NJC 2022 H2 Chemistry Prelim P4 Ans

1 NJC SH2 Prelim Exam 9729 / 04/ 22 [Turn over NATIONAL JUNIOR COLLEGE SH2 Year-End Practical Examination Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 4 Practical Candidates answer on the Question paper Additional Materials: As listed in the Confidential Instructions 9729/04 16 August 2022 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST Write your identification number and name. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. Qualitative Analysis Notes are printed on pages 22 and 23. At the end of the examination, fasten all your work se curely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 23 printed pages including this cover page. Shift Laboratory For Examiner’s use 1 / 9 2 / 21 3 / 17 4 / 8 Total / 55

2 NJC SH2 Prelim Exam 9729 / 04/ 22 [Turn over Answer all the questions in the spaces provided. 1 Determination of the enthalpy change of a reaction, Hr FA 1 is 1.00 mol dm–3 sodium hydrogen carbonate, NaHCO3 (to be used for Q3 as well) FA 2 is 2.00 mol dm–3 sodium hydroxide, NaOH FA 3 is 2.00 mol dm–3 sulfuric acid, H2SO4 An acid -base neutralisation reaction involves reacting the two solutions, to produce water molecules. The equation for this neutralisation reaction is given below. H+(aq) + OH–(aq) → H2O(l) You will follow the instructions to perform two experiments, Experiment A and Experiment B. Record your results in Tables 1.1 and 1.2. Experiment A Reaction between FA 1, NaHCO3, and FA 2, NaOH. Reaction 1 NaHCO3(aq) + NaOH(aq) → Na2CO3(aq) + H2O(l) ΔHreaction1 The molar enthalpy change for reaction 1 , Δ Hreaction1, is the enthalpy change when 1.00 mol of NaHCO3 reacts completely with NaOH. 1 Using a 50 cm3 measuring cylinder, transfer 30.0 cm3 of FA 1 into a Styrofoam cup. Place this cup inside a second Styrofoam cup, which is placed in a 250 cm3 glass beaker. Place the lid on the cup. 2 Stir and measure the temperature of this FA 1, TFA1. 3 Using another 50 cm3 measuring cylinder, measure 20.0 cm3 of FA 2. 4 Stir and measure the temperature of this FA 2, TFA2. 5 Add FA 2 from the measuring cylinder to the FA 1 in the Styrofoam cup. Immediately replace the lid. 6 Using the thermometer, stir the mixture continuously until a maximum temperature is reached. Read and record this temperature Tmax. 7 Calculate the weighted average initial temperature, Taverage, of FA 1 and FA 2 using the formula given below: Taverage = (𝑉𝐹𝐴1×𝑇𝐹𝐴1)+(𝑉𝐹𝐴2×𝑇𝐹𝐴2) (𝑉𝐹𝐴1 + 𝑉𝐹𝐴2 ) Experiment A TFA1 /°C TFA2 /°C Taverage /°C Tmax /°C ΔTmax /°C Table 1.1

3 NJC SH2 Prelim Exam 9729 / 04/ 22 [Turn over Experiment B Reaction between FA 1, NaHCO3, and FA 3, H2SO4. Reaction 2 NaHCO3(aq) + ½H2SO4(aq) → ½Na2SO4(aq) + H2O(l) + CO2 (g) ΔHreaction2 The molar enthalpy change for reaction 2 , Δ Hreaction 2 , is the enthalpy change when 1.00 mol of NaHCO3 reacts completely with H2SO4. 1 Using a measuring cylinder, transfer 30.0 cm3 of FA 1 into a Styrofoam cup. Place this cup inside a second Styrofoam cup, which is placed in a 250 cm 3 glass beaker. Place the lid on the cup. 2 Stir and measure the temperature of this FA 1, TFA1. 3 Using another measuring cylinder, measure 20.0 cm3 of FA 3. 4 Stir and measure the temperature of this FA 3, TFA3. 5 Add slowly, the FA 3 from the measuring cylinder to the FA 1 in the Styrofoam cup. Immediately replace the lid. 6 Using the thermometer, stir the mixture continuously until a minimum temperature is reached. Read and record this temperature Tmin. 7 Calculate the weighted average initial temperature, Taverage, of FA 1 and FA 3 using the formula given below: Taverage = (𝑉𝐹𝐴1×𝑇𝐹𝐴1)+(𝑉𝐹𝐴3×𝑇𝐹𝐴3) (𝑉𝐹𝐴1 + 𝑉𝐹𝐴3) Experiment B TFA1 /°C TFA3 /°C Taverage /°C Tmin /°C ΔTmax /°C Table 1.2 [2] For both Tables 1.1 & 1.2: 1m - All temperature readings are recorded to the nearest 0.1 °C. 1m - ΔTmax/min must be correctly calculated (with sign -ve) i.e. Tmax – average initial temperature.

4 NJC SH2 Prelim Exam 9729 / 04/ 22 [Turn over (a) For the purpose of calculations, you should assume that the mixture has a density of 1.00 g cm–3 and specific heat capacity, c, of 4.18 J g–1 K–1. (i) Use your results from Table 1.1 to calculate a value for the molar enthalpy change for reaction 1, ∆Hreaction1. ∆Hreaction1 = ……………………………. [2] (ii) Use your results from Table 1.2 to calculate a value for the molar enthalpy change for reaction 2, ∆Hreaction2. ∆Hreaction2 = ……………………………. [2] q = (20.0 + 30.0)(1.00) x 4.18 x ∆Tmax 1m n(NaHCO3) = 1.00 x 30.0/1000 = 0.03 mol ∆Hreaction1 = – q / n(NaHCO3) (this reaction is exothermic) 1m (with unit) q = (20.0 + 30.0)(1.00) x 4.18 x ∆Tmin 1m n(NaHCO3) = 1.00 x 30.0/1000 = 0.03 mol ∆Hreaction1 = + q / n(NaHCO 3) (this reaction is endothermic) 1m (do not penalise if no units if already penalised in part 1)

5 NJC SH2 Prelim Exam 9729 / 04/ 22 [Turn over (b) Ionic equations for neutralisation, reaction 1, and reaction 2 are shown below. H+(aq) + OH−(aq) ⟶ H2O(l) ∆Hneu= –57.1 kJ mol–1 HCO3−(aq) + OH−(aq)→ CO32−(aq) + H2O(l) ∆Hreaction1 HCO3−(aq) + H+(aq) → H2O(l) + CO2(g) ∆Hreaction2 Carbon dioxide reacts with solutions of carbonate ions according to the following equation. reaction 3 CO2(g) + CO32−(aq) + H2O(l) ⟶ 2HCO3−(aq) ∆Hreaction3 Using your calculated answers in (a), together with the given value of enthalpy change of neutralisation, ∆ Hneu, construct an energy cycle to determine a value for the enthalpy change for this reaction, ∆Hreaction3. ∆Hreaction3 = (– (a)(ii)) + (– (a)(i)) + (–57.1) 1m – cycle 1m – substitute values correctly in eqn 1m – final ans ∆Hreaction3 = ……………………………….………. [3] [Total: 9]

6 NJC SH2 Prelim Exam 9729 / 04/ 22 [Turn over 2 Qualitative analysis of some organic and inorganic compounds You are provided with liquids FA 4, FA 5, FA 6, FA 7, and FA 8. You are to perform the tests described in Tables 2.1 and 2.3 and record your observations. At each stage of any test, you are to record details of the following: • colour changes seen • the formation of any precipitate and its solubility in an excess of the reagent added • the formation of any gas and its identification by a suitable test • if there is no observable change, write no observable change You should indicate clearly at which stage in a test a change occurs, recording your observations alongside the relevant tests. In all tests, the reagents should be added gradually until no further change is observed unless you are instructed otherwise. If any solution is warmed, a boiling tube must be used. Rinse and reuse test-tubes