2022 ZSS Chem Prelims P2

Candidate Name: ___________________________ Index No: ____ Class: _______ Paper 2 6092 / 02 25 August 2022 1 hour 45 minutes Candidates answer on the Question Paper. No Additional Materials are required. Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary Sch ool Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary School Zhenghua Secondary School READ THESE INSTRUCTIONS FIRST Write your index number, class and name 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 in the spaces provided. Section B Answer all three questions, the last question is in the form either/or. Answer all questions in the spaces provided. The number of marks is given in brackets [ ] at the end of each question or part question. A copy of the Periodic Table is printed on page 23. The use of an approved scientific calculator is expected, where appropriate. Name of Setter: Ms Natalie Tan Li Er This document consists of 23 printed pages including this Cover Page and the Periodic Table. [Turn over For Examiner’s Use Total ZHENGHUA SECONDARY SCHOOL PRELIMINARY EXAMINATION 2022 SECONDARY FOUR EXPRESS CHEMISTRY

2 Section A Answer all questions in this section in the spaces provided. The total mark for this section is 50. A1 Choose from the following list of substances to answer the questions below. nitrogen sodium oxide carbon dioxide silicon dioxide iron(II) chloride silver chloride zinc oxide diamond Each substance can be used once, more than once or not at all. (a) Which element has a giant covalent structure? …………………………………………………………………………………………...[1] (b) Which substance will react with potassium hydroxide to form precipitate which will change colour on standing? …………………………………………………………………………………………...[1] (c) Which compound exists as discrete molecules? …………………………………………………………………………………………...[1] (d) Which substance will dissolve to form an acidic solution? …………………………………………………………………………………………...[1] (e) Which substance can be added in excess to hydrochloric acid to form a soluble salt? …………………………………………………………………………………………...[1] [Total: 5]

3 A2 Silver dichromate, Ag2Cr2O7, is a red insoluble salt. Silver dichromate can be made by reacting silver nitrate solution with ammonium dichromate solution as shown by the equation below. 2AgNO3 + (NH4)2Cr2O7 → 2NH4NO3 + Ag2Cr2O7 (a) Describe how you can obtain pure and dry solid silver dichromate after mixing silver nitrate solution and ammonium dichromate solution. ……………………………………………………………………………………………… ……………………………………………………………………………………………… …………………………………………………………………………………….……..[2] (b) (i) Deduce the oxidation state of chromium in Ag2Cr2O7. .……………………………………………………………………………………..[1] (ii) Write the ionic equation with state symbols for the formation of silver dichromate in this reaction. ………………………………………..…………………………………………….[2] (c) The apparatus shown below is set up. After five minutes, a red solid appeared at the position marked S on the diagram. (i) Explain why a red solid appeared at the position marked S. ……………………………………………………………………………………...… …………………………………………………………………………………..….… ………………………………………………………………………………….…..[2] solid ammonium dichromate plastic trough solid silver nitrate water S solid ammonium dichromate

4 (ii) The experiment is then repeated at a higher temperature. What effect, if any, would this have on the time taken for the red solid to appear? Explain your answer. …………………………………………………………………………………..….… ………………………………………………………………………………….…..[1] [Total: 8] A3 Fig. 3.1 shows an article from a textbook about elderberries. Fig. 3.1 (a) The chromatogram in Fig. 3.2 was obtained when water was added to a drop of elderberries extract at the centre of a filter paper. Fig. 3.2 (i) Predict the colour of the elderberries extract at pH 7. ………………………………………………………………………………………[1] Elderberries are widely used in wine making. Extract of elderberries is a useful pH indicator and it can be separated by chromatography. As an indicator, the colour of the extract changes to pink at a pH of 2 – 3 and to blue at a pH of 11 – 12. dark blue dark pink light blue

5 (ii) An alternative set-up for the above experiment was shown in Fig. 3.3 below. Fig. 3.3 Given that the distance travelled by the dark pink dye is 1.00 cm, complete the table above by stating the distance travelled by the light blue and dark blue dyes. [1] (b) Fig. 3.4 shows how pH values changed during a titration when an acid was added from a burette into an alkali. A few drops of elderberries extract were added at the start of titration to monitor the pH of the solution. Fig. 3.4 (i) State and explain what could be one possible chemical formula of the alkali used. …………………………………………………………………………..……………. ………………………………………………………………………………...……[2] 5 10 15 20 25 30 35 40 14 12 10 8 6 4 2 0 stopper boiling tube volume of acid added / cm3 pH solvent front filter paper strip position of extract at the start of the experiment water

6 (ii) State the colour of the elderberries extract when the volume of acid added was 20 cm3 …………………………… 30 cm3 ……………………………. [2] [Total: 6] A4 This question is about compounds containing chlorine. The equation shows the reaction of aqueous methanol with dilute hydrochloric acid. CH3OH (aq) + HCl (aq) CH3Cl (g) + H2O (l) The progress of this reaction can be followed by taking small samples of the reaction mixture every hour and determining the concentration of dilute hydrochloric acid. (a) (i) Draw a ‘dot and cross’ diagram to show the bonding in a molecule of methanol. Draw only the outer shell electrons. [2] (ii) Explain, using ideas about structure and bonding, why methanol exists in liquid state at room temperature and pressure. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. ………………………………………………………………………………………[2]

7 (b) The graph shows how the concentration of hydrochloric acid changes as the reaction proceeds. (i) Sketch the graph on the axes above when the temperature is decreased. Label your graph “Y”. [1] 1.80 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 concentration of HCl / mol/cm3 1 5 10 15 20 25 30 35 time / h

8 (ii) Using data from the graph, describe how speed varies with time from A to D. ………………………………………………………………………………………. ………………………………………………………………………………………. ………………………………………………………………………………………. ………………………………………………………………………………………. ……………