qtss comb chem prelim 2024 p3 answer

This document consists of 9 printed pages. Name: ………………………………………………….. ( ) Class: Sec 4 SG 5 / 6 Queenstown Secondary School Preliminary Examination 2024 Secondary Four Normal (Academic) Science (Chemistry) 5105/04 30 July 2024 Time: 1015 – 1130h Tuesday Papers 3 and 4: 1 hour 15 minutes Setter: Additional Materials: Candidates answer on the Question Paper. No Additional Materials are required. READ THESE INSTRUCTIONS FIRST Write your name, class and index number on all the work you hand in. Write in dark 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. Section A Answer all questions. Write your answers in the spaces provided. Section B Answer one question. Write your answers in the spaces provided. The use of an approved scientific calculator is expected, where appropriate. In calculations, you should show all the steps in your working, giving your answer at each stage. You are advised to spend no longer than 30 minutes on Paper 3. You may proceed to answer Paper 4 as soon as you have completed Paper 3. A copy of the Periodic Table is printed on page 9. At the end of the examination hand in your answers to Paper 3 and Paper 4 separately. The number of marks is given in brackets [ ] at the end of each question or part question. Examiner’s Use Section A /22 Section B /8 TOTAL /30

2 Queenstown Secondary School [Turn over Section A Answer all questions. 1 Fig. 1 shows the apparatus that is used to fractionally distill petroleum into different fractions. Fig. 1 (a) (i) Name the physical property that allows the fractions to be separated. The fractions are miscible liquids with different boiling points. [1] (ii) State why the fractions condense in the condenser. The fractions have a lower boiling point than the water in the condenser. [1] (b) The petrol fraction has a boiling point range of 35oC to 70oC. (i) Explain why the petrol fraction is collected over a range of temperatures rather than at a single temperature. The petrol fraction is a mixture. [1]

3 Queenstown Secondary School [Turn over (ii) Explain what happens when a liquid boils. Your answer should explain boiling in terms of the kinetic particle theory and of the energy changes involved. When a liquid boils, it gains energy. During boiling, the particles move further apart, from being close together [liquid] to far apart [gas]. [arrangement] During boiling, the particles move faster, from sliding over each other [liquid] to moving randomly [gas]. [movement] [1] [1] [1] [Total: 6] 2 A student carries out an experiment to study the reaction between dilute hydrochloric acid, HCl, and excess magnesium carbonate, MgCO3. The carbon dioxide gas produced in this experiment is collected and its volume is measured at one-minute intervals. The results obtained from the experiment are shown in the table below. Table 2 time / min 0 1 2 3 4 5 6 7 8 9 volume of carbon dioxide / cm3 0 15 20 29 33 37 39 40 40 40 (a) (i) Name the apparatus used to measure the volume of gas collected. gas syringe [1] (ii) Describe a positive test to identify the gas. test Deliver the gas into limewater. result Gas evolved gives white ppt. with limewater. [1]

4 Queenstown Secondary School [Turn over (iii) Write a balanced chemical equation for the reaction between dilute hydrochloric acid and magnesium carbonate. 2HCl + MgCO3 → MgCl2 + H2O + CO2 [1] (b) At the start of the experiment there is 29.4 g of magnesium carbonate. Calculate the amount of magnesium carbonate, in moles. [relative atomic masses, Ar: Mg, 24; C, 12; O, 16] no. of mol. of MgCO3 = 29.4 g ÷ 84 = 0.35 mol amount of magnesium carbonate = 0.35 mol [1] (c) Plot a graph of these results, marking each point with a cross (). Draw a best-fit curve taking into account all the plotted points. [2] volume of carbon dioxide gas / cm3 time / min

5 Queenstown Secondary School [Turn over (d) Suggest why the volume of gas stops increasing after 7 minutes. All the hydrochloric acid had fully reacted. [1] [Total: 7] 3 The table shows some information about the elements in Group 1 of the Periodic Table. Table 3 element atomic number melting point / oC density in g / cm3 lithium 3 180 0.53 sodium 11 98 0.97 potassium 19 63 0.89 rubidium 37 39 1.53 caesium 55 (a) (i) Explain why the elements are in Group 1 of the Periodic Table. The atoms of these elements contain 1 outer (valence) electron. [1] (ii) Explain why potassium is in period 4 of the Periodic Table. The atom of potassium occupies 4 electron shells. [1] (b) (i) Describe one observation when a piece of sodium is added to water. The piece of sodium darts around the surface of water. [1] (ii) Describe what you would observe when a few drops of Universal Indicator are added to the reaction mixture. The Universal Indicator turns violet. [accept = blue] [1] (iii) Write a balanced chemical equation for the reaction between sodium and water. 2Na + 2H2O → 2NaOH + H2 [1] [Total: 5]

6 Queenstown Secondary School [Turn over 4 Vegetable oil can be converted to margarine in the industries. There are several types of polyunsaturated fatty acids in vegetable oils that can be made into margarine. The structure shown below is linoleic acid, a common acid found in vegetable oils. Fig. 4 (a) Explain, with reference to its structure, why linoleic acid is polyunsaturated. It contains many C=C double bonds. [1] (b) (i) Name the reagent required to convert vegetable oil to margarine. hydrogen [1] (ii) Describe a simple test to distinguish between a sample of polyunsaturated vegetable oil and a sample of saturated margarine. test Deliver aqueous bromine into both samples. result with vegetable oil The red-brown aqueous bromine turns colourless immediately. result with margarine No visible reaction. [1] [1] [Total: 4]

7 Queenstown Secondary School [Turn over Section B Answer one question from this section. 1 The most common isotope of carbon in carbon dioxide is C6 12 . (a) Define the term isotopes. Isotopes are atoms of the same element with the same number of protons but different number of neutrons. [1] (b) Another isotope of carbon has the symbol C6 13 . Determine the number of sub-atomic particles in one atom of C6 13 . number of protons = 6 number of neutrons = 13 – 6 = 7 [1] (c) Ethene burns completely in oxygen to produce carbon dioxide, CO2. (i) Complete and balance the equation for the complete combustion of ethene. C2H4 + 3 O2 → 2 CO2 + 2 H2O 1m – for correct formula of H2O 1m – for correct balancing [2] (ii) Draw a ‘dot and cross’ diagram to show the bonding in one molecule of carbon dioxide, CO2. Show only outer shell electrons. [covalent; O=C=O] [2] (d) Carbon monoxide, CO, is an atmospheric pollutant. (i) Explain how carbon monoxide is formed. It is formed from the incomplete combustion of fuel in cars. [1] (ii) Describe the effect of carbon monoxide on health. When inhaled, the CO combines with haemoglobin, causing t