exemplar candidate work twenty first century science

TWENTY FIRST CENTURY SCIENCE CHEMISTRY B

QualificationAccredited

www.ocr.org.uk/chemistry

GCSE (9-1)

Exemplar Candidate Work

J258For first teaching in 2015

J258/03 Summer 2018 examination seriesVersion 1

http://www.ocr.org.uk/chemistry


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GCSE (9-1) Twenty First Century Science Suite Chemistry B

© OCR 2018

ContentsIntroduction 3

Question 1(a) 4

Question 1(b) 5

Question 1(c) 6

Question 1(d) 7

Question 2(a) 8

Question 2(b) 9

Question 2(c)(i) 10

Question 2(c)(ii) 11

Question 2(d) 12

Question 3(a) 13

Question 3(b) 14

Question 3(c) 15

Question 4(a)(i) 16

Question 4(a)(ii) 17

Question 4(a)(iii) 18

Question 4(b) 19

Question 5(a) 20

Question 6(a) 21

Question 6(b) 22

Question 6(c) 23

Question 6(d) 24

Question 7(a)(i) 25

Question 7(a)(i) 26


Question 7(b)(ii) 28

Question 7(c) 29

Question 8(a)(i) 30


Question 8(b) 32

Question 9(a) 33

Question 9(b)(i) 34


Question 9(c)(i) 36

Question 9(c)(ii) 37

Question 10(a) 38

Question 10(b) 39

Question 10(c) 40

Question 11(a) 41

Question 11(b) 42

Question 11(c) 43

Question 12(a) 44

Question 12(b)(i) 45


Question 12(b)(iii) 47


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© OCR 2018

IntroductionThese exemplar answers have been chosen from the summer 2018 examination series.

OCR is open to a wide variety of approaches and all answers are considered on their merits. These exemplars, therefore, should not be seen as the only way to answer questions but do illustrate how the mark scheme has been applied.

Please always refer to the specification https://www.ocr.org.uk/qualifications/gcse/twenty-first-century-science-suite-chemistry-b-j258-from-2016/ for full details of the assessment for this qualification. These exemplar answers should also be read in conjunction with the sample assessment materials and the June 2018 Examiners’ report or Report to Centres available from Interchange https://interchange.ocr.org.uk/Home.mvc/Index

The question paper, mark scheme and any resource booklet(s) will be available on the OCR website from summer 2019. Until then, they are available on OCR Interchange (school exams officers will have a login for this and are able to set up teachers with specific logins – see the following link for further information http://www.ocr.org.uk/administration/support-and-tools/interchange/managing-user-accounts/).

It is important to note that approaches to question setting and marking will remain consistent. At the same time OCR reviews all its qualifications annually and may make small adjustments to improve the performance of its assessments. We will let you know of any substantive changes.

https://www.ocr.org.uk/qualifications/gcse/twenty-first-century-science-suite-chemistry-b-j258-from-2016/



https://interchange.ocr.org.uk/Home.mvc/Index

https://interchange.ocr.org.uk/Home.mvc/Index

http://www.ocr.org.uk/administration/support-and-tools/interchange/managing-user-accounts/




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© OCR 2018

Question 1(a)

Examiners’ commentary A fully correct answer with working clearly shown. As the candidate had to read a value from a graph to use in a calculation, a range of 0.07-0.09 was allowed for the final correct answer.

Exemplar 1 2 marks


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Question 1(b)Exemplar 1 2 marks

Exemplar 2 0 marks

Examiners’ commentary Both marks are earned. The candidate has identified two factors (variables) to be controlled: the temperature and the ‘size’ of calcium carbonate to ensure that the surface area is constant. A good answer which earns both marks.

Examiners’ commentary This answer gained no marks. It was common to restate the question (in this case ‘volume’ was given in the question as already controlled). Notice also that this candidate has said that the same apparatus should be used (the beaker and the syringe) rather than identifying control factors. These are usually assumed in this type of question. Where control factors are asked for candidates should identify variables to be controlled rather than giving lower level answers such as ‘use the same apparatus’.


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Question 1(c)Exemplar 1 1 mark

Examiners’ commentary The candidate earns one mark for ‘more frequent collisions’ (not that just ‘more collisions’ alone would not have gained credit). However this is not linked to the concentration. To earn both marks, the candidates needed to explain how concentration affects the collisions of particles, for example by stating that they are closer together.


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Question 1(d)Exemplar 1 2 marks

Examiners’ commentary Although the candidate has done the calculation correctly, the question asks that the answer be recorded to ‘2 significant figures’. The addition of a zero after the decimal point means that this answer cannot earn the significant figures mark. The insistence on mathematical skills being tested to GCSE level is new to these specifications and candidates need to take care to check all of the instructions in calculations to ensure that they earn every mark.


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Question 2(a)Exemplar 1 2 marks

Examiners’ commentary Most candidates realised that carbon dioxide leaves the test tube and this results in a loss of mass. Hence the most common mark for this question was (1). However, this answer has continued to explain what the law of conservation of mass is and why this situation appears not to obey it. This is a higher level answer and earns both marks.


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Examiners’ commentary A fully correct answer. Notice how clearly the candidate has laid out the answer so that, even if all marks were not earned, the examiner would be able to find any partially correct working. Notice also that the candidate has correctly given their answer to the 1 decimal place demanded in the question.


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Question 2(c)(i)Exemplar 1 1 mark

Examiners’ commentary This is a relatively simple calculation, set early in the paper. The values given enable candidates to work out the mass of carbon dioxide by ratio, rather than needing to use a full calculation based on reacting masses. The correct answer is given and earns a mark.


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Question 2(c)(ii)Exemplar 1 1 mark

Examiners’ commentary This is a clear, correct answer. Notice that there are no instructions here to inform candidates to report their answer to a particular number of decimal places or significant figures.


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Examiners’ commentary This is fully correct. Notice how clearly the candidate has shown the outer electron shells. The candidate has followed the instructions precisely, only showing the outer shell (empty in the case of Mg, but a missing outer shell was given full credit). Notice that the electrons are represented as both dots and crosses and that the overall charges on each ion are clearly shown.


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Examiners’ commentary This answer is characterised by the candidate processing (rather than repeating) the information in the table. For each polymer, the candidate has identified salient points related to the use of the polymer for cups. Polymer B softens below the boiling point of water. A has the highest breaking strength. A has the highest softening temperature. These are clear points and earn all 3 marks.


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Examiners’ commentary The correct polymer is chosen and the strength of intermolecular forces has been correctly linked to the softening temperature, with an explanation in terms of energy to overcome the intermolecular forces. A very good answer.

http://www.ocr.org.uk/Images/170850-unit-031-1-the-local-business-environment-sample-assessment-material.pdf


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Examiners’ commentary The monomer, with its double bond, is drawn fully correctly.


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Question 4(a)(i)Exemplar 1 2 marks

Examiners’ commentary In questions such as this, candidates need to access the detail in any given data (in this case the data is given as a graph). Weaker answers such as ‘it increases’ are unlikely to gain credit. Answers which quote many values, rather than focus on the most important, are similarly not such high level answers. In this case the candidate has described the pattern of the change succinctly (very low, then a rapid increase, followed by fluctuations) and has read the most important data from the graph (the rapid increase happened 2.5 billion years ago). This is a very sophisticated description of the data.


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Question 4(a)(ii)Exemplar 1 1 mark

Examiners’ commentary The candidate has read the values correctly from the graph and processed them to work out that the increase. As there was a tolerance allowed in the reading of the graph, all answers within a range of 1.7 - 2.0 were accepted as correct.


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Question 4(a)(iii)Exemplar 1 2 marks

Examiners’ commentary The candidates understand that both photosynthesis and respiration control the balance of oxygen in the atmosphere. There is a slight misconception here which was not penalised; the answer implies that plants do not respire.


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Question 4(b)Exemplar 1 1 mark

Examiners’ commentary Although this is a challenging equation to balance, as the formulae were given, with spaces for numbers, most candidates managed to complete the balancing correctly, as this candidate has done.


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Exemplar 2 0 marks

Examiners’ commentary This answer correctly explains that chlorine is more reactive than bromine and so can displace it, earning 2 marks. However, in common with most other candidates, the correct ionic equation was not given. Candidates typically find ionic equations for halogen displacement very challenging, it is a specification area that is not well understood.

Examiners’ commentary The unfortunate use of ‘chloride’ and ‘bromide’ rather than ‘chlorine’ and ‘bromine’ means that this answer cannot earn any credit. It is incorrect to compare the reactivity of ions or to discuss halide ions displacing each other. This confusion of language is an unfortunate but common error.


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Question 6(a)Exemplar 1 1 mark

Examiners’ commentary The high surface area to volume ratio is responsible for almost all of the properties of nanoparticles, so it is important that candidates learn this idea thoroughly as it is likely to gain a mark in almost every nanoparticle question.


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Examiners’ commentary This is a well thought out and well expressed answer. The candidate addresses both ‘sides’ of the argument, taking information from the question to express ideas about the use of nanoparticles to ensure complete combustion, discussing the outcomes of this on air pollutants. The candidate further draws on their own knowledge to discuss the drawbacks of nanoparticles. This is a very good answer.


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Examiners’ commentary Many candidates found it difficult to express the idea that charges must balance in a formula. This answer both shows that the total negative charges are -4 but also expresses the idea that the charge of the cerium ion must be ‘equal and opposite’. This is a very clear answer.


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Examiners’ commentary Although the answer is not fully correct, the clearly laid out calculation has allowed the candidate to gain 2 of the available 3 marks. The candidate has stated the number of particles in a mole and has the first stage of the calculation; they have clearly shown the ratio of volumes of nanoparticles to molecules.


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Question 7(a)(i)Exemplar 1 1 mark

Examiners’ commentary The candidate has clearly linked the ‘forwards and backwards’ reversible reaction to the arrow.


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Question 7(a)(ii)Exemplar 1 2 marks

Examiners’ commentary Most candidates knew that equilibrium reactions go in both directions. This answer, however, clearly gives the second important idea; that these reactions happen at the same rate at equilibrium, resulting in no visible changes.


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Question 7(b)(ii)Exemplar 1 2 marks

Exemplar 2 0 marks

Examiners’ commentary This answer is at a very high level. Most candidates confused position of equilibrium with rate, and discussed how increased pressure would ‘speed up the rate’ or even ‘speed up the equilibrium’. This candidate has expressed how the ratio of volume of products to reactants is important and has correctly predicted the effect of increased pressure on the position of equilibrium.

Examiners’ commentary This answer is unfortunate because the candidate clearly understands how pressure affects equilibrium position. However, the question asks about ‘the reaction’ i.e. the reaction in the question. Hence the answers need to discuss how a change in pressure impacts on the equilibrium in this specific reaction, rather than in general terms. The candidate has also discussed rate, which is not relevant to the question.


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Question 7(c)Exemplar 1 3 marks

Examiners’ commentary Firstly, this answer is well structured. The candidate has paused for thought and has written reasons for and against in the correct places so that it is very clear to read. Both ‘for’ reasons come from line 2 (no by-products and 100% atom economy). Notice that the prompts at the start of the lines are to help candidates to structure their answers, and are not used as barriers to scoring; two correct responses on one line earn two marks. The candidate has also identified an important ‘against’. Cost arguments alone (‘it is cheaper/more expensive) are not usually credited marks. Notice in this case that the cost is linked to a process condition – the high temperature.


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Question 8(a)(i)Exemplar 1 1 mark

Examiners’ commentary Although the candidate has correctly followed the instructions to ‘include state symbols’, one of the symbols is incorrect. Carbon has been represented as a gas (g).


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Question 8(a)(ii)Exemplar 1 2 marks

Examiners’ commentary The question asks candidates to ‘use ideas about reactivity and reduction’. Many candidates did not follow this instruction. In this answer, the candidate has faithfully addressed both issues and so gains both marks. Very good examination technique is shown.


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Examiners’ commentary There were several common errors which many candidates made when answering this question. Many discussed protons. Some talked about positive and negative ions. This answer gains all 3 marks, but it is not ‘perfect’. The candidate has identified that the particles are positive ions and that the electrons are delocalised. The answer also states that these have strong forces of attraction for each other. However, notice that at the start they imply that the positive ions can move. This was ignored here, because the description in this case (‘slide over each other’) is an explanation for malleability, but candidates who stated clearly that the positive ions are mobile would not gain all 3 marks.


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Question 9(b)(i)Exemplar 1 2 marks

Examiners’ commentary This is an excellent answer, showing that the candidate knows exactly what an emission spectrum looks like (lines of colours on a black background). Other candidates typically knew that emission spectra involve ‘colours’ but did not typically describe the arrangement of lines so clearly.


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Question 9(b)(ii)Exemplar 1 1 mark

Examiners’ commentary Although (1) mark was earned, this is a ‘medium’ level response because the answer only discusses ‘comparing spectra’. The idea of a comparison was enough to earn a mark but was not the ‘best’ answer seen. A higher level answer would state that the positions of the lines on the spectra should be compared and matched.


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Question 9(c)(i)Exemplar 1 1 mark

Examiners’ commentary ‘White precipitate’ is a common response to all ion tests. In this case it is correct but some candidates use this observation for every question which asks for observations on mixing reagents, whether or not it is correct.


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Question 9(c)(ii)Exemplar 1 2 marks

Examiners’ commentary This answer is fully correct. Most candidates gave the correct formulae for both products, but the ‘high level’ mark was to balance the equation. It was common for candidates to omit the ‘2’ before ‘NaCl’.


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Examiners’ commentary Although this candidate has earned all 3 marks, this is a ‘medium’ level response because some higher level responses were seen. The candidate has correctly calculated the energy needed to break bonds, and that given out when bonds form. The layout is very clear, allowing partial credit should the final answer be incorrect. However, best answers gave the sign for the enthalpy change (in this case +). In this case, as the change was endothermic, ‘490’ alone was accepted as the value was positive. However, for similar calculations where the outcome is exothermic, candidates would be expected to show the negative sign if the overall change demanded it.


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Examiners’ commentary All 3 marks were earned; the shape of the curve and the position of the final products is correct and all is labelled. The activation energy clearly ‘starts’ at the reactants and ‘ends’ at the very top of the hump. However, the activation has been shown with a double headed arrow. At this level this is accepted as correct, but to completely obey convention, energy changes such as energy changes of reaction and activation energies should have a correct direction and value. A ‘perfect’ answer would show the arrow pointing upwards to show that energy is taken in.


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Examiners’ commentary Two perfect equations; it was common for candidates to know the aluminium equation but most could not write the equation for the discharge of oxygen ions. In this case both are fully correct with the correct numbers of electrons.


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Examiners’ commentary This is a very good description of the mechanism for conductivity in molten ionic compounds. A very common error in this question was to discuss the movement of electrons, rather than ions, showing confusion of the conduction of ionic compounds with that of metals.


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Examiners’ commentary The most demanding questions are often placed at the very end of the paper. This is a demanding calculation. Notice that this answer is very clearly laid out, allowing for partial credit even if all marks were not credited. Notice also that, without prompting, this candidate has rounded the answer to an appropriate number of significant figures. This is a very high level of response to this lengthy calculation.


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Question 12(b)(ii)Exemplar 1 3 marks

Examiners’ commentary This is another example of a more demanding calculation, placed towards the end of the paper. The equation is given, but a unit conversion is needed, which this candidate does routinely. Notice again how clearly the calculation is laid out, enabling partial credit if the correct answer had not been reached.


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Question 12(b)(iii)Exemplar 1 3 marks

Examiners’ commentary There were a number of challenges in this question. The candidate needed to transfer their calculated value from 12bii; then to use this value to calculate the number of moles of sulfuric acid used and then from this to calculate the concentration of the sulfuric acid. Finally there was an instruction to give the answer to 2 significant figures. At the end of this long paper, this candidate worked steadily, logically and clearly through every step, with all working shown, to give a fully correct answer. This is a very high level response.

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