AQA CERTIFICATE CHEMISTRY

Transcription

1 AQA CERTIFICATE CHEMISTRY Paper (8402/) Mark scheme 8402/ June 204 Version:.0 Final

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 204 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 Level /2 Certificate in Chemistry Paper 8402/ June 204. General Information to Examiners 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 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 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. 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. Example : What is the ph of an acidic solution? ( mark) Student Response Marks awarded green, red*, 5 3 red*, of 20

4 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June 204 Example 2: Name two planets in the solar system. (2 marks) Student Response Marks awarded Neptune, Mars, Moon 2 Neptune, Sun, Mars, Moon 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 e.c.f. 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 Ignore / Insufficient / Do not allow Ignore of insufficient is used when the information given is irrelevant to the question or not enough to gain the 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 of 20

5 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June Quality of Communication and levels marking In Question 4 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 Level /2 Certificate in Chemistry Paper 8402/ June 204 (a)(i) an electron AO.2c,.2j (a)(ii) a neutron AO.2c (a)(iii) A AO2.2f (a)(iv) boron AO2.2g (b)(i) GH 3 AO2 2.g (b)(ii) covalent AO2 Total 5 2.h 6 of 20

7 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June 204 2(a)(i) precipitation AO 5.2c 2(a)(ii) (aq) on left hand side AO (s) on right hand side.3g, 5.2c 2(a)(iii) potassium iodide AO2 potassium nitrate.3f, 5.2c 2(a)(iv) filtration AO 4.2a 2(b)(i) diffusion AO.b 2(b)(ii) iodide ions move / diffuse faster than lead ions or travel further in the same time Must be a comparison Accept converse AO2/AO3.b because the lead iodide forms much closer to the lead nitrate (or X) than the potassium iodide (or Y). allow because iodide ions are smaller than lead ions allow references to potassium iodide and lead nitrate 2(b)(iii) the particles / ions move / diffuse faster because they have more energy or will collide / meet sooner ignore which particles / ions the student refers to ignore reference to frequency of collisions AO/AO3 7c Total 7 of 20

8 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June 204 3(a)(i) Na allow sodium / phonetic spelling if more than one answer is given apply list principle 3(a)(ii) Fe allow iron / phonetic spelling if more than one answer is given apply list principle 3(a)(iii) Na or S allow sodium or sulfur / sulphur / phonetic spelling if more than one answer is given apply list principle 3(a)(iv) S allow sulfur / sulphur / phonetic spelling if more than one answer is given apply list principle 3(a)(v) Na allow sodium / phonetic spelling if more than one answer is given apply list principle AO 4.2a AO 4.2d, 6.4a AO2.2j AO 3.c AO2 2.b, 2.c 3(b)(i) any three from: effervescence / fizzing or bubbles or gas produced sodium melts or turns into a ball sodium moves (on the surface) steam / mist / vapour is produced sodium gets smaller / disappears colour of indicator is darker / more intense near the sodium do not allow incorrectly named gas ignore heat / temperature / flame / spark allow dissolves Must be linked to near the sodium. 3 AO 4.2a Question 3 continues on the next page 8 of 20

9 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June 204 Question 3 continued 3(b)(ii) hydroxide or OH allow OH without a charge AO2 do not allow OH + 4.2a, 5.a 3(c) diagram showing electron configuration of ion is 2,8 charge on ion is + AO 2.c, 2.d Bracket not necessary. [2,8] + is worth mark as there is no diagram Total 9 of 20

10 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June marks 4AO / 2AO2 4.a 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 Any description of a method used and / or a result given Description of workable methods used, with results to identify positive or negative ions Description of methods used to identify both positive and negative ions, with relevant results examples of the points made in the response Test: add (platinum / nichrome) wire (for the flame test) Result: the sodium compounds result in a yellow / orange / gold flame or the potassium compound results in a lilac / purple / mauve flame Test: add dilute nitric acid to all four solutions extra information accept any method of introducing the solution into the flame, eg a splint soaked in the solution or sprayed from a bottle student could state that potassium carbonate gives a different colour to the three sodium compounds as long as it is clear that the flame test colour comes from Na + or K + allow any acid Result: sodium carbonate and potassium carbonate will effervesce or sodium chloride and sodium iodide will not effervesce Test: add dilute nitric acid followed by silver nitrate Result: sodium chloride and sodium iodide produce a precipitate or sodium chloride produces a white precipitate and sodium iodide produces a yellow precipitate accept sodium carbonate and potassium carbonate do not produce a precipitate 0 of 20

11 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June 204 5(a)(i) lit splint or ignite the gas AO (squeaky) pop / explosion 6.c 5(a)(ii) because it provides energy (for the reaction) to break bonds (in the reactants) or so the particles collide successfully ignore reference to frequency or rate of collisions AO 7b because it provides the activation energy gains 2 marks 5(b)(i).67(g) allow correct answer (to 3 significant figures) with or without working gains 3 marks if answer incorrect allow up to 2 marks for the following steps: / 24 or moles magnesium = / 24 or 0.04(7) multiply by 40 allow ecf from incorrect ratio or incorrect number of moles 3 AO2.3i Question 5 continues on the next page of 20

12 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June 204 Question 5 continued 5(b)(ii) if correct answer from (b)(i) used 89.8 or 90 allow ecf from (b)(i) 2 AO2.3k if.82 g used 82.4 or 82 correct answer with or without working gains 2 marks if answer incorrect, allow the following for mark:.50 /.67 (or their answer from (b)(i)) if.82 g used:.50 /.82 5(b)(iii) any one from: ignore measurement errors AO not all the magnesium reacted allow the reaction may be reversible.3j some of the magnesium oxide / product may have been left in the tube or may have been lost ignore magnesium lost different / unexpected reaction magnesium not pure Total 0 2 of 20

13 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June 204 6(a)(i) points correctly plotted (± ½ small square) straight line of best fit using full range of points from 0,0 four points = 2 marks three points = mark Max 2 2AO2 / AO3 2.2b 6(a)(ii) any one from: the solution may not have been properly stirred must explain why the point is below the line AO3 2.2b the electrodes may have been a larger distance apart the drop of sodium chloride may have been a smaller volume / smaller allow not enough sodium chloride added allow smaller amount of sodium chloride do not allow too few drops added ignore the student may have misread the conductivity meter 6(a)(iii) any one from: AO3 the volume of pure water allow amount 2.2b the concentration (of the solutions added) the volume (of the drops) of solution added ignore number of drops the distance between the electrodes the same electrodes or electrodes made of the same material same depth or surface area of electrodes in the water constant power supply ignore current. stirred Question 6 continues on the next page 3 of 20

14 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June 204 Question 6 continued 6(b)(i) because (pure) water is covalent / molecular (simple) or contains molecules AO 2.2e therefore (pure) water has no free / mobile electrons or ions molecules do not have a charge or molecules do not contain ions gains 2 marks 6(b)(ii) because there are ions in sodium chloride allow Na + and / or Cl (ions) or ionic bonding. Ignore particles other than ions for MP. AO 2.2b, 3a which can move or carry the current / charge MP2 must be linked to ions only. 6(b)(iii) Hydrogen allow H 2 / H AO2 3j Total 0 4 of 20

15 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June 204 7(a)(i) D AO2 8.2a, 8.2b 7(a)(ii) B AO2 0.2a 7(a)(iii) A AO2 9.b, 9.c 7(a)(iv) E AO2.3e, 0.a 7(a)(v) E AO2 0.3a 7(b)(i) high temperature catalyst or steam ignore hot / heat allow temperature quoted (range ºC) AO 9.a 7(b)(ii) C 8 H 8 smaller molecule It = C 8 H 8 AO / AO2 therefore there are weaker intermolecular forces allow intermolecular bonds do not accept breaking covalent bonds / bonds 2.2d or weaker intermolecular forces in C 8 H 8 () allow intermolecular bonds so less energy to break () 7(c) add bromine water AO turns (from orange / yellow / red / brown) to colourless or decolourises do not accept discoloured ignore clear 9.d incorrect test = 0 marks Question 7 continues on the next page 5 of 20

16 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June 204 Question 7 continued 7(d) single C C bond AO four carbon-hydrogen bonds in place and two trailing bonds 9.2a structure in brackets and n at bottom right Total 4 6 of 20

17 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June 204 8(a)(i) any one from: AO one electron in the outer shell / energy level form ions with a + charge 4.b, 4.2a 8(a)(ii) any one from: hydrogen is a non-metal (at RTP) hydrogen is a gas hydrogen does not react with water hydrogen has only one electron shell / energy level hydrogen can gain an electron or hydrogen can form a negative / hydride / H ion hydrogen forms covalent bonds or shares electrons accept answers in terms of the Group elements AO 2.c, 2.d 8(b)(i) (bromine) gains electrons it = bromine do not accept bromide ion gains electrons ignore loss of oxygen AO 3d 8(b)(ii) I 2 must both be on the right hand + 2e side of the equation 2I 2e I 2 for 2 marks AO2 3f Question 8 continues on the next page 7 of 20

18 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June 204 Question 8 continued 8(b)(iii) fluorine is the smallest atom in Group 7 or has the fewest energy levels in Group 7 or has the smallest distance between outer shell and nucleus the outer shell must be mentioned to score 3 marks AO 4.b, 4.2f fluorine has the least shielding or the greatest attraction between the nucleus and the outer shell therefore fluorine can gain an electron (into the outer shell) more easily Total 8 8 of 20

19 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June Divide by A r : Na = 22.8 / 23 B = 2.8 / O = 55.4 / 6 Values if student has calculated moles upside down they can score mp 3 mp4 and mp 5 as follows: Na 23 / 22.8 B / 2.8 O 6 / 55.4 AO2.3e Divide by the smallest : 2 : 3.5 Whole number ratio 2 : 4 : 7 Empirical formula Divide by the smallest () 3.5 :.75 : Whole number ratio () 4 : 7 : 4 Empirical formula () Na 4 B 7 O 4 Na 2 B 4 O 7 if no working shown allow 4 marks for Na 2 B 4 O 7 Total 5 9 of 20

20 MARK SCHEME AQA Level /2 Certificate in Chemistry Paper 8402/ June 204 0(a) the forward and backward reactions occur at (exactly) the same rate allow reversible AO 2c in a closed system allow therefore the concentrations / amounts of the reactants and products remain the same 0(b)(i) increasing the temperature would lower the yield of ethanol or the (position of) equilibrium moves to the left if student has stated that increasing the temperature increases the yield then award 0 marks AO / Ao2 2d, 2e, 2f since the backwards reaction is endothermic or the forward reaction is exothermic 0(b)(ii) increasing the pressure would increase the yield of ethanol or the (position of) equilibrium moves to the right if student has stated that increasing the pressure decreases the yield then award 0 marks AO / AO2 2d, 2g because the position (of equilibrium) moves in the direction of the lower number of moles (of gas) 2 (moles / molecules / volumes / particles) on lhs / (mole / molecule / volume / particle) on rhs 0(c) (a catalyst) provides an alternative pathway with lower activation energy AO.2e or (a catalyst) lowers the activation energy () so less energy is needed to react or more particles react () Total 9 20 of 20