TKGS 2025 Sec 4 Prelim Paper 3 (answers)

TANJONG KATONG GIRLS’ SCHOOL PRELIMINARY EXAMINATION SECONDARY FOUR EXPRESS CANDIDATE NAME ANSWERS CLASS 4 INDEX NUMBER CHEMISTRY 6092/03 Paper 3 Practical Test 5 August 2025 1 hour 50 minutes Candidates answer on the Question Paper. READ THESE INSTRUCTIONS FIRST Write your name, class and register number on all the work you hand in. Write in dark blue or black pen. You may use a 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 page 13. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 13 printed pages and 1 blank page. Shift Laboratory For Examiner’s Use 1 2 3 Total

Tanjong Katong Girls’ School Preliminary Examination 2025 2 1 You are going to investigate the reaction between aqueous copper( II) nitrate and aqueous sodium carbonate. Step 1: Label the test-tubes 1, 2, 3, 4 and 5 and place them in order in the test-tube rack. Step 2: Use the measuring cylinder to pour 5.0 cm3 of the 1 mol/dm3 of aqueous copper(II) nitrate into each of the five test-tubes. Step 3: Using another measuring cylinder, add 1.0 cm3 of 1 mol/dm3 of aqueous sodium carbonate to test-tube 1. Using the glass rod, carefully stir the contents of the test-tube. Repeat for steps 4 to 7. Rinse the glass rod with distilled water before stirring it in each test-tube. Step 4: Add 2.0 cm3 of 1 mol/dm3 of aqueous sodium carbonate to test-tube 2. Step 5: Add 3.0 cm3 of 1 mol/dm3 of aqueous sodium carbonate to test-tube 3. Step 6: Add 4.0 cm3 of 1 mol/dm3 of aqueous sodium carbonate to test-tube 4. Step 7: Add 5.0 cm3 of 1 mol/dm3 of aqueous sodium carbonate to test-tube 5. Leave the contents of the test-tubes to stand for 15 minutes. You should start Question 2 while you are waiting for the solid formed in the test- tubes to settle. (a) After 15 minutes, use a ruler to measure the height of the solid formed in each test-tube. In Table 1, record the volume of 1 mol/dm 3 of aqueous sodium carbonate added, and the height (in cm) of the solid formed in each test -tube. Include appropriate headings in Table 1. Table 1 test-tube number Volume of Na2CO3 (aq) / cm3 Height of solid / cm 1 1.0 1.5 2 2.0 2.2 3 3.0 4.5 4 4.0 5.2 5 5.0 7.0 [4]

Tanjong Katong Girls’ School Preliminary Examination 2025 3 Volume of Na2CO3 (aq) in cm3 measured to 1 d.p [1] Height of solid in cm measured to 1 d.p. (allow mm measurement to nearest whole number) [1] Appropriate heading and correct units [1] Full set of data shows • correct volumes of Na2CO3 (aq), according to question • height increasing with test-tube number [1] (b) On the grid provided, plot a graph of height of solid against volume of aqueous sodium carbonate added. Axis labels and correct units (based on table of values) [1] Appropriate axis and scale must include (0,0) origin (vertical: height of solid / cm; horizontal: volume of aqueous Na2CO3 added / cm3). [1] All points plotted correctly from table of values [1] Best fit line that uses all/most points and starts from origin Line can be straight or curved, whichever is more suitable (ECF if already penalised for axis not including origin) [1] 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 [4] Volume of Na2CO3 (cm3) Height of solid (cm)

Tanjong Katong Girls’ School Preliminary Examination 2025 4 (c) Describe the trend in the height of the solids formed in test-tubes 1 to 5. The height of the precipitate generally increases as the volume of aqueous sodium carbonate added increases [1]. ………………………………………………………………………………………..[1] (d) From the graph, deduce the height of the solid formed when 0.004 5 moles of aqueous sodium carbonate is added to 0.005 moles of copper(II) nitrate. Value from graph with unit (indicated with dotted line) when 4.5 cm3 of aqueous sodium carbonate is added to 5 cm 3 of copper(II) nitrate. ………………………………………………………………………………………..[1] (e) Suggest two improvements that can be made in order to improve the accuracy of the results. Use a burette to add the copper(II) nitrate to the test tubes. Repeat experiment twice and calculate average height of precipitate for each test tube. Allow the precipitate a longer time to settle to the bottom of the test-tube. [Any two] ………………………………………………………………………………………..[2] [Total: 12]

Tanjong Katong Girls’ School Preliminary Examination 2025 5 2 Water of crystallisation refers to the water molecules bonded to the lattice structure of ionic compounds. You are going to investigate the enthalpy change, ∆Hhyd, when anhydrous sodium carbonate, Na 2CO3, converts to hydrated sodium carbonate, Na2CO3.10H2O. One way to determine the enthalpy change is to measure the changes in temperature when samples of anhydrous sodium carbonate and hydrated sodium carbonate are added separately to excess hydrochloric acid. Read all the instructions carefully before starting the experiments in Question 2. Instructions You are going to carry out two experiments. X is 2.2 g of anhydrous sodium carbonate, Na2CO3. Y is 5.5 g of hydrated sodium carbonate, Na2CO3.10H2O Z is 1.50 mol/dm3 hydrochloric acid, HCl. (a) Experiment 1 Place a Styrofoam cup into a 250 cm3 beaker. Transfer 25.0 cm3 of Z into the cup using the 25 cm3 measuring cylinder. Measure the temperature of Z and record the value in the table. Pour all of X into the acid in the cup. Stir, using the thermometer, and measure the highest or lowest temperature reached. Record this temperature in the table. Experiment 2 Repeat Experiment 1 using the Styrofoam cup, but use Y in place of X.

Tanjong Katong Girls’ School Preliminary Examination 2025 6 Complete Table 2 and calculate the temperature change for each experiment. Table 2 All four thermometer readings recorded to .0 oC or .5 oC. ∆T calculated correctly using Tfinal - Tinitial with negative sign for Experiment 2. Experiment 1 temperature increase AND experiment 2 temperature decrease Full set of data 2 correct answers= 1mk [3] (b) (i) The amount of heat transferred in the experiments can be calculated using the expression shown. Heat transferred (J) = mass of solution (g) x maximum temperature change (oC) x 4.2 Use this expression to calculate, in J, the amount of heat transferred in each experiment. Assume that 1.0 cm3 of solution has a mass of 1.0 g. Heat transferred in Experiment 1 = 25 x 4.2 x 8.5 = 892.5 J [1] Heat transferred in Experiment 2 = 25 x 4.2 x 9.0 = 945 J [1] Allow ECF from incorrect ∆T Answers to be in 2-3 sf Experiment 1 with X: heat transferred = ......................J Experiment 2 with Y: heat transferred = .....................J [2] experiment initial temperature of Z /oC highest or lowest temperature of Z /oC temperature change /oC 1 30.0 38.5 8.5 2 30.0 21.0 -9.0

Tanjong Katong Girls’ School Preliminary Examination 2025 7 (ii) Calculate the enthalpy change, ∆H, in kJ/mol, when all the solid reacts with hydrochloric acid in each experiment. [Mr: Na2CO3, 106, Na2CO3.10H2O, 286] Enthalpy change, ∆H = Heat transferred in kJ Number of moles of solid ∆H1 = 892.5 x 106 2.2 x 1000