Hkcee Chemistry 2007 free essay sample

The products of cracking contained unsaturated (hydrocarbons) / alkenes / C=C / ethane / reasonable name of alkene, which decolourised the bromine water immediately by addition reaction (2) (II) The student then dropped more bromine water into the boiling tube until the brown colour of the bromine water persisted. After about 10 minutes, the brown colour disappeared. Why? The products of cracking also contained saturated (hydrocarbons) / alkanes / methane/reasonable name or molecular formula of alkane, which decolourised the bromine water slowly by substitution reaction. 2) 3. The diagram below shows the set-up used in performing a microscale experiment. A drop of hydrochloric acid was added to the drop of chlorine bleach in a petri dish was immediately covered with is lid. Chlorine gas formed by the reaction between chlorine bleach and hydrochloric acid eventually filled up the whole set-up. a) Write a chemical equation for the reaction between chlorine bleach and hydrochloric acid. (1) b) State the expected observation at position C and position D. In each case, write a relevant chemical equation if a reaction occurred. i. Position C Change (from colourless) to brown/yellow (1) (1) ii. Position D No observable change/reaction (1) c) Suggest one advantage of replacing test tube experiments with morescale experiments. Less chemicals are used / less harmful to environment / less dangerous / easy to handle / save money / save time (1) 4. A student learnt from a book that an ancient chemical cell could be made by immersing an iron rod in a liquid placed inside a copper can. The liquid used was vinegar but not wine. The diagram below shows the set-up designed by him in simulation the cell. ) Explain, in terms of structure and property of particles why the liquid inside the ancient chemical cell was vinegar but not wine. Both vinegar and wine contain molecules. (1) Only vinegar (ethanoic acid) can ionize in water / contains (mobile) ions for conducting electricity. (1) b) The student found that the iron rod dissolved gradually, and colourless gas bubbles were given out on the inner wall of the copper can. i. Write a half equation, involving iron, for the reaction that occurred at the iron rod. (1) ii. Write a half equation for the reaction that occurred on the inner wall of the copper can. 1) c) The student found that colourless gas bubbles were also given out at the surface of the iron rod that immersed in vinegar. Explain the observation. Iron reacts directly with vinegar/H+ (aq) giving out hydrogen gas. (1) 5. A solid sample contains zinc and copper only. The composition of the solid sample was analysed experimentally as outlined below: 2. 00 g of the solid sample was added to excess dilute hydrochloric acid in a beaker. Upon completion of reaction, the mixture inside the beaker was filtered. The residue obtained was first washed with distilled water, and then dried. The mass of the dried residue was 1. 75 g. a) Write a chemical equation for the reaction involved. (1) b) How can one know that the reaction has been completed? No further gas evolved (1) c) Explain why it is necessary to wash the residue obtained. To wash away Zn2+ / Cl- / H+ / ZnCl2 / HCl /acid left behind (1) d) Explain why it is NOT appropriate to dry the residue with a Bunsen flame after washing. Copper will be oxidized / become copper(II) oxide / copper reacts with oxygen (or air) (1) e) Assuming negligible experimental errors, calculate the percentage of zinc by mass in the solid sample. Percentage of zinc by mass in the sample = (2. 00 – 1. 75)/2. 00 x 100% = 12. 5% (2) 6. Read the paragraph below and answer the questions that follow. Magnesium is a useful metal. Scientists adopt different methods to extract magnesium form magnesium oxide. In 1828, a scientist obtained magnesium in two steps. In the first step, magnesium oxide reacts with chlorine and carbon to form magnesium chloride. In the second step, the magnesium chloride formed reacts with potassium to give magnesium. In 1951, some scientists adopted another chemical process to obtain magnesium form magnesium chloride. Potassium is not used in this process, and there is even no need to use any other chemicals. | a) Write a chemical equation for the reaction that occurred in the first step of the method used by the scientist in 1828. (1) b) Name the type of reaction between potassium and magnesium chloride. Why can potassium react with magnesium chloride to give magnesium? Redox (reaction) / displacement (reaction) (1) Potassium is a more powerful reducing agent / more reactive than magnesium (1) c) I. What would be the chemical process that can obtain magnesium from magnesium chloride, without using potassium or other chemicals, in 1951? Electrolysis (1) II. What property does magnesium chloride possess so as to make the chemical process possible? Magnesium chloride is an ionic compound / electrolyte/conduct electricity in molten state/contains mobile ions. (1) d) Suggest one use of magnesium in daily life. Sacrificial protection / making alloy / firework / flash (1) 7. This question involves how to distinguish four unlabelled test tubes, each containing one of the following colourless liquids: a) By heating a small amount of each of the colourless liquids to dryness, ONE of the liquids can be distinguished. Suggest which liquid can be distinguished, and state the observation involved. Concentrated sodium hydroxide solution (1) Solid / powder left (1) b) By applying a flame directly to a small amount of each of the colourless liquids, TWO of the liquids would catch fire. i. Suggest which two liquids would catch fire. Methanol and hexane (1) ii. For the two liquids that would catch fire, the observations involved during combustion are different. Suggest the difference in these observations, and explain your answer. Methanol burns with a blue flame while hexane burns with a yellow flame / hexane burns with a more sooty flame than methanol (1) Carbon content in hexane is higher than that in methanol (1) iii. Without using other chemicals apart from the above colourless liquids, suggest another method to distinguish the two liquids that would catch fire. State the expected observation. (Smelling is not accepted. ) Add distilled water / conc. Sodium hydroxide solution separately to methanol and hexane. Methanol is miscible with distilled water. Conc. Sodium hydroxide solution while hexane is not. (1) OR Just mix then together. Two layers are observed. Upper layer is hexane while lower layer is methanol. OR Carry out boiling point test. The one with higher boiling point is hexane. 8. a) Teflon is a plastic that can be used to make artificial hip joints. Teflon is an addition polymer of linear structure consisting of carbon and fluorine only. The ratio of the number of carbon atoms to the number of fluorine atoms in the polymer is 1 : 2. i. Draw a portion of the Teflon structure with 10 carbon atoms. (1) ii. Write the repeating unit of Teflon, and suggest a possible monomer of Teflon. Repeating Unit: (1) Monomer: CF2— CF2 / Tetrafluoroethene (1) b) Nylon is a polymer that can be used to make carpets. A portion of the nylon structure is shown below: i. With reference to the given structure, explain whether nylon is a thermoplastic or thermosetting plastic. Nylon is a thermoplastic as there are no cross links. (1) ii. Suggest one reason why recycling of used carpets to recover nylon is difficult. Carpets may be made of a variety of materials. Separating nylon from carpets may be difficult. (1) iii. State one disadvantage of disposing of nylon carpets by incineration. Poisonous gas / NO2 / NO / CO / HCN / soot may evolve. (1) For question 9, candidates are required to give answers in paragraph form. For the question, 6 marks will be awarded for chemical knowledge and 3 marks for effective communication. 9. A certain brand of rust remover contains an acid of high concentration. The rust remover can be used for removing tough rust stains; while the rust remover, after dilution, can be used for removing comparatively light rust stains. Write some instructions, with reasons, on how the rust remover can be used safely at home. Two sentences have been given below as an introduction. The rust remover should be kept out of reach from children as it contains an acid of high concentration. The rust remover should not be swallowed because it is harmful. Chemical knowledge (6) Corrosive property * Safety glasses / goggles / rubber gloves / protective clothings / avoid contact with skin or eyes / wash with plenty of water it contacted with skin. * Because acid of high concentration is corrosive. Dilution process * Add slowly small amount of rust remover into a large amount of water with stirring * Because large amount of heat given out in dilution of the rust remover (acid of high concentration) / avoid rust remover (acid) splashing out. Other potential dangers Use plastic container instead of metal / do not use to clean marble / do not mix with chlorine bleach or caustic soda / do not put in warm place * Because will damage metal container / damage marble / toxic gas evolves if mix with chlorine bleach / large amount of heat releases if mix with caustic soda / acidic gas evolves if put in warm place. Effective communication (3) Section B 10. In an experiment to determine the concentration of phosphoric acid (H3PO4), 10. 0 cm3 of the acid was first diluted 250. 0 cm3 with distilled water. 25. 0 cm3 of the diluted solution was then transferred to a conical flask and titrated with a 0. 25 M sodium hydroxide solution using phenolphthalein as indicator. 17. 60 cm3 of sodium hydroxide solution was needed to reach the end point. a) Describe briefly how 10. 0 cm3 of phosphoric and can be diluted to 250. 0 cm3 with distilled water in the laboratory. 10. 0 cm3 of the acid is transferred into a 250. 0 cm3 volumetric flask using a pipette. (1) Distilled water is added up to the graduation mark. (1) b) Phosphoric acid reacts with sodium hydroxide in the titration according to the following equation: Calculate the molarity of the original phosphoric acid before dilution. No. of moles of NaOH = 0. 0176 x 0. 025 = 4. 40 x 10-4 No. f moles of H3PO4 in 25. 0 cm3 of the dil. soln. = 4. 40 x 10-4 / 2 = 2. 20 x 10-4 Molarity of the original phosphoric acid = 2. 20 x 10 -4 x 10 / (10/1000) = 0. 22 M (3) c) ‘At the beginning of titration, the solution in the conical flask turned pink upon the addition of sodium hydroxide solution but because colourless immediately upon swirling. However, pear the end point, the solution took longer time to become colourless upon swirling. ’ Explain why the time needed for the solution to become colourless is different at the two stages mentioned above. Neutralization is a quick process. 1) As titration proceeds, concentration of acid decreases, less chance of NaOH to contact with the acid / rate of reaction decreases. (1) d) In the titration, the 0. 025 M sodium hydroxide solution was used a standard solution. i. What does the term ‘standard solution’ mean? A solution of known concentration. (1) ii. Comment whether it is appropriate to prepare a standard solution of sodium hydroxide by the following procedure: ‘Weigh a sample of solid sodium hydroxide, dissolve it in some distilled wa ter and make up to a known volume of solution. ’ Not appropriale. Solid sodium hydroxide absorbs water/CO2 readily in air. 1) 11. In a chemical piant, extraction of copper from its ores involves roasting copper(I) sulphide with air inside a high temperature fumace. Copper(I) sulphide reacts with oxygen in air according to the following equation: The copper so extracted contains impurities including metals such as silver, iron, zinc and gold. The impure copper is then purified by electrolysis as illustrated in the diagram below: a) With reference to the reaction between copper(I) sulphide and ixygen, identify the species undergoing oxidation and the species undergoing reduction. Explain your answers in terms of changes in oxidation numbers. Species undergo oxidation is sulphide ion, O. N. of S changes from -2 to -4. (1) Species undergo reduction are copper(I) ion and oxygen, O. N. of Cu changes from +1 to 0 and O. N. of O changes from 0 to -2. (1) b) Explain briefly bow impure copper can be purified by electrolysis as illustrated in the diagram above. The impure copper anode (+ve electrode) becomes copper(II) ions / Cu Cu2- + 2e- (1) Copper(II) ions in the solution discharge on the pure copper cathode (-ve electrode) / Cu2- + 2e- Cu (1) c) Insoluble impurities deposit under the impure copper anode as ‘anode sludge’. According to the information given, suggest what substances the anode sludge would contain. Explain your answer. Silver and gold (1) They are less reactive than copper (less readily to dissolve as ions when compared with copper) (1) d) ‘The concentration of copper(II) ions in copper(II) sulphate solution remains UNCNGED in the above electrolysis. ’ Is this statement correct? Explain your answer. Not correct. Concentration of copper (II) ions drops gradually. At anode, iron/zinc dissolve as ions because they become ions more readily then copper. (1) However at cathode, copper (II) ions are always preferentially discharged. 1) e) State TWO advantages of building a factory in which contact process is carried out near the chemical plant mentioned above * SO2 available as resource for contact process. * The cost of transportation of SO2 is minimized * Prevent air pollution induced by SO2. (any 2 points above, 1 mark for each point, 2) 12. Organic compound Z contains carbon, h ydrogen and oxygen only. Analysis of Z gives the following results: i. 1. 0 g of Z contains 0. 401 g of carbon, 0. 068 g of hydrogen and 0. 531 g of oxygen. ii. 1. 0 g of Z, upon complete vapourisation, occupies 400 cm3 at room temperature and pressure. ii. There are no observable changes when potassium carbonate solution is added to Z. iv. Brown colour of bromine remains unchanged when several drops of bromine in organic solvent are added to Z. (Molar volume of gas at room temperature and pressure = 24 dm3) a) Calculate the empirical formula of Z. (2) b) Deduce the molecular formula of Z. (2) c) i. Suggest a possible structure of Z. Explain your answer. HCOOCH3 (1) Explanations: From (III): Z is not an acid. (1) From (IV): No C=C double bond in Z. (1) ii. Five the systematic name for the compound represented by the structure you suggested in (i). Methyl methanoate (1) For question 13, candidates are required to give answers in paragraph form. For this question, 6 marks will be awarded for chemical knowledge and 3 marks for effective communication. 13. Discuss the similarities and differences between soapy detergents and soapless detergents with reference to their raw materials, structures and properties. Chemical Knowledge (6) * Both soapy and soapless detergents have ionic group / head and long hydrocarbon chain / tail. * Both soapy and soapless detergents have hydrophilic property and hydrophobic property. Soapy detergents made from fats / oils, while soapless detergents made from petroleum. * Soapy detergents have —COO- group, while soapless detergents have —SO3 / —OSO3- group. * Soapy detergents cannot wark with hard water / acidic medium, wile soapless detergents can. * Bother soapy and soapless detergents act as wetting agents. * Both soapy and soapless detergents act as emulsifying agents * Soapy d etergents are wsually biodegradable, while soapless detergents usually are not. * Soapless detergents can be tailor-made, while soapy detergents cannot. Effective communication (3)