating with aluminum and aqueous sodium hydroxide 1 1 2(d)(i) ACE Mass lost = 180.06+2 – 181.82 = 0.24 g 1

7 2(d)(ii) ACE CuCO3 + 2HNO3  Cu(NO3)2 + CO2 + H2O No of mol of CO2 = 0.24 / 44 = 0.005455 mol 1 mol CO2 ≡ 1 mol CuCO3 0.005455 mol CO 2 ≡ 0.005455 mol CuCO3 Mass of CuCO3 = 0.005455 x 124 = 0.6764 g Percentage Purity = 0.6764 / 2 x 100% = 33.8 % (to 3sf) 1 1 2(e) MMO All mass lost is due to carbon dioxide 1 2(f) MMO Plug the mouth of the conical flask with cotton wool. OR Keep weighing the flask until a constant mass is achieved. 1 2(g) ACE To remove any carbonate ions (that may give a false positive/interfere with the reaction.) 1 Total: 17 3 P 1. Measure a fixed volume e.g 20 cm3, using a measuring cylinder, of 1.0 mol/dm3 of acidified sodium iodate and transfer into a conical flask/beaker. 2. Add a few drops of starch solution indicator. 3. Measure a fixed volume e.g 20 cm3, using a measuring cylinder, of 1.0 mol/dm3 of aqueous potassium iodide and add it to the conical flask/beaker. 4. Start the stopwatch and measure the time taken to the nearest second for blue-black colour to appear. 5. Repeat step 1 to 4 using same volume and concentration of acidified sodium iodate and same volume but varying concentrations e.g. 2.0 mol/dm3 ,0.5 mol/dm3 of aqueous potassium iodide. 6. The highest concentration would give the colour change first, while the lowest concentration would give the colour change last. Q1: Constant quantities of KI and NaIO3 and constant concentration of NaIO3 Q2: Varying concentrations of KI A: Suitably sized container, stopwatch, measuring cylinder/burette/pipette M: Take the time using the stopwatch, describe how time is related to concentration. 1 1 1 1 Total: 4 Skill Marks PDO 10 MMO 12 ACE 14 P 4