QTSS Prelim 2024 Sec 4Exp Chemistry P2 [6092] Ans

This document consists of 24 printed pages. Name: ………………………………………………….. ( ) Class: Sec 4 SG 1 / 2 Queenstown Secondary School Preliminary Examination 2024 Secondary Four Express Chemistry 6092/02 20 August 2024 Time: 1115 – 1300h Tuesday Duration: 1 hour 45 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 number of marks is given in brackets [ ] at the end of each question. A copy of the Periodic Table is printed on page 24. The use of an approved scientific calculator is expected, where appropriate. Examiner’s Use Section A /70 Section B /10 B9 B10 TOTAL /80

2 Queenstown Secondary School [Turn over Section A Answer all questions. A1 Use the list of substances to answer the questions. calcium hydroxide carbon monoxide copper hydrogen magnesium nitrogen nitrogen dioxide sulfur dioxide zinc oxide Each substance can be used once, more than once or not at all. Name the substance(s) which (a) is added to soil to increase the pH, calcium hydroxide [1] (b) helps prevent iron in underwater pipes from rusting, magnesium [1] (c) lead to the formation of acid rain that corrodes limestone buildings, sulfur dioxide and nitrogen dioxide [1] (d) react in a 1:3 ratio in the Haber Process to produce ammonia. nitrogen and hydrogen [1] [Total: 4]

3 Queenstown Secondary School [Turn over A2 Esters are commercially used as solvents for cosmetics, perfumes and glue. (a) Propyl ethanoate has a characteristic odour of pears and is commonly used in fragrances. A bottle of perfume containing propyl ethanoate is left opened in a laboratory and it diffused throughout the laboratory. (i) Give the names and draw the full structural formulae of the carboxylic acid and the alcohol that react to form propyl ethanoate. H O | || H – C – C – O – H | H ethanoic acid H H H | | | H – C – C – C – O – H | | | H H H propanol [2] (ii) The temperature of the laboratory is increased. Explain, in terms of collisions between reacting particles, what effect would this have on the rate of diffusion of propyl ethanoate. At higher temperatures, the reacting particles move faster leading to a higher frequency of effective collisions between the particles, resulting in a higher rate of diffusion. [1] [1] (b) The table shows some information about different esters. Table 2 name formula relative molecular mass methyl methanoate HCO2CH3 60 ethyl methanoate HCO2C2H5 74 propyl methanoate HCO2C3H7 88 butyl methanoate HCO2C4H9 102 pentyl methanoate HCO2C5H11 116

4 Queenstown Secondary School [Turn over (i) Use information in the table to give one piece of evidence that suggests that the esters belong to the same homologous series. The esters have the same general formula, HCO2CnH2n+1. / The esters have the same functional group, HCO2. [1] (ii) Which ester has the greatest rate of diffusion at room temperature and pressure? Explain your answer. Methyl methanoate It has the lowest relative molecular mass of 60. [1] [1] [Total: 7] A3 Seawater constitutes a rich source of various commercially important elements. (a) Magnesium bromide is an important salt found in seawater. Table 3 shows some information about two isotopes of bromine. Table 3 symbol number of protons number of neutrons number of electrons Br35 79 35 44 35 Br35 81 - 35 46 36 (i) Complete the table. [1] (ii) Use data from the table to explain the term isotopes. Isotopes have the same number of protons (i.e. 35), but different number of neutrons (i.e. 44, 46). [1] (iii) Chlorine is treated with a sample of seawater containing magnesium bromide. Write the ionic equation for the reaction that occurs. Cl2 (g) + 2Br- (aq) → 2Cl- (aq) + Br2 (l) [1]

5 Queenstown Secondary School [Turn over (iv) Explain why astatine does not react with aqueous magnesium bromide. Astatine, being less reactive than bromine, is unable to displace bromine from its salt. [1] (b) The distinctive smell of the seaside was thought to be due to ozone (O3). Ozone reacts with aqueous potassium iodide as shown in the equation below. 2I- (aq) + O3 (g) + 2H+ (aq) → I2 (aq) + O2 (g) + H2O (l) (i) Describe what is observed when ozone is bubbled into aqueous potassium iodide. The colourless solution turns dark-brown. [1] (ii) Explain, in terms of oxidation states, why this is a redox reaction. The iodine in potassium iodide undergoes oxidation, as the oxidation state of iodine increases from -1 in I- to 0 in I2. The oxygen in ozone undergoes reduction, as the oxidation state of oxygen decreases from 0 in O3 to -2 in H2O. [1] [1] (c) It is now known that the smell of the seaside is due to the presence of dimethyl sulfide, (CH3)2S. (i) Draw a ‘dot-and-cross’ diagram for dimethyl sulfide, (CH3)2S. covalent H H | | H – C – S – C – H | | H H [2] (ii) Explain, in terms of bonding and structure, why dimethyl sulfide is a gas at room temperature and pressure. Dimethyl sulfuide is a simple covalent molecule. A small amount of heat energy is required to overcome the weak intermolecular forces of attraction between molecules. [1] [Total: 10]

6 Queenstown Secondary School [Turn over A4 (a) Complete the table. Table 4.1 electrolyte state electrode half-equation at anode effect on electrolyte silver nitrate aqueous silver Ag (s) → Ag+ (aq) + e- no change copper(II) chloride molten graphite 2Cl- (l) → Cl2 (g) + 2e- fully decomposed [1] [1] (b) The diagram below shows the electrolysis of concentrated aqueous sodium chloride. H+, OH-, Na+, Cl- Fig. 4.2 (i) Write the half equation for the reaction that occurs at the negative electrode. 2H+ (aq) + 2e- → H2 (g) [1] (ii) The volumes of products 1 and 2 should theoretically be the same. Explain why the volume of product 1 is lesser. As chlorine is slightly soluble in water, the volume collected is lesser. [1]

7 Queenstown Secondary School [Turn over (iii) What happens to the resulting solution during the electrolysis? Explain your reasoning. The resulting solution becomes increasingly alkaline. With the removal of H+ and Cl- ions, the remaining Na+ and OH- combine to form the strong alkali, NaOH. [1] (iv) The same apparatus can be used to electrolyse dilute aqueous sodium chloride. Give one similarity and one difference between the products of the electrolysis of concentrated and dilute aqueous sodium chloride. similarity At the cathode, H+