Chemistry 9701/36 October/November 2019: question paper and mark scheme

Mark scheme

1(a) I All the following data recorded 1 • two burette readings and titre for the rough titration • initial and final burette readings for two (or more) accurate titrations II Titre values shown, for accurate titrations and Appropriate headings and units in the accurate titration table 1 • initial / start and (burette) reading / volume • final / end and (burette) reading / volume • titre or volume / FB3 and used / added (not ‘difference’, ‘amount’, ‘total’) unit: / cm3 or (cm3) or in cm3 (for each heading) or cm3 unit given for each volume recorded III All accurate burette readings are to the nearest 0.05 cm3. 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 Award V if δ ⩽ 0.80 (cm3) (Where δ is difference to the supervisor’s value) 1 Award VI if δ ⩽ 0.50 (cm3) 1 Award VII if δ ⩽ 0.30 (cm3) 1 1(b) Candidate calculates the mean correctly. 1 • Candidate must take the average of two (or more) titres that are within a total spread of not more than 0.20 cm3. • Working / explanation must be shown or ticks must be put next to the two (or more) accurate readings selected. • The mean should be quoted to 2 dp and be rounded to nearest 0.01 cm3. 1(c)(i) All quoted answers in (ii)–(v) are expressed to 3 or 4 sig fig. 1 Minimum of 3 answers displayed to qualify for this mark. 1(c)(ii) Correctly calculates no of moles of HCl used. 1 25 answer (b) No of moles HCl = 0.5 × /250 × /1000 1(c)(iii) Ca(OH)2 + 2 HCl → CaCl2 + 2H2O and Correctly calculates number of moles of Ca(OH)2 = 0.5 × answer in (ii) 1 1(c)(iv) Correct use of (iii) to calculate concentration of Ca(OH)2 in FB 1 1 Concentration Ca(OH)2 in FB 1 = ans (iii) × 40 1(c)(v) Correct use to obtain mass of Ca(OH)2 1 Mass of Ca(OH)2 = answer (iv) × 74.1

Mark scheme

2(a)(i) Six pieces of data shown, with unambiguous headings and correct units. 1 • (mass of) container and FB 4 • (mass of) container (plus residue) • (mass of) FB 4 used • first / start / initial (temperature) / T of FB 6 • final / highest (temperature) • (temperature) rise / change Precision of readings shown in 2(a) and 2(b) 1 • all four thermometer readings are shown to 0.0 or 0.5 °C • both balance readings for each experiment are shown to same number of d.p. • masses and temperatures subtracted correctly Accuracy (Q) mark 1 If δ is less than or equal to 2.0 °C, award this mark 2(a)(ii) Correctly calculated answer 1 Energy change = 40 × 4.2 × temp rise 2(a)(iii) Correctly calculated moles of Ca(OH)2 1 mass used Moles of FB 4 = /74.1 2(a)(iv) Correct use 1 • ∆ H = (ii) / (iii) × 1000 • Negative sign must be shown on answer line. • Answer should be expressed to 2, 3 or 4 sig fig 2(b)(i) Readings written in space provided 1 • Two thermometer readings are recorded both above 10 °C • Two masses are recorded giving mass of CaO between 0.5–2.0 g. Two accuracy marks 2 • Calculate the difference between corrected candidate’s and supervisor’s temp rise (δ) See table for accuracy marks. Sup ∆Tmax ⩾ 25.5 °C 25.0–15.5 °C 15.0–10.5 °C 10.0–5.5 °C < 5.5 °C 1 mark δ ⩽ 3.0 °C δ ⩽ 2.5 °C δ ⩽ 2.0 °C δ ⩽ 1.5 °C δ ⩽ 0.5 °C 2 marks δ ⩽ 2.0 °C δ ⩽ 1.5 °C δ ⩽ 1.0 °C δ ⩽ 0.5 °C not available 2(b)(ii) Correct expressions shown for enthalpy change 1 • Energy released (40 × 4.2 × temp rise) mass of FB 4 used • No of moles used = /56.1 • ∆H = energy/no of moles × 1000 • An attempted answer, quoted to 2 or more sig fig • Negative sign in answer 2(c) Possible working: 1 Attempt at Hess Cycle diagram or clear working shown • Downward arrow from Ca(OH)2 • Downward arrow from CaO OR Use of equations Ca(OH)2(s) + 2HCl(aq) → CaCl2(aq) + 2H2O(l) CaO(s) + 2HCl (aq) → CaCl2(aq) + H2O(l) Correctly uses values for ∆H, with sign correct 1 ∆H = (a)(iv) – (b)(ii) Correct answer with some working gains both marks. Correct answer with no working gains one mark. Wrong answer – look at working and award one mark if appropriate. If default values are used, ∆H = + 27 kJ mol-1 2(d)(i) Temperature rise / change would be the same and because acid is used in excess / CaO is the limiting factor 1 2(d)(ii) No, because there is no ‘frothing up’ / spitting out of reagents / no acid spray 1 or No, because a taller cup would not (significantly) reduce heat loss FB 7 is ZnCO3; FB 8 is KI; FB 9 is KNO2

Mark scheme

3(a) Observations when FB 7 is heated 2 • Condensation / water droplets / steam produced • solid becomes fluidised / powder jumps around • solid turns yellow / yellow-green (when hot) • solid goes white or paler when cooled Award 1 mark for two correct points from the list, award 2 marks for three or more correct points from the list. (Gas) turns limewater milky / chalky / white precipitate / cloudy white 1 3(b)(i) Observations (see also the table shown on the next page) 5 Award 1 mark for two correct points (*) in the table 3(b)(i) Table of Observations for 3(b)(i) test observations with FB 8 observations with FB 9 To a 1 cm depth in a test-tube, add a yellow / yellow-brown / brown / KMnO4 decolorised / purple to few drops of acidified potassium orange / orange-brown / red-brown colourless * manganate(VII) coloration (formed) * To a 1 cm depth in a test-tube, add an no (visible) reaction / no change / no no (visible) reaction / no change / no equal volume of dilute nitric acid, then precipitate / solution remains precipitate / solution remains add a few drops of aqueous barium colourless / goes (pale) yellow * colourless nitrate, then and add a few drops of aqueous silver (pale) yellow precipitate (formed) * no (visible) reaction / no change / no nitrate. precipitate / solution remains colourless * To a 1 cm depth in a boiling tube, add no (visible) reaction / no change / no no (visible) reaction / no change / no an equal volume of aqueous sodium precipitate / solution remains precipitate / solution remains hydroxide and warm carefully, then colourless / no gas * colourless / no gas * add a strip of aluminium foil. fizzing / bubbling / effervescence Fizzing / bubbling / effervescence * or gas / NH3 turns litmus blue * gas / H2 pops with lighted splint * litmus turns blue is CON any other ‘positive’ gas test is CON 3(b)(ii) Ag+(aq) + I–(aq) → AgI(s) 1 3(b)(iii) Al / aluminium is oxidised (not ‘aluminium ions’) 1 and Nitrite/nitrate ion is reduced / N5+ / N3+ (not ‘FB 9 reduced’) 3(c)(i) At first: no change / stays colourless / goes yellow / light brown 1 and With acid: goes brown / orange-brown / red-brown / yellow-brown / yellow 3(c)(ii) I2 (formula required) 1 3(c)(iii) Add starch 1 and mixture goes dark blue / blue-black (ignore state) OR Add sodium thiosulfate and brown colour disappears / brown decolourised / iodine colour fades / solution goes colourless