Ammonia Production: Effects of Temperature, Pressure, and Catalysts, Schemes and Mind Maps of English

Download Ammonia Production: Effects of Temperature, Pressure, and Catalysts and more Schemes and Mind Maps English in PDF only on Docsity! The flow diagram shows the Haber process. In the Haber process ammonia is produced from nitrogen and hydrogen.   (a)     The word equation for the production of ammonia is:     nitrogen + hydrogen   ammonia Draw a ring around the correct answer to complete the sentence.           exothermic.   The symbol   in the word equation shows the reaction is reversible.     slow. (1) (b)     The reactor contains iron. Complete the sentence. The iron speeds up the reaction because it is a .............................................. (1) 1 (c)     What happens to the unreacted nitrogen and hydrogen? ........................................................................................................................ ........................................................................................................................ (1) Page 2 of 66 (d)     The sentences describe how ammonia is produced in the Haber process. The sentences are in the wrong order. P        Ammonia is separated as a liquid. Q        Nitrogen and hydrogen are mixed together. R        A mixture of gases enters the condenser. S        Nitrogen and hydrogen react to produce ammonia. Complete the boxes below to show the correct order of the sentences. The first box has been done for you.   (2) (Total 5 marks)           Ammonia is made using the Haber process. (a)     How is ammonia separated from unreacted nitrogen and hydrogen in the separator? ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ (2) 2 Page 3 of 66 (ii)     Give two reasons why it does not matter that the percentage yield of ammonia is low. Use the flow diagram at the start of this question to help you. ............................................................................................................... ............................................................................................................... ............................................................................................................... ............................................................................................................... (2) (Total 14 marks) Page 6 of 66 This question is about the Haber process. The diagram below shows a flow diagram for the Haber process. (a)     (i)      Nitrogen gas and hydrogen gas are obtained from different sources. Draw one line from each gas to its source. (2) 3 (ii)     Explain why iron is used in the reactor for the Haber process. ................................................................................................................... ................................................................................................................... ................................................................................................................... (2) Page 7 of 66 (iii)    Describe how the ammonia is separated from the other gases. ................................................................................................................... ................................................................................................................... ................................................................................................................... (2) (iv)     What happens to the mixture of unreacted gases (nitrogen and hydrogen)? ................................................................................................................... ................................................................................................................... ................................................................................................................... (1) (b)     The reaction to produce ammonia is reversible. Complete the word equation for this reaction. nitrogen     +     ................................................................................................... (2) (Total 9 marks) The flow diagram shows the Haber process. In the Haber process, ammonia (NH3) is produced from nitrogen (N2) and hydrogen (H2).   (a)     Which raw material is nitrogen obtained from? ........................................................................................................................ (1) 4 Page 8 of 66 (b)     Ammonia can be used to make ammonium nitrate (NH4NO3). (i)      Draw a ring around the correct answer to complete the sentence.     Ammonium nitrate can be made by reacting ammonia with ethanoic hydrochloric nitric acid. (1) (ii)     State one use of ammonium nitrate. ............................................................................................................... (1) (iii)    Calculate the relative formula mass (Mr) of ammonium nitrate (NH4NO3). Relative atomic masses: H = 1; N = 14; O = 16. ............................................................................................................... ............................................................................................................... Relative formula mass (Mr) = ................................................. (2) (iv)      Calculate the percentage by mass of nitrogen in ammonium nitrate. ............................................................................................................... ............................................................................................................... ............................................................................................................... ............................................................................................................... Percentage by mass of nitrogen = ............................ % (2) Page 11 of 66 (c)     In this question you will be assessed on using good English, organising information clearly and using specialist terms where appropriate. Ammonia is manufactured from nitrogen and hydrogen by the Haber process: N2(g)    +    3H2(g)         2 NH3(g) The forward reaction is exothermic. The conditions used in the Haber process are: •        200 atmospheres pressure •        450 °C •        iron catalyst. Use the equation and your knowledge of reversible reactions to explain why these conditions are used in the Haber process. To get full marks you must consider both yield and rate of reaction in your answer. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... (6) (Total 14 marks) Page 12 of 66           Ammonia is produced by the Haber process. In the process nitrogen and hydrogen are mixed. The pressure is increased to about 200 atmospheres. The gases are passed over an iron catalyst at about 450°C. The equation for the reaction is: N2(g)     +     3H2(g)             2NH3(g) 6           The reaction between nitrogen and hydrogen is reversible. This affects the amount of ammonia that it is possible to obtain from the process. The graph below shows how the pressure and temperature affect the percentage of ammonia that can be produced.           Use this information, together with your knowledge of the process, to explain why many industrial ammonia plants operate at 200 atmospheres and 450°C. ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... (Total 5 marks)     Page 13 of 66           The diagram shows some details of the manufacturing process.           The graph shows the percentage of reacting gases converted into ammonia at different temperatures and pressures.           At room temperature and pressure, the reaction is very slow and only a small percentage of the reacting gases is converted to ammonia.           Use the information on the diagram and graph to: Page 16 of 66 (a)     describe the conditions used in the manufacture of ammonia to increase the rate of reaction. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. (4) (b)     describe and explain the conditions used in the manufacture of ammonia to increase the yield. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. (7) (Total 11 marks)           Ammonia is manufactured in the Haber Process, from nitrogen and hydrogen. (a)     Balance this symbol equation for the process. (2) 9 Page 17 of 66 (b)     The graph below shows the percentage of reacting gases converted into ammonia, at different temperatures and pressures. (i)      What does the graph suggest about the temperature and pressure needed to convert the maximum percentage of reacting gases into ammonia? ................................................................................................................... ................................................................................................................... ................................................................................................................... (2) (ii)     Suggest reasons why the manufacture of ammonia in the Haber Process is usually carried out at about 400°C and 200 atmospheres pressure. ................................................................................................................... ................................................................................................................... ................................................................................................................... (2) (Total 6 marks)           (a)     In industry ammonia is produced from nitrogen and hydrogen. The equation for the reaction is:10 (i)      What does the symbol (g) represent? ................................................................................................................... (1) Page 18 of 66 (b)     The percentage yield of ammonia depends on the temperature and pressure inside the reaction vessel. The set of graphs show this. (i)      MPa is the symbol for which unit? ................................................................................................................... (1) (ii)     What is the percentage yield of ammonia produced at a temperature of 450 °C and a pressure of 20 MPa? ................................................................................................................... (1) (iii)     Suggest what changes the chemical engineers should make to both the temperature and the pressure to increase the percentage yield of ammonia. Temperature ..................................................................................................... Pressure ............................................................................................................ (1) Page 21 of 66 (iv)    How can the rate of ammonia production be increased without changing the temperature or pressure or the mass of hydrogen and nitrogen? ................................................................................................................... ................................................................................................................... (1) (c)     About four-fifths of ammonia production is used to produce fertilisers. One of them is known as NPK. It is made in the following way. •        Some ammonia is converted to nitric acid which is then mixed with phosphoric acid. •        The mixture is neutralised with more ammonia and the solution is partly evaporated. •        Potassium chloride is added to form granules. •        The granules are coated to make the fertiliser free-flowing. Complete the flow-chart for the production of NPK by writing in the names of the correct chemicals in the six boxes. (2) (Total 10 marks) Page 22 of 66           The Haber process is used to make ammonia (NH3) which is an important substance.           The equation below shows the reaction in which ammonia is formed.           The graph below shows how temperature and pressure affect how much ammonia is produced in the reaction.           In the industrial process a mixture of nitrogen and hydrogen is passed over iron at a temperature of about 450 °C and 200 atmospheres pressure. 12 N2(g)    +   3H2(g)              2NH3(g)   +     Heat (a)     Use the graph to find the percentage of ammonia present when the temperature and pressure are 450 °C and 200 atmospheres. ................................................................ % (2) (b)     Explain why the nitrogen and hydrogen mixture is passed over iron. ............................................................................................................................. ............................................................................................................................. (2) Page 23 of 66 (ii)     Use your graphs and your knowledge of the Haber process to explain why a temperature of 450°C and a pressure of 200 atmospheres are used in industry. ................................................................................................................... ................................................................................................................... ................................................................................................................... ................................................................................................................... ................................................................................................................... ................................................................................................................... (5) (c)     (i)      Ammonium nitrate is one type of artificial fertiliser. Calculate the relative formula mass of ammonium nitrate NH4NO3. (Relative atomic masses: H = 1, N = 14, O = 16.) ................................................................................................................... ................................................................................................................... (1) (ii)     Use your answer to part (c)(i) to help you calculate the percentage by mass of nitrogen present in ammonium nitrate NH4NO3. ................................................................................................................... ................................................................................................................... (2) (Total 15 marks)           Early atmospheres on Earth contained ammonia (NH3).           (a)     (i)      Complete the sentence. Our atmosphere today is made up of about .............................. % nitrogen. (1) 14 (ii)     Today we convert nitrogen back to ammonia mainly for the production of fertilisers. What do plants convert the nitrogen in these fertilisers into? ................................................................................................................... ................................................................................................................... (1) Page 26 of 66 (b)     The conversion of nitrogen to ammonia is shown. (i)      When making ammonia, what is one source of hydrogen? ................................................................................................................... (1) (ii)     Apart from ammonia, name one other product formed during this conversion. ................................................................................................................... (1) (c)     The main reaction is the formation of ammonia from nitrogen and hydrogen. (i)      Complete and balance the equation for this reaction. .......................(g)   +   .......................(g) →   .........NH3(g) (2) (ii)     Name the metal catalyst used in this reaction. ................................................................................................................... (1) (iii)     This reaction does not work successfully at room temperature (20 °C) and needs a much higher temperature of 400 °C. Explain why. ................................................................................................................... ................................................................................................................... ................................................................................................................... (2) Page 27 of 66 (d)     Draw a diagram to show the arrangement of the electrons in a molecule of ammonia. The electron arrangement of each atom is hydrogen 1 and nitrogen 2.5.           (2) (Total 11 marks)           The Haber process is named after the German chemist, Fritz Haber. The diagram shows the main stages in the Haber process. Reproduced with the permission of Nelson Thornes Ltd from PATRICK FULLICK et al, ISBN 0-7487-9644- 4. First published in 2006 (a)     Use the diagram to help you to answer these questions. 15 Page 28 of 66 (b)     Figure 1 shows how the yield of ammonia at 300 °C changes with pressure. Figure 1                                 Pressure in atmospheres Describe how the yield of ammonia changes as the pressure increases. ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ (3) Page 31 of 66 (c)     Figure 2 represents the Haber process. Figure 2 How does the Haber process avoid wasting nitrogen and hydrogen? ........................................................................................................................ ........................................................................................................................ (1) (d)     Before the Haber process, nitrates had been mined in South America. Nitrates are used for making fertilisers. The Haber process allowed nitrates to be produced on a large scale, anywhere in the world. (i)      Suggest what effect the Haber process had on the miners in South America. ............................................................................................................... ............................................................................................................... (1) (ii)     Suggest one advantage of producing nitrates on a large scale. ............................................................................................................... ............................................................................................................... (1) (Total 8 marks) Page 32 of 66 A flow diagram of the Haber process is shown below. The Haber process produces ammonia from nitrogen and hydrogen. (a)     Use the correct answer from the box to complete the sentence.     air limestone natural gas Hydrogen is obtained from ............................................................ . (1) 17 (b)     In the reactor, nitrogen and hydrogen at a high pressure are heated and passed over a catalyst. (i)      Use the correct answer from the box to complete the sentence.     25 100 450 The temperature in the reactor is ........................................ °C (1) (ii)     Use the correct answer from the box to complete the sentence.     copper iron nickel The catalyst used in the reactor is ........................................ . (1) Page 33 of 66 (b)     The graph in Figure 2 shows the percentage yield of ammonia using different conditions. Figure 2 (i)      Use Figure 2 to suggest the conditions that produce the greatest yield of ammonia. ................................................................................................................... ................................................................................................................... (1) Page 36 of 66 (ii)      Use Figure 2 to suggest and explain why the conditions used to produce ammonia in the Haber process are a temperature of 450 °C and a pressure of 200 atmospheres. ................................................................................................................... ................................................................................................................... ................................................................................................................... ................................................................................................................... ................................................................................................................... ................................................................................................................... ................................................................................................................... ................................................................................................................... ................................................................................................................... ................................................................................................................... ................................................................................................................... ................................................................................................................... ................................................................................................................... ................................................................................................................... (5) (Total 12 marks) Page 37 of 66 Mark schemes (a)    reversible 11 (b)     catalyst 1 (c)     recycled allow re-used 1 (d)     (Q) S R P allow 1mark if one letter in correct place. 2 [5]           (a)     mixture is cooled / cooling 12 so ammonia / it condenses or so ammonia turns into a liquid (but nitrogen and hydrogen remain as gases) 1 (b)     (i)      exothermic reaction accept reverse reaction is endothermic or equilibrium / reaction moves in the direction which raises the temperature ignore answers based on rate or collisions 1 (ii)     they / particles / molecules move faster or have more (kinetic) energy allow atoms instead of particles ignore particles move more / vibrate do not accept electrons (max1) 1 any one from: •        particles / molecules collide more often / more frequently / more likely to collide ignore collide faster ignore more collisions •        more of the collisions are successful or particles collide with more energy / harder or more of the particles have the activation energy accept more successful collisions 1 Page 38 of 66 (d)                        allow converse arguments                    ignore references to compromise because a higher temperature would reduce (equilibrium) yield allow higher temperature favours backward reaction 1 because a lower temperature would reduce rate 1 (e)     (i)      (energy of) reactants greater than (energy of) products allow converse allow (overall) energy decreases allow energy required to break bonds is less than the energy released making bonds 1 (ii)     line starting and finishing at same levels but with lower peak 1 [8] (a)     three bonding pairs do not allow non-bonding electrons in hydrogen ignore any inner shells on nitrogen 1 5 two non-bonding electrons allow either dots and crosses or combination of both 1 (b)     (i)      nitric 1 (ii)     fertilisers / explosives ignore other uses 1 (iii)    80 correct answer with or without working gains 2 marks if answer incorrect, allow 14 + (1 × 4) + 14 + (16 × 3) for 1 mark 2 (iv)    35 allow ecf from (b)(iii) allow ecf for 1 mark for correct working but incorrect answer. if answer incorrect, allow 28 / 80 × 100 for 1 mark if answer is 17.5 % allow 1 mark 2 Page 41 of 66 (c)     Marks awarded for this answer will be determined by the Quality of Communication (QoC) as well as the standard of the scientific response. Examiners should also refer to the information on page 5, and apply a ‘best−fit’ approach to the marking. 0 marks No relevant content Level 1 (1 − 2 marks) There are statements about the conditions used. There is no correct explanation of the link between rate or yield and the conditions. Level 2 (3 − 4 marks) There is a correct explanation of the conditions used that links the conditions to rate or yield Level 3 (5 − 6 marks) There is an explanation covering at least temperature and pressure, which shows understanding of the compromise between rate and yield examples of chemistry points made in the response: 200 atmospheres pressure •        high pressure gives a high yield of ammonia •        too high a pressure causes risk of explosion •        high pressure costly to maintain •        a high pressure will cause the rate to be higher •        4 moles of gas become 2 (or fewer moles of gas in products) 450 °C •        high temperature increases the rate of reaction •        optimum temperature •        (forward reaction is exothermic so) a high yield of ammonia requires a low temperature •        but too low a temperature causes the rate of reaction to be too slow iron catalyst •        a catalyst speeds up the reaction •        an iron catalyst allows a lower temperature to be used (saving energy and causing a higher yield) •        iron catalyst increases the rate of reaction equally in both reactions others •        compromise conditions •        unreacted nitrogen and hydrogen is recycled 6 [14] Page 42 of 66           Effect of pressure •        high pressure increases yield for 1 mark •        either because less product molecules (Le Chatelier) or but high pressure increases cost/safety for 1 mark 6 Effect of temperature •        low temperature increases yield for 1 mark •        either because exothermic reaction (Le Chatelier) for 1 mark •        or but at low temperature rate is slow/catalyst does not work Compromise •        optimum conditions to balance rate and % yield for 1 mark •        or rate is slow (at higher temperature) so need a catalyst or low percentage conversion so recycle untreated gases [5]   Page 43 of 66           (a)     (i)      gas accept they are all gases 1 (ii)     reversible (reaction) accept can go either way accept ammonia can be decomposed (to nitrogen and hydrogen) accept could be (an) equilibrium do not credit just ‘equilibrium’ 1 10 (iii)     (liquid) air or atmosphere 1 (iv)    same number or amount or weight (of atoms) on each side (of the equation) accept “sums” for each side accept same amounts of elements on each side do not credit molecules or compounds do not credit both sides are the same unless explained 1 of the same type or gives a correct example ‘e.g. six hydrogen atoms’ (on each side) 1           (b)     (i)      nitrate or sulphate or phosphate if first left blank, second may be awarded do not credit chloride nitric or sulphuric or phosphoric 1          (only if correct above, exception is for ammonium chloride followed by hydrochloric acid (1 mark)) as appropriate if only the formula is given this should be credited only if it is correct in every detail i.e. NH4NO3 HNO3 (NH4)2SO4 H2SO4 accept correct name with an incorrect version of the formula do not credit a correct formula with an incorrect version of the name e.g. ‘nitrate/sulphite’ etc 1 Page 46 of 66          any one of          * (solution) can be sprayed (on the fields or crops) accept more even distribution          * dissolves in soil water or rain (water) accept soaks into soil (because soaks implies water)          * can be taken up by (plant) roots do not credit can be added to water to “feed” the plants 1           (c)     (i)      elements or different atoms are bonded or joined or combined or reacted do not credit just ‘atoms’ do not credit added or mixed 1 (ii)     (pairs of) electrons are shared do not credit an electron is shared 1 [10]            (a)     (i)      atmosphere or (fractional distillation of liquid) air 1 (ii)     either more (chance) of them colliding/ not just ‘faster’ coming into contact or the volume of the product / the ammonia is less than / only half the volume of the reactants / the nitrogen and hydrogen 1 11 (iii)     3 × (1 ×2) of hydrogen → 2 × (14 +1 ×3) of ammonia accept 6 parts of hydrogen →34 parts of ammonia or similar i.e. candidate uses the atomic masses and works correctly from the equation 1 = 225 (tonnes/t) unit not required 1 Page 47 of 66 (b)     (i)      megapascal(s) accept million pascal(s) 1 (ii)     28 (%) accept any answer in the range 28.0 to 28.5 inclusive 1 (iii)     reduce the temperature and increase the pressure both required 1 (iv)    either use a catalyst accept use iron as a catalyst accept use iron which has been more finely divided accept use iron / catalyst with a bigger (surface) area accept use a better catalyst 1          or remove the ammonia (as it is produced) accept react the ammonia with or dissolve the ammonia in water (as it is produced) 1 (c)     ammonia nitric acid phosphoric acid all three on the left correct ammonia potassium chloride all three on the right correct water or water vapour accept ‘steam’ 1 [10]               (a)     16% for 2 marks           (attempt by drawing lines etc gains 1 mark) 2 12 Page 48 of 66 (b)     (i)      natural gas accept methane (CH4) accept water (H2O) 1 (ii)     carbon dioxide 1 (c)     (i)      N2 + H2 1 correct balancing 1 + 3 → 2 award only if reactants are correct 1 (ii)     iron accept Fe 1 (iii)     at low temperatures rate of reaction is too slow accept very few collisions at low temperatures accept converse          particles need enough (activation) energy to react accept particles need enough energy for bonds to break accept converse 1 (d)     all three covalent bonds displayed correctly as electron pairs 1           two lone electrons displayed not necessarily as a pair 1 [11]   Page 51 of 66 (a) (i) nitrogen + hydrogen → ammonia accept full correct balanced equation 1 (ii)     reversible (reaction) (owtte) do not allow just ‘backwards’ (unqualified) 1 (iii)     catalyst / speed up reaction accept to lower activation energy 1 (iv)    boiling point 1 (v)     recycled (owtte) 1 15 (b)     (i)      used to make explosives (owtte) used to make medicines (owtte) 1 (ii)     used to make fertilisers (owtte) 1 (c)     (i)      sensible answers such as          provides workers (owtte)          good transport links ignore reference to raw materials 1 Page 52 of 66 (ii)     sensible idea 1          linked reason           idea linked reason          eg escape of chemicals /fumes /waste gases / pollution harmful to health / environmental damage owtte do not allow harmful / damage / smell (unqualified)          risk of explosion because of high pressures / may endanger local people / dangerous          risk of fire because of high temperatures / may endanger local people          noise any detrimental effect on quality of life or night and day          lorries / traffic danger / noise / pollution etc          unsightly detrimental effect on quality of life / house prices / reduced tourism          uses a lot of land loss of habitats 1 [10]   Page 53 of 66 higher pressures would involve high cost / energy 1 ignore risk / explosion [12] Page 56 of 66 Examiner reports (a)    The great majority of students knew the meaning of the reversible reaction symbol. (b)     About three quarters of students answered correctly. 1 (c)     The majority of the students scored a mark here. However, there were some who stated that nitrogen and hydrogen were released into the air or condensed. (d)     Although most students managed to score one mark, disappointingly only about a third managed to sequence the statements entirely correctly for two marks. Page 57 of 66           (a)     Many students gained a mark for indicating that ammonia is turned into a liquid in the separator but fewer gained a mark for stating that this is achieved by cooling the mixture. Common misconceptions were that the mixture is filtered or that the ammonia is heavier than the nitrogen and hydrogen. Some students stated that the mixture is heated so that the nitrogen and hydrogen evaporate off, leaving the liquid ammonia. This did not gain credit. (b)     (i)      Despite the reversible nature of the Haber process being quite frequently referred to, only a minority of students were able to link the fact that the forward reaction is exothermic to the effect of a decrease in temperature on the position of equilibrium. Some students confused exothermic and endothermic; other non-scoring answers attempted an explanation based on Le Chatelier's principle but only restated the information given in the question. Some weaker students attempted an explanation based on rate of reaction. (ii)     Many excellent, comprehensive answers to this rate of reaction question were seen. Despite the fact that there were some references to particles vibrating which were ignored, most students gained a mark for stating that the particles have more energy or move faster when the temperature is increased. Fewer students, but still the majority, gained a mark for referring to the increased frequency of collisions or the increased proportion of collisions that result in a reaction. A small number of students appear to have the misconception that the activation energy increases when more energy is put into a system. (iii)    Students found this question very difficult. Many students gave an explanation in terms of rate of reaction and did not make the link between the increased yield of ammonia and the numbers of reactant and product molecules in the chemical equation. A few students gave detailed answers in terms of Le Chatelier’s principle which were, of course, accepted. The idea of the particles being squashed closer together was often given but did not gain credit. (iv)     Approximately half of the students gained this mark with the most common response being a reference to the cost implications of using a much higher pressure. References to the risk of explosion were sufficient to convey the idea of such a high pressure being difficult to contain but responses such as “it would be too dangerous” were considered to be too vague to gain credit. (c)     (i)      Approximately half of the students were able to use the mole ratio in the chemical equation to correctly calculate the volume of hydrogen needed. The most common incorrect answer was 20. (ii)     Many of the students found this type of calculation very difficult but almost a fifth of students gained all three marks. The correct answer gained full credit even if there was no working but students should always be encouraged to show their working. It would be helpful to examiners if students would set out their working clearly since it is difficult to award marks to a jumble of numbers. The question revealed that a significant number of students do not fully understand the use of numbers and formulae in chemical equations; a common error was finding the mass of 2NH3 to be (2 × 14) + (3 × 1) = 31. (d)     (i)      Most students correctly calculated the percentage yield of ammonia. (ii)     The majority of students gained a mark for the idea that the unreacted nitrogen and 2 Page 58 of 66 (a)     The majority of students knew the formula of ammonia and were also able to balance the equation. Only a few students scored just one mark. The most common incorrect answer, although there were many others, was N2H6. (b)     (i)      The vast majority of students were able to interpret the graph correctly for this question. (ii)     A majority of candidate answered correctly but a very significant minority tried to answer, incorrectly, in terms of yield. (iii)    About two thirds of students answered correctly. Given that the question presented a restricted choice of answers it would seem that students found it significantly more difficult than question (i). (vi)    A majority of students answered correctly. The fact that answers in terms of either rate or yield were acceptable perhaps explains why students performed better in this question than in question (ii). (c)     About half of students gained at least one mark; only about a fifth gained both. The mark for the condensation of ammonia was much more common than the mark for explaining that this is achieved by cooling. Some confused the separation of the gases with their reaction, giving the conditions in the reactor. Many referred to nitrogen and hydrogen being recycled. 7           Part (a) was done reasonably well by most candidates but few candidates gained more than two or three marks in part (b). This part of the question was frequently answered in terms of rate rather than yield. Also constraints on increasing the yield - i.e. the increased cost of the plant and its decreased cost-effectiveness as the pressure increases and the slower rate of reaction if the temperature is too low - were seldom mentioned. 8           (The diagram incorrectly indicated that unreacted ammonia rather than unreacted hydrogen is recycled along with unreacted nitrogen. Candidates were, therefore, given full credit for quoting the re-cycling of the materials shown on the diagram as one aspect of increasing the yield.)               (a)     Though a sizeable minority correctly balanced the equation, many – some of whom did not attempt the item – did not.9 (b)     Most candidates gained one mark for specifying the optimum temperature and pressure shown; many gained the second mark for identifying the trends.     Page 61 of 66           Foundation Tier           Some candidates knew what the (g) meant and others used their imagination suggesting ’gen’ in nitrogen, for example. Most candidates were able to explain the reversed arrows and the better ones gave air, or the atmosphere, as the source of the nitrogen. Only a minority of candidates were able to write a convincing explanation of how they could tell that the equation is balanced. The minority who could name an appropriate ammonium salt were usually able to name the required acid. Many candidates realised the value of solubility of ammonium fertilisers but a common erroneous belief was that plants feed on fertiliser. Few candidates were able to describe convincingly the nature of compounds with many references to chemicals and substances rather than to elements or different atoms. Only a small minority could say what a covalent bond is. Amongst those who made a start, a very common mistake was to write about one electron being shared. 10 Higher Tier           Both of the symbols were well known. Some gave grams instead of gas for (g). However, air was seldom given as the raw material for nitrogen. Most could give good explanations of why the equation was balanced. Surprisingly, many gave a wrong acid for a suitable ammonium salt. Candidates could apply their knowledge to suggest why the solubility of ammonia is a useful property of a fertiliser.           When specific explanations were required, weaker candidates did not give clear enough answers to gain marks. For example when completing the sentence defming a chemical compound, common mistakes were to fail to specify that the atoms were different or to refer only to mixing.           Similarly with the covalent bond, which was well answered by many, a number only referred to sharing one electron.              Parts (a)(i) and (ii) were generally well answered but only a minority appeared to have any idea about how to proceed with the calculation in part (iii).          Few candidates recognised the symbol for megapascals and some, without any regard for the context, identified it as a millipascal.          The remainder of the question was well answered except that, in (c), whilst all the other boxes were often correctly filled, the nature of whatever was leaving the evaporator was a mystery to many.     11 Page 62 of 66           Parts (a) and (b) were usually well answered. In (c) candidates often found this question very difficult and some scored very few marks, despite giving quite long answers. It was hoped that the graph would give the candidates a starting point upon which they could build an answer, based on their knowledge of syllabus section 3.3.5. Two marks were available for simply interpreting the graph and stating that a high yield of ammonia would be obtained at low temperature and high pressure. A number of candidates clearly did not understand the graph and gave very confused answers. It was also hoped that the equation at the start of the question would jog their memories to the reversible nature of the reaction, but many candidates made no reference to this. A wide range of answers was accepted in the mark scheme. More able candidates often gave excellent answers which sometimes went far beyond the requirements of the syllabus.     12           The graph in part (a)(i) was usually well drawn with many candidates gaining all four marks. Some candidates found difficulty with the scales which were sometimes non-linear whilst others made the intervals complicated so that it was difficult for them to plot the points accurately. Best lines of fit were usually drawn well although some candidates join the dots with straight lines or draw multiple lines. The vast majority of candidates answered parts (a)(ii) and (a)(iii) correctly. 13           Most candidates were able to gain the mark for part (b)(i) and the quality of answers for (b)(ii) was significantly better than last year. Some candidates write about temperature and pressure at the same time so that the answer becomes confused and difficult to give credit. Others remembered that the conditions used are a compromise but fail to explain why. A minority of candidates seemed very confused about the reasons for the conditions used and gave answers such as ‘the yield is kept low so that they do not make too much ammonia’ or ‘the yield is kept low so that some of the nitrogen and hydrogen can be re-used’. It is pleasing to report that many candidates were able to explain the reason why a low temperature gives a high yield of ammonia in terms of the reaction being exothermic and similarly why pressure affects the yield in terms of the relative number of reactant and product molecules. Questions of this type are best answered when the candidate structures the answer with separate paragraphs on the effect of temperature, the effect of pressure and a concluding paragraph which explains the compromise or the fact that the yield is not too important because the unreacted gases can be recycled.           The calculations of part (c) were usually well answered. In part (c)(i) some candidates lost the mark because they did not understand the formula and interpreted NH4 as (NH)4 and NO3 as (NO)3. The percentage calculation was very well done. A number of candidates managed to complete this calculation even when they did not calculate the Mr correctly in part (c)(i). Some candidates calculated (28/100 x 80) rather than (28/80 x 100).     Page 63 of 66