Copy of 2024 CGS Chemistry Prelim P3 Mark Scheme

CRESCENT GIRLS’ SCHOOL SECONDARY FOUR PRELIMINARY EXAMINATION MARK SCHEME 6092/03 CHEMISTRY 14 AUGUST 2024 PAPER 3 Practical 1 hour 50 minutes Shift Laboratory This document consists of 11 printed papers. For Examiner’s Use Q1 /17 Q2 /15 Q3 /8 Total /40 Class: Register No: Name: READ THESE INSTRUCTIONS FIRST Write your name, register number and class in the spaces provided at the top of this page. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use a 2B 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 working or if you do not use appropriate units. Qualitative Analysis Notes are printed on page 12. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question.

2 CRESCENT/PRELIM/P3 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 1 You are to determine the enthalpy change of neutralisation of sodium hydroxide by an acid and the concentration of hydrogen ions in the acid. These can be found by measuring the temperature change when solutions of the acid and alkali are mixed. Read all the instructions below carefully before starting the experiments in Question 1. Instructions You are going to carry out seven experiments. P is 1.50 mol/dm3 sodium hydroxide, NaOH. Q is an aqueous solution of a strong acid. (a) Experiment 1 Place a Styrofoam cup into a 250 cm3 glass beaker. Put P into a burette and measure 40.00 cm 3 of P into the Styrofoam cup. Measure the temperature of P and record the value in the table. Use a measuring cylinder to measure 10.0 cm3 of Q. Pour this volume of Q into the Styrofoam cup containing P. Stir gently, using the thermometer, and measure the highest temperature reached. Record this temperature in the table. Empty the Styrofoam cup and rinse it with deionised water. Experiments 2 to 7 Repeat Experiment 1 but use different volumes of P and Q for each experiment given in the table. Complete the table by calculating the temperature rise for each experiment. 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 10.0 31.0 35.5 4.5 2 35.00 15.0 32.0 37.0 5.0 3 30.00 20.0 31.5 39.0 7.5 4 25.00 25.0 31.5 41.0 8.5 5 20.00 30.0 31.5 39.5 8.0 6 15.00 35.0 31.5 38.0 6.5 7 10.00 40.0 31.0 36.0 5.0 ALL temperature recorded up to 1 dp [1] Correct temperature rise calculated [1] Trend in temperature rise. Increases and then decreases. [1] [3]

3 CRESCENT/PRELIM/P3 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN (b) Plot a graph of temperature rise against volume of P on the grid below. Use these points to draw two intersecting straight lines of best fit. [4] Scale – suitable scale is used. E.g x axis is interval of 5 cm 3 , y axis in interval of 1 cm3 [1] Line of best fit [1] Axis – correct heading and units [1] Points – correct plotting of points. Measure up to half the smallest division. [1] (c) Use the temperature rise obtained when the two lines meet to calculate the maximum amount of heat energy produced during the reaction. [Assume that 4.3 J are required to raise the temperature of 1 cm 3 of any solution by 1°C] Let (y) be the temperature rise.

4 CRESCENT/PRELIM/P3 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN Amount of heat = 4.3 x 50 x (y) Correct value of (y) AND amount of heat energy released [1] amount of heat produced = _____________J (d) From your graph, read the volume of P where the two lines intersect. Show clearly on the graph how you obtain your answer. volume of P = _____________ cm3 [1] (e) Calculate the number of moles of P in (d). No. of moles of P = volume of P (cm3) in (d) x 1.50 number of moles of P = _____________ mol [1] (f) The ionic equation for the reaction between P and Q is as given. OH-(aq) + H+(aq) → H2O(l) (i) Calculate the volume of Q that reacts with volume of P in (d). Volume of Q = 50 – (answer in (d)) = x cm3 volume of Q = _____________cm3 (ii) Hence, calculate the concentration of hydrogen ions, H+, in Q in mol/ dm3. Concentration of hydrogen ions = (answer in (e)) ÷ 𝒙 𝟏𝟎𝟎𝟎 concentration of hydrogen ions = ________________ [2] Answer in 3 sf [1] Correct unit [1] (g) If Q is sulfuric acid, state the concentration of the acid in mol/ dm3. Explain your reasoning. concentration = answer in f(ii) ÷ 𝟐 [1] reasoning Sulfuric acid is a dibasic acid. 1 mole of the acid dissociates fully in water to form 2 moles of hydrogen ions AND concentration of sulfuric acid is half concentration of hydrogen ions [1]

5 CRESCENT/PRELIM/P3 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN [2] (h) A student mentioned that the experiment was not accurate because the temperature changes measured were small. Suggest and state how one modification to the experimental method can give larger changes in temperature. Insulate the Styrofoam cup/wrap the Styrofoam cup with cloth [1] Reduces heat loss to the surrounding AND highest temperature rise can be recorded. [1] OR Use of magnetic stirrer [1] To ensure the solution is well mixed so that maximum amount of energy can be released OR Well-mixed solution to allow even distribution of heat so highest temperature rise can be recorded more accurately [1] [2] [Total: 15]

6 CRESCENT/PRELIM/P3 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 2 You are given a hydrated salt, M.6H2O. On heating, hydrated M.6H2O loses its water of crystallisation. M.6H2O (s) → M(s) + 6H2O(g) Read all the instructions carefully before starting the experiments. Carry out the following tests on solid M.6H2O, recording all of your observations in the spaces provided. (a) Instructions 1. Use an electronic balance to measure and record mass of an empty boiling tube. 2. Add about 1.00 g of M.6H2O into the boiling tube. 3. Measure and record the total mass of the boiling tube and M.6H2O. 4. Gently heat the contents for 1 minute. 5. Record your observation in (a)(i). 6. Leave the boiling tube to co