2015 O LEVEL CHEMISTRY 5073/02

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1 OF SCIENCE 2015 O LEVEL CHEMISTRY 5073/02 SECTION A 1. The table shows some common oxidation states for some elements in their compounds. element common oxidation states metal / non-metal A 2 non-metal B +2;+3;+4;+6;+7 metal C +1 non-metal D +3 metal E 1 non-metal (a) Complete the table by indicating whether each element listed is a metal or non-metal. [1] [No marks is awarded for any wrong indication] (b) Answer the following questions using the letter of the elements. [3] (i) Which element is likely to be hydrogen? C (ii) Which element is likely to be in Group VI? E (iii) Which element is likely to form coloured compounds? B (c) No elements from Group 0 are listed in the table. Use the information in the table to explain why. [2] Elements in Group 0 have full valence electron shell and do not have to gain or lose electrons to obtain stable noble gas configuration, hence will not form compounds. [To use information in table is to state no compound is formed (for 1mark)] 2. Metals can be extracted from their oxides by reduction reactions. The table given below gives the minimum temperature needed for reduction of some metal oxides by carbon. metal oxide min. temperature needed ( ) ( calcium oxide 2100 zinc oxide 900 copper oxide 100 magnesium oxide 1600 lead oxide 400 (a) (i) How does the temperature needed relate to the reactivity of the metal? [1] The more reactive the metal, the higher the minimum temperature needed for reduction.

2 OF SCIENCE (ii) Predict the minimum temperature needed for reduction of iron oxide by carbon. Explain your answer. [2] 600. Iron is less reactive than zinc, but more reactive than lead as it is between zinc and lead in the reactivity series, thus the temperature is between the 2 metals. [Any value between 400 and 900 ] (b) Metal oxides also react with metals. Use the list below for the following questions. copper zinc magnesium iron silver oxide sodium oxide calcium oxide potassium oxide Which metal and metal oxide pair will likely react to give the most vigorous reaction? [1] Magnesium and silver oxide. [Both substances must be correct for 1mark] (c) Zinc metal reacts with steam. Gives the names of the two products. [2] Zinc oxide and hydrogen gas. [No marks awarded for chemical formulae] 3. (a) The table shows some information of certain organic compounds. Fill in the blanks for missing names, formula and processes. [5] name structural formula process(es) for making compound ethene Fractional distillation of crude oil followed by catalytic cracking poly(ethene) Addition polymerisation of ethene. ethanol Two processes. Process 1: Catalytic addition of steam to ethene ethanoic acid Oxidation of ethanol. [0.5mark each]

3 OF SCIENCE (b) Dilute ethanoic acid and hydrochloric acid react with metal oxides. (i) How are the reactions between metal oxide and each of the two acids similar? [1] Both reactions produce a salt and water as the only products. (ii) The rate of the reaction of dilute ethanoic acid with metal oxides is slower than that of dilute hydrochloric acid of the same concentration and temperature. Why is this so? [2] Ethanoic acid is a weak acid with partial ionisation in water to form ions, but hydrochloric acid is a strong acid with complete ionisation in water to form ions. Hence, ethanoic acid contains lesser ions, resulting in a slower reaction. 4. Aluminium is used to make drink cans. The metal used can be obtained either from recycling or by extraction from bauxite by electrolysis. process separation recycling Physical sorting. Steel and aluminium cans are separated using a magnet. Aluminium cans are shredded into small pieces. electrolysis Physical extraction and breaking up of ore. Concentrated sodium hydroxide is used to dissolve aluminium oxide to separate it from insoluble metal oxide impurities. main process Heating to 700. Dissolving in ionic solvent at 900. Electrolysis to form molten aluminium. Carbon dioxide is formed in the process. finishing Cooling and shaping of molten aluminium. Cooling and shaping of molten aluminium. (a) Use the information above to estimate the melting point of aluminium. [1] 700 [Actual melting point = 660 ] (b) The extraction of aluminium uses 95% more energy than the recycling process. Use the information to explain why. [2] The amount of energy required to dissolve aluminium in ionic solvent is almost 30% (900 vs 700 ) more than that needed to heat aluminium cans. And, a huge amount of energy is required to ensure bauxite stays molten for extraction to occur. [Mention of 30% is the enhancement for the centre s students]

OF SCIENCE (c) The extraction of aluminium creates waste that can harm the environment. Two such waste are concentrated sodium hydroxide and carbon dioxide.

4 OF SCIENCE (c) The extraction of aluminium creates waste that can harm the environment. Two such waste are concentrated sodium hydroxide and carbon dioxide. Why would each of these wastes cause harm when released into the environment? [2] NaOH is a strong alkali and corrosive. When leaked into the sea, it will raise ph of water greatly above 7, 7 killing marine lifes that don t survive in alkaline conditions. CO is a greenhouse gas that raise global temperature, melting polar caps, raising sea-level and causes floodings. (d) Recycling uses less energy and produces less waste than extraction. Give one other reason why recycling metals is important. [1] Recycling reduces the need for more extraction of metals such as aluminium which are finite resources and will deplete over time. (e) Some aluminium is used to make duralumin. It is commonly used to make aircraft bodies. Duralumin is a mixture of aluminium and other elements such as copper, manganese and magnesium. (i) What is the name given to mixtures such as duralumin? [1] Alloy (ii) Suggest why duralumin is more useful as aircraft bodies than aluminium. [1] It is stronger and harder than aluminium as layers of atoms in duralumin are less likely to slide. 5. Dilute sodium chloride solution forms hydrogen and oxygen gases during electrolysis. (a) Write ionic equations for the reactions at the cathode and anode. [2] Cathode: 2 ( )+2 ( ) Anode: 4 ( ) ( )+2 ( )+4

5 OF SCIENCE (b) The gases are collected and measured. Theoretically, the ratio of the volume of hydrogen to oxygen collected should be 2:1. Oxygen gas is more soluble than hydrogen in water. This changes the ratio of gases that are collected. (i) Explain the theoretical volume of the two gases. [2] For 2moles of electrons form 1mole of, hence for the same circuit,2moles of electrons(anode) form 0.5moles of, resulting in twice the volume of than. (ii) Explain how and why the solubility of oxygen affects the ratio of hydrogen to oxygen that is collected. [2] When some dissolves, the volume collected decreases and is less than half that of. As such, the actual yield is lower, resulting in a greater ratio difference between the 2 gases. (iii) The difference from the expected ratio is greater when the electrolysis starts but less noticeable after the electrolysis has been running for some time. Suggest why. [1] The volume of oxygen gas produced exceeds the volume of space in container at anode. Hence, over time, the difference remains consistent. (c) What happens to the concentration of sodium chloride during electrolysis? Explain. [1] The concentration increases. The amount of water in electrolyte decreases as and ions are preferentially discharged, removing water while amount of sodium chloride remains unchanged. (d) The same apparatus can be used to electrolyse concentrated aqueous sodium chloride. Give one similarity and one difference between the products of the two electrolysis. [2] Similarity Difference Dilute electrolyte Concentrated electrolyte Both processes produced hydrogen gas at the cathodes. Oxygen gas produced at anode. Chlorine gas produced at anode. (e) Platinum metal electrodes are used. Why is platinum a suitable material? [1] Platinum is inert and as anode, does not oxidised and breaks down, allowing the Respective anions to be preferentially discharged instead.

6 OF SCIENCE 6. Farmers often buy soil improvers that contain substances that neutralise acidity. The table below shows some common substances found in soil improvers. substance chemical effectiveness in composition neutralising limestone CaCO fair quicklime CaO Very high slaked lime Ca(OH) Very high other points Insoluble in water. Needs to be ground to a very fine powder. Made by heating limestone to a high temperature. Reacts exothermically with water to make an alkaline solution. Made by adding water to quicklime. Slaked lime is an alkali. blast furnace slag Mixture of CaCO and CaSiO with other impurities fair Insoluble in water. Impurities include silicon oxides and other non-metal compounds. Composition of mixture varies. (a) Use the information in the table to suggest why limestone is less effective at neutralizing acidity than quicklime and slaked lime. [2] Limestone is insoluble and hence does not dissociate in water to form hydroxide ions that can effectively neutralise the hydrogen ions in acids. Therefore, it is limited in contact with acids as solids unlike the mobile ions in the other two to allow faster reaction. (b) (i) Suggest an advantage of making improvers from slag rather than the other substances. [1] Slag contains other non-metal compounds that may include components such as nitrogen or phosphorus, which are essential for good plant growth. [Accept idea of slag as waste that have no demands in market and increase costs from disposal] (ii) Slag may contain impurities of Group IV and V oxides, making it less effective at neutralizing acidity. Explain why this is so. [2] These oxides are non-metal oxides that are mostly acidic oxides. Therefore Increasing acidity and require more improvers to be used.

7 OF SCIENCE (c) The calcium content of the substances is important because it adds to the mineral content of the soil. Show by calculation that quicklime has a higher percentage by mass of calcium than both of limestone and slaked lime. [3] Percentage by mass of calcium in quicklime = 100% = 71.4% (3sf) Percentage by mass of calcium in limestone = 100% = 40% Percentage by mass of calcium in slaked lime = ( ) 100% = 54.1% (3sf) Therefore, quicklime has a higher percentage by mass of calcium than limestone and slaked lime. (d) Quicklime is made by strongly heating limestone. The reaction produces carbon dioxide. Assuming that limestone is pure calcium carbonate, calculate the volume of carbon dioxide that is produced. [3] No. of moles of limestone = = mol Equation : CaCO CaO+CO So, CaCO CO ratio 1 : 1 moles Volume of carbon dioxide = = 6.0

8 OF SCIENCE SECTION B 7. Table 1 shows the melting and boiling points of alkanes. alkane formula melting point ( ) boiling point ( ) ethane C H propane C H butane C H pentane C H hexane C H octane C H Table 1 Graph 1 below shows the graph of melting/boiling points plotted against number of carbon atoms in alkane.

9 OF SCIENCE The flashpoint of a compound is the minimum temperature at which that compound gives off enough vapour to burn in air. It is important when considering the amount of hydrocarbons blended to make fuels. Compounds with lower flashpoints evaporate and burn more easily at lower temperatures and burn more smoothly in car engines. In a petrol refinery, isomerization is used to convert straight chain alkanes into branched alkanes. The flashpoints of some straight chain and branched alkanes are provided in Table 2 and Table 3 respectively. alkane formula flashpoint ( ) propane C H 104 butane C H 71 pentane C H 49 hexane C H 23 heptane C H 4 octane C H 13 Table 2 number of carbon atoms formula flashpoint ( ) branched alkane branched alkane branched alkane Table 3 (a) What trends are shown by the data in Table 1 and Graph 1? 1 [1] Generally, as the number of carbon atoms increases, the melting and boiling points increases. Both the melting and boiling points do not increase in value linearly. [Note the plural in trends] (b) One of the alkanes appears to have a melting point that is different from the expected value. Name the alkane and explain your choice. [1] Propane. The expected value of the melting point of propane is and the listed value is much lower to give a clear trend in melting points. (c) Predict the melting and boiling of heptane, C H. [1] Melting point : 79 and boiling point : 90 [Acceptable : in range of 70 s and 90 s respectively] [No marks for just 2 values given without stating melting and boiling point]

10 OF SCIENCE (d) The boiling points and flashpoints of straight chain alkanes have similar trends. Describe these trends. [1] Both boiling points and flashpoints increase as the number of carbon atoms per molecule increases. s. (e) How is the flashpoint of a straight chain alkane affected by isomerisation? Use the information in the table to explain. [4] When isomerise, the flashpoint decreases as compared to a straightchain alkane of the same number of carbon atoms. By comparing pentane and branched alkane 1, the flashpoint decreased from 49 to 57. Also, the more the number of branchings, the lower the flashpoint of the same number of carbon atoms. By comparing branched alkane 1 and 2, when the no. of branchings increased from 1 to 2, the flashpoint dropped from 57 to 65. [2mark marks for comparing straightchain to isomer; 2mark2 marks for comparing no. of branchings per isomer. Respective examples must be quoted from the table] (f) Petrol contains straight chain octane. Other compounds are added to improve the performance of petrol. In cold countries, winter blend petrol is sold. This petrol contains increased amounts of compounds that include pentane and branched alkane 2. (i) Explain how and why this mixture works better than pure octane alone. [2] Pure octane has a higher flashpoint and requires more energy for fuel to burn. When mixed, petrol will have a lowered melting point where it s flashpoint can go below, making the fuel more efficient in burning. (ii) Engine tanks in cars are not air tight. Winter blend petrol is not sold in very hot weather conditions because it leads to an increased loss of petrol from the tank. Explain how and why petrol is lost. [2] Winter blend petrol has boiling point lower than room temperature during hot weather conditions. Hence, it vapourises very readily and escapes uncombusted in engines in the form of unburnt hydrocarbons, resulting in mass loss.

11 OF SCIENCE 8. Car engines are adjusted to work at a particular air:fuel ratio. The amount of air that is mixed with the fuel affects the temperature of the engine, the amount of pollutant gases that form and how efficiently the catalytic converter works. Two major pollutants are carbon monoxide and nitrogen monoxide. (a) A lean burn engine runs with a higher ratio of air to fuel than a normal car engine. This means that the mixture contains a higher amount of air compared to fuel. One effect of this is a lower running temperature of the engine. How will a lean burn engine affect the amount of carbon monoxide and nitrogen monoxide production compared to a normal car engine? Explain. [3] Production of CO decreases in lean burn engine. With higher air:fuel ratio, the presence of more oxygen allows for greater chance of complete combustion. Thus, reducing formation of CO. At a lower running temperature of engine, the amount of NO produced decreases. Nitrogen gas is inert and only combusts under high temperature. As such, lesser tendency of formation of NO in lean burn engine. (b) A catalytic converter removes pollutants by redox reactions. from oxidising agent CO+[O] CO to reducing agent 2NO N +2[O] Write an overall equation for the two reactions above. [1] 2CO+2NO 2CO +N (c) The amount of air in pollutants that enter the converter affects the reactions. The graph shows the percentage of pollutants successfully removed. (i) Describe and explain how increased amounts of air affect the removal of carbon monoxide and nitrogen monoxide. [3] Initially, as amount of air increases, % of CO removed increases with maximum amt. of NO removed. When maximum amount of CO is removed, increasing ing amt. of air will cause the % of NO removed to decrease with no effects on CO. At lower amount of air, NO can be reduced to more effectively without combusting in air. However, increasing amt. of air allows for oxidation of CO to. Eventually, in the lack of CO but more air, formed oxidises back into NO, reducing % of removal of NO.

12 OF SCIENCE (ii) In the converter, apart from reacting with each other, carbon monoxide and nitrogen monoxide react with other substances as well. How does the graph show that? [1] Both the pollutants do not have zero percent removal, which indicates that there is presence of both gases in converter reacting with other substances. EITHER 9. The diagram shows the energy changes when lithium chloride and sodium chloride are dissolved in water. (a) Describe the differences in the energy changes and temperature changes that happen as each compound dissolves. [4] When lithium chloride dissolves, the reaction is exothermic and the surrounding temp. increases. As the energy level of product is less than that of reactant, more heat is released into surrounding. However, when sodium chloride dissolves, the reaction is endothermic and the surrounding temperature decreases. The energy level of product is greater than that of reactant, hence more heat is absorbed from the surrounding. (b) A student measured the temperature change when 4.0g of potassium chloride was dissolved in excess water. temperature at start ( ) ( 20 lowest temperature recorded after dissolving ( ) ( 12 calculated energy change (J) +720 (i) Explain why the calculated energy change includes a + sign. [1] The reaction is endothermic, hence the enthalpy change is positive. (ii) Use the results to calculate the enthalpy change when one mole of potassium chloride dissolves in excess water. Give your answer in kj/mol to 3 significant figures. [3] No. of moles of KC. =. = mol M1 H per mole of KC = ( ). M1 = kj/mol (3s.f.) M1

13 OF SCIENCE (iii) Describe what happens to the arrangement and movement of the particles in potassium chloride during dissolving. [2] The ions are closely-packed together in an orderly arrangement, vibrating about their fixed position as solids. When added to water, the ions become widely separated amongst the water molecules,, vibrating randomly and sliding over one another. [Note that excess water means a dilute solution] OR 9. Fluorine is in Group VII. (a) A jet of fluorine gas is aimed at a filter paper soaked with potassium bromide solution. The solution on the paper quickly turns brown. (i) Explain why this happens. Include an ionic equation to support your answer. [3] Fluorine is more reactive than bromine, hence it displaces bromine from potassium bromide to form the brown bromine solution. Ionic equation : F ( )+2B ( ) 2F ( )+Br ( ) (ii) The experiment is repeated with chlorine and iodine gases replacing fluorine in different setups with potassium bromide solution. State and explain what you would expect to see in each setup. [3] Chlorine is more reactive than bromine, hence it displaces bromine from potassium bromide. When added, the colourless solution turns brown. But, iodine is less reactive than bromine, hence it doesn not displace bromine from its solution. When added, iodine gas dissolves in solution and turns it yellowish-brown brown. [Avoid brown for iodine and confuse with the bromine colour stated] (b) Fluorine also reacts with iron. When a jet of fluorine is aimed at some iron wool, the wool glows and appears to burn. An ash of iron(iii) fluoride is left behind. (i) Write an equation, with state symbols, for this reaction. [2] 3F ( )+2Fe( ) 2FeF ( ) (ii) Explain, using ideas of oxidation state and electron transfer, why fluorine is considered an oxidising agent in the reaction. [2] Fluorine caused iron to be oxidised as the oxidation state of iron increased from 0 in Fe to +3 in FeF, and is itself reduced to FeF. Also, each molecule of fluorine gains an electron from iron, causing iron to be reduced to FeF. Therefore, fluorine acts as an oxidising agent. E N D O F P A P E R

14 OF SCIENCE

Chemical reactions and electrolysis

Chemical reactions and electrolysis Higher Revision Questions Name: Class: Date: Time: 95 minutes Marks: 95 marks Comments: Page of 29 (a) Magnesium metal is shaped to make magnesium ribbon. Explain why