AQA CERTIFICATE Chemistry

Transcription

1 AQA CERTIFICATE Chemistry 8402/2 Mark scheme 8402 June 206 Version/Stage:.0 Final Mark Scheme

2 Mark schemes are prepared by the Lead Assessment Writer and considered, together with the relevant questions, by a panel of subject teachers. This mark scheme includes any amendments made at the standardisation events which all associates participate in and is the scheme which was used by them in this examination. The standardisation process ensures that the mark scheme covers the students responses to questions and that every associate understands and applies it in the same correct way. As preparation for standardisation each associate analyses a number of students scripts. Alternative answers not already covered by the mark scheme are discussed and legislated for. If, after the standardisation process, associates encounter unusual answers which have not been raised they are required to refer these to the Lead Assessment Writer. It must be stressed that a mark scheme is a working document, in many cases further developed and expanded on the basis of students reactions to a particular paper. Assumptions about future mark schemes on the basis of one year s document should be avoided; whilst the guiding principles of assessment remain constant, details will change, depending on the content of a particular examination paper. Further copies of this mark scheme are available from aqa.org.uk Copyright 206 AQA and its licensors. All rights reserved. AQA retains the copyright on all its publications. However, registered schools/colleges for AQA are permitted to copy material from this booklet for their own internal use, with the following important exception: AQA cannot give permission to schools/colleges to photocopy any material that is acknowledged to a third party even for internal use within the centre.

3 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 Information to Examiners. General The mark scheme for each question shows: the marks available for each part of the question the total marks available for the question the typical answer or answers which are expected extra information to help the Examiner make his or her judgement and help to delineate what is acceptable or not worthy of credit or, in discursive answers, to give an overview of the area in which a mark or marks may be awarded. the Assessment Objectives and specification content that each question is intended to cover. The extra information is aligned to the appropriate answer in the left-hand part of the mark scheme and should only be applied to that item in the mark scheme. At the beginning of a part of a question a reminder may be given, for example: where consequential marking needs to be considered in a calculation; or the answer may be on the diagram or at a different place on the script. In general the right-hand side of the mark scheme is there to provide those extra details which confuse the main part of the mark scheme yet may be helpful in ensuring that marking is straightforward and consistent. 2. Emboldening and underlining 2. In a list of acceptable answers where more than one mark is available any two from is used, with the number of marks emboldened. Each of the following bullet points is a potential mark. 2.2 A bold and is used to indicate that both parts of the answer are required to award the mark. 2.3 Alternative answers acceptable for a mark are indicated by the use of or. Different terms in the mark scheme are shown by a / ; eg allow smooth / free movement. 2.4 Any wording that is underlined is essential for the marking point to be awarded. 3. Marking points 3. Marking of lists This applies to questions requiring a set number of responses, but for which students have provided extra responses. The general principle to be followed in such a situation is that right + wrong = wrong. Each error / contradiction negates each correct response. So, if the number of error / contradictions equals or exceeds the number of marks available for the question, no marks can be awarded. However, responses considered to be neutral (indicated as * in example ) are not penalised. 3 of 20

4 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 Example : What is the ph of an acidic solution? Student Response Marks awarded green, red*, 5 3 red*, 8 0 [ mark] Example 2: Name two planets in the solar system. Student Response Marks awarded Pluto, Mars, Moon 2 Pluto, Sun, Mars, Moon 0 [2 marks] 3.2 Use of chemical symbols / formulae If a student writes a chemical symbol / formula instead of a required chemical name, full credit can be given if the symbol / formula is correct and if, in the context of the question, such action is appropriate. 3.3 Marking procedure for calculations Full marks can be given for a correct numerical answer, without any working shown. However, if the answer is incorrect, mark(s) can be gained by correct substitution / working and this is shown in the extra information column or by each stage of a longer calculation. 3.4 Interpretation of it Answers using the word it should be given credit only if it is clear that the it refers to the correct subject. 3.5 Errors carried forward Any error in the answers to a structured question should be penalised once only. Papers should be constructed in such a way that the number of times errors can be carried forward are kept to a minimum. Allowances for errors carried forward are most likely to be restricted to calculation questions and should be shown by the abbreviation ecf in the marking scheme. 3.6 Phonetic spelling The phonetic spelling of correct scientific terminology should be credited unless there is a possible confusion with another technical term. 3.7 Brackets (..) are used to indicate information which is not essential for the mark to be awarded but is included to help the examiner identify the sense of the answer required. 3.8 Accept / allow Accept is used to indicate an equivalent answer to that given on the left-hand side of the mark scheme. Allow is used to denote lower-level responses that just gain credit. 4 of 20

5 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE Ignore / Insufficient / Do not allow Ignore or insufficient is used when the information given is irrelevant to the question or not enough to gain a marking point. Any further correct amplification could gain the marking point. Do not allow means that this is a wrong answer which, even if the correct answer is given, will still mean that the mark is not awarded. 4. Quality of Written Communication and levels marking In Question 3(c) students are required to produce extended written material in English, and will be assessed on the quality of their written communication as well as the standard of the scientific response. Students will be required to: use good English organise information clearly use specialist vocabulary where appropriate. The following general criteria should be used to assign marks to a level. Level : Basic Knowledge of basic information Simple understanding The answer is poorly organised, with almost no specialist terms and their use demonstrating a general lack of understanding of their meaning, little or no detail The spelling, punctuation and grammar are very weak. Level 2: Clear Knowledge of accurate information Clear understanding The answer has some structure and organisation, use of specialist terms has been attempted but not always accurately, some detail is given There is reasonable accuracy in spelling, punctuation and grammar, although there may still be some errors. Level 3: Detailed Knowledge of accurate information appropriately contextualised Detailed understanding, supported by relevant evidence and examples Answer is coherent and in an organised, logical sequence, containing a wide range of appropriate or relevant specialist terms used accurately The answer shows almost faultless spelling, punctuation and grammar. 5 of 20

6 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 Question Answers Extra information Mark AO / Spec. Ref. (a)(i) C AO 6.3b (a)(ii) A AO.a (a)(iii) B AO 2.a (b)(i) 0 AO3 4.c (b)(ii) The atoms have a stable arrangement of electrons. AO 4.c (c) first row ticked AO 2.2a, 2.2b Total 6 6 of 20

7 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 Question Answers Extra information Mark AO / Spec. Ref. 2(a) (s), (g), (s) must be in this order all 3 correct gains 2 marks or 2 in correct position gains mark 2 AO / AO2.3g 2(b)(i) To let air in. AO3 3.d, 3db 2(b)(ii) To make sure that all the calcium had reacted. AO3.3h, 3.d, 3db 2(b)(iii) weighing / masses / the balance do not allow the idea of loss of product/ca when opening the lid this is a systematic error ignore references to time, impure Ca, surface area, amount AO3.3h, 3dj 2(b)(iv) straight line drawn through origin line is best fit - equal distribution of points either side of line allow a leeway of +/- half a small square on going through (0,0) an acceptable straight line through (0,0) will pass through or above the point with a mass of 0.6 and through or below the point at a mass of 0.4 AO2.3h, 3dg,3e9 allow second mark even if line does not go through (0.0). Line must be straight 2(b)(v) correct reading from their graph of mass of calcium oxide produced from 0.35 g calcium minus 0.35 g this must be ecf from the graph line AO2.3h, 3dl, 3e2 correct answer with or without working gains 2 marks 7 of 20

8 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 2(b)(vi) (direct) proportion do not allow inverse proportion accept quantitative statements such as if the mass of calcium doubles, so does the mass of calcium oxide allow mark for any answer that indicates as one increases, so does the other (eg positive correlation) 2 AO2 / AO3.3h, 3dk, 3e6 Total 8 of 20

9 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 Question Answers Extra information Mark AO / Spec. Ref. 3(a)(i) Filtration AO2 4.2a, 3dl 3(a)(ii) add water (and stir) then filter/decant do not allow the use of ethanol dependent on addition of water or it is clear they have a solution AO2 4.2a, 3da evaporate (the water) to obtain crystals/solid or crystallise the potassium iodide dependent on the addition of water or it is clear have a solution allow evaporate to dryness or boil off the water ignore references to trying to dry the potassium iodide do not allow use of fractional distillation or melting. 3(b)(i) start line should be in pencil because ink is soluble or will affect results or will run or will wash away or pencil is insoluble AO / AO3 4.2b, 3db the solvent should be water or the solvent should be something else because the colouring is not soluble in propanone or the colouring is soluble in water or so chromatography can occur allow so it will run ignore so it will work use a solvent in which it can dissolve gains 2 marks use water as the solvent so the dye can separate/dissolve gains 2 marks 9 of 20

10 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 3(b)(ii) (solvent moves) 43 (mm) and (spot moves) 9.5 (mm) correct calculation of R f based on measured distances to 2 sig figs or more allow range of 42.5 to 43.5 allow range of 7 to 2 allow anything in range 0.39 to 0.49 for R f if first mark awarded ecf on measurements 2AO2 / AO3 4.2b, 3dm, 3dl, 3e3 (R f ) value close to 0.43 or (R f )value close (R f ) value of yellow 3 allow idea of spot being just under half way (to solvent front) so 0.43 is the nearest if measurements incorrect allow R f value obtained is not close to 0.43 so it cannot be yellow 3 because the R f value is wrong 3(b)(iii) R f value close to that of Yellow 2/4 or other colourings may have same R f value AO3 4.2b, 3do, 3dl Question 3 continues on the next page 0 of 20

11 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 Question 3 continued Question Answers Extra information Mark AO / Spec. Ref. 3(c) 6 2AO / 4AO3 4.2b,c,d, 3dm Marks awarded for this answer will be determined by the Quality of Communication (QC) 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 Level ( 2 marks) Level 2 (3 4 marks) Level 3 (5 6 marks) No relevant content. At least one relevant statement is made. Relevant statements are made. Uses at least two of the data sources to make at least two correct conclusions. Relevant statements and conclusions are made. Information from different figure should have been correctly combined. Most of the data sources must be considered. examples of the points made in the response Figure 7 all substances are soluble (in ethanol)) (because no spot left on start line) there are (at least) four (ethanol) soluble substances (because 4 spots on chromatogram) extra information Figure 8 not all substances are soluble (in water) (because there is a spot on the start line) there is (at least) one (water-) soluble substances (because there is one spot that has moved) not all got same R f (because some left on start line) Figure 9 4 substances in mixture (because there are 4 peaks) C is major substance in mixture (because peak is biggest) B makes up smallest part of mixture (because peak is smallest) Table 4 substance D has the largest M r (because molecular ion peak at biggest value) substance C has the smallest M r (because molecular ion peak a lowest value) of 20

12 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 Combining data sources more substances soluble in ethanol than water (or vice versa) three substances not soluble in water / have same R f all substances soluble in ethanol as 4 spots on Figure 7 and 4 peaks in Figure 9 separation greater in ethanol than water no link between retention time and M r Total 8 2 of 20

13 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 Question Answers Extra information Mark AO / Spec. Ref. 4(a)(i) ring drawn around 62 or Experiment 4 AO3 3dh 4(a)(ii) any one from: one (or both) of the solutions were warmer or at a higher temperature must be a reason for the reaction time being lower (not just being different) AO3 3dh the solutions had been stirred a lower volume of one of the solutions had been added accept started timer late or stopped timer early the student was not looking from the top of the beaker accept a wider/bigger beaker was used 4(b)(i) any three from: volume/size of drop temperature (of solution or lab temperature) compound = catalyst allow amount of catalyst or volume of catalyst ignore mass of catalyst 3 AO3 3dd concentration of solutions, E, F, G or H concentration of sodium thiosulfate or concentration of iron(iii) nitrate volume of sodium thiosulfate or iron(iii) nitrate allow concentration(s) of solutions/reagents/substances allow volume(s) of solutions/reagents/substances. Ignore same amount. rate of stirring (if any) same size/shape beaker use same cross on paper allow same person looking Question 4 continues on the next page 3 of 20

14 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 Question 4 continued Question Answers Extra information Mark AO / Spec. Ref. 4(b)(ii) G (addition of drop of G causes the cross to become visible in the) shortest time or the rate of reaction increases the most or time reduces the most allow 2 dependent on selecting G allow statements that talk about time taken for the cross to disappear AO3 3dm 4(c)(i) the enthalpy change ( H) L the activation energy (E a ) J AO.2d 4(c)(ii) a single line showing the catalysed reaction has lower activation energy (showing intermediate or transition state) must start at the level of the reactants and end at the level of the products AO.2d,e,h 4(d) the reaction is endothermic exothermic = 0 AO2 / AO3.2d because the products have more energy than the reactants or the products (line) is above the reactants (line) accept enthalpy change is positive or H positive for either one of the statements Total 2 4 of 20

15 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 Question Answers Extra information Mark AO / Spec. Ref. 5(a)(i) before: 20 after: 9 change: ignore trailing zeros ecf from recorded temperatures AO2.a, 3dg, 3e3 ignore the sign of the temperature change 5(a)(ii) Endothermic AO3.a,c, 3dm 5(b)(i) total mass not 20 g or wrong mass of citric acid used or used 22g accept should be 2g citric acid AO3 5.3b,.c, 3dh 5(b)(ii) all 4 points plotted correctly curve of best fit drawn line extended appropriately to meet printed line 2 or 3 points correct gains mark ignore point plotted at (4,-) do not award for straight line accept extrapolation of straight line 2 AO2 5.3b,.c, 3dg, 3e9, 3e 5(b)(iii) mass citric acid = 0.4(g) mass sodium carbonate = 9.6(g) ecf on graph line ecf from mass citric acid total mass must be 20 g AO2 / AO3 5.3b,.c, 3dl 5(b)(iv) as mass of citric acid increases temperature goes down (more) and then decrease gets less ignore numbers given in the answer allow rises again or then get warmer/higher for mark 2 AO2 5.3b,.c, 3dk 5(b)(v) the curved line is extended/extrapolated from the reading at 8 g construction line showing reading at 9 g extrapolation need not go to the correct temperature allow point drawn at 9g on extrapolated line AO2 / AO3 5.3b,.c,3dl, 3dm Question 5 continues on the next page 5 of 20

16 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 Question 5 continued Question Answers Extra information Mark AO / Spec. Ref. 5(b)(vi) temperature lower / temperature changes more polystyrene is an insulator / polystyrene is a poor(er) (heat) conductor (than glass) / polystyrene prevents heat gain stand-alone mark ignore prevents heat loss AO3 5.3b,.c, 3db, 3do 5(b)(vii) temperatures would be higher or would not change or changes would be less (water bath) increases heat gain or provides energy to reactants / mixture/beaker or prevents the (reaction mixture) cooling or heats the mixture ignore comments on rate of reaction AO3 5.3b,.c, 3db, 3do Total 20 6 of 20

17 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 Question Answers Extra information Mark AO / Spec. Ref. 6(a)(i) add (a few drops) of a (named) indicator allow universal or ph indicator AO 5.h (add the) ethanoic acid (from a) burette do not allow biuret until the indicator changes colour colour change does not need to be stated or correct then any two further points from: record the volume before and after addition repeat (to get concordant results) swirl/stir the flask add the acid dropwise (near the endpoint) read burette at eye level allow record the volume used 2 6(a)(ii) 0.4(0) (moles per dm 3 ) correct answer with or without working gains 2 marks if answer incorrect award mark for the number of moles of sodium hydroxide = or allow 0.2 x 25/000 2 AO2 5.i, 3e3 Question 6 continues on the next page 7 of 20

18 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 Question 6 continued Question Answers Extra information Mark AO / Spec. Ref. 6(b)(i) Test add a named reactive metal (magnesium, aluminium, zinc, iron or tin) Result slower effervescence with ethanoic acid apply list principle result dependent on test being correct or a near miss do not allow a group metal 2 AO 0.2b add a named carbonate or hydrogencarbonate slower effervescence with ethanoic acid add any named alcohol (with a strong acid catalyst) sweet smelling substance produced with ethanoic acid or no reaction with HCl ignore ph meter add universal indicator red with hydrochloric acid or orange / yellow with ethanoic acid add silver nitrate (white) precipitate with hydrochloric acid or no precipitate with ethanoic acid allow converse throughout for hydrochloric acid allow any other chemical test and result that will work 8 of 20

19 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 6(b)(ii) Test Result apply list principle 2 AO 4.e add silver nitrate (solution) sodium chloride produces a white (precipitate) or sodium iodide produces a yellow (precipitate) or the precipitate formed from sodium chloride dissolves in (dilute or concentrated) ammonia addition of dilute nitric acid not necessary. Do not award first marking point if candidate has acidified using dilute hydrochloric acid add bromine (water) goes brown/darker with sodium iodide or no reaction/change with sodium chloride add chlorine water goes brown with iodide or no change with chloride allow any other chemical test and result that will work 6(b)(iii) add (dilute) sodium hydroxide both produce a (white) precipitate AO 4.b but the precipitate formed with aluminium (sulfate) dissolves in excess/more sodium hydroxide allow becomes colourless for dissolves or add (dilute) sodium hydroxide allow lots of in excess no ppt with aluminium (sulfate) / (white) ppt with magnesium (sulfate) Total 4 9 of 20

20 MARK SCHEME AQA CERTIFICATE IN CHEMISTRY 8402/2 JUNE 206 Question Answers Extra information Mark AO / Spec. Ref. 7(a) error heat loss (to the atmosphere) improvement any way of insulating the apparatus or use a (metal) calorimeter (rather than a beaker) or use a bomb calorimeter or use a lid or move the spirit burner closer to the beaker or draught shields mark both parts together do not allow polystyrene cup, or insulation if specify insulating the bottom AO3.2a, 3dj, 3dp 7(b)(i) 040 (kj per mole) correct answer with or without working gains 3 marks allow 2 marks for (+)040 (kj per mole) if answer incorrect award up to 2 marks for any two of the steps below: bonds formed: ( ) = 5768 (kj per mole) or ( ) = 5304 (kj per mole) 4728 bonds formed (correct method used) 3 AO2.2e, 3e3 allow ecf for arithmetical errors 7(b)(ii) (the reaction is exothermic because) the energy released from forming (new) bonds answer must convey explanation of why exothermic AO.2f is greater than the energy needed to break (existing) bonds 7(c) the energy produced from forming (4) extra C=O and (4) O H allow for mark more bonds are made (in the product) so more energy is released. AO2 / AO3.2f is greater than the extra energy required to break the extra bonds in the reactants Total 9 20 of 20