CHEMISTRY Unit 2 Energy, Rate and Chemistry of Carbon Compounds

Transcription

1 Surname Centre Number Candidate Number Other Names 2 GCE AS/A Level 2410U20-1 NEW AS S U20-1 CHEMISTRY Unit 2 Energy, Rate and Chemistry of Carbon Compounds P.M. FRIDAY, 10 June hour 30 minutes For Examiner s use only Question Maximum Mark Mark Awarded Section A 1. to Section B ADDITIONAL MATERIALS In addition to this examination paper, you will need a: calculator; Data Booklet supplied by WJEC Total U INSTRUCTIONS TO CANDIDATES Use black ink or black ball-point pen. Do not use gel pen or correction fluid. Write your name, centre number and candidate number in the spaces at the top of this page. Section A Answer all questions in the spaces provided. Section B Answer all questions in the spaces provided. Candidates are advised to allocate their time appropriately between Section A (10 marks) and Section B (70 marks). INFORMATION FOR CANDIDATES The number of marks is given in brackets at the end of each question or part-question. The maximum mark for this paper is 80. Your answers must be relevant and must make full use of the information given to be awarded full marks for a question. The assessment of the quality of extended response (QER) will take place in Q.8(b). If you run out of space, use the additional page(s) at the back of the booklet, taking care to number the question(s) correctly. JUN162410U20101 SM*(S U20-1)

2 2 SECTION A Examiner only Answer all questions in the spaces provided. 1. (a) Name the compound whose formula is shown below. [1] CH 3 H COOH H C 3 C C C CH 3 H Cl H (b) Draw the skeletal formula of the compound shown in (a). [1] 2. Respiration involves the release of energy by using foods. The equation for the respiration of glucose is shown below. C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O ΔH = 2900 kj mol 1 Photosynthesis is the process by which plants store energy in foods. It can be thought of as the reverse of respiration. On the axes below sketch and label the energy profile for the photosynthesis process. You should label the enthalpy change, ΔH, and the activation energy, E a. [2] Energy Progress of reaction 02

3 3 3. Draw a section of the addition polymer formed from the monomer below. Examiner only CH 3 CH(OH)CH CHCl You should show two repeat units. [1] 4. Bromine reacts with methanoic acid according to the equation below. Br 2 (aq) + HCOOH(aq) 2Br (aq) + 2H + (aq) + CO 2 (g) A student wanted to follow the rate of the reaction and mixed solutions of known concentration of bromine and methanoic acid. Suggest two methods which the student could use to follow the rate of this reaction as it proceeds. [2] 2410U Method 1... Method Turn over.

4 4 5. (a) State what feature of a molecule gives rise to E-Z isomerism. [1] Examiner only (b) Does the molecule shown below show E-Z isomerism? Explain your answer. [1] CH 3 CH 2 CH C(CH 3 ) 2 6. On warming, copper(ii) oxide reacts with aqueous methanoic acid. Write the equation for this reaction. [1] 10 04

5 5 BLANK PAGE 2410U PLEASE DO NOT WRITE ON THIS PAGE 05 Turn over.

6 6 SECTION B Examiner only Answer all questions in the spaces provided. 7. Sian was given the following instructions for an experiment to find the enthalpy change for the reaction below. Mg(s) + 2HCl(aq) MgCl 2 (aq) + H 2 (g) Instructions Weigh the magnesium. Place 25.0 cm 3 of 0.5 mol dm 3 hydrochloric acid in an insulating plastic container. Measure the initial temperature of the acid in the container. Add the magnesium to the acid and stir. Measure the highest temperature reached. (a) Draw a labelled diagram showing how the apparatus is set up to carry out this experiment. You should show how the heat losses could be minimised. [3] 06

7 7 (b) Sian recorded the following data. Examiner only Mass of magnesium strip = 0.1 g Initial temperature of acid = 21.0 C Highest temperature reached = 35.5 C Use these data to calculate the enthalpy change for the reaction, ΔH. Mg(s) + 2HCl(aq) MgCl 2 (aq) + H 2 (g) Assume that 4.18 J is needed to raise the temperature of 1.0 cm 3 of all aqueous solutions by 1.0 C and that the hydrochloric acid was in excess. Give your answer in kj mol 1. [3] 2410U ΔH =... kj mol 1 (c) Efan repeated the experiment using 0.20 g of magnesium and 50.0 cm 3 of 0.1 mol dm 3 hydrochloric acid. Although he used his data correctly to calculate the enthalpy change of reaction for 1 mol of magnesium, his answer was numerically much less than that obtained by Sian. By using the data above and that in part (b), explain why Efan obtained a different value for ΔH. [2] 07 Turn over.

8 8 (d) Sian used a one-decimal place balance to weigh the magnesium. Calculate the maximum percentage error in the mass of magnesium she used. Show clearly how you obtained your answer. [2] Examiner only Maximum percentage error =... % (e) Burning magnesium, when placed in a gas-jar containing chlorine, forms magnesium chloride and energy is released. Mg(s) + Cl 2 (g) MgCl 2 (s) Suggest why this reaction cannot be used to measure the enthalpy change for the reaction that forms magnesium chloride from magnesium. [1] 11 08

9 9 θ 8. (a) (i) Define the term standard enthalpy change of combustion, Δ c H. [2] Examiner only (ii) The standard enthalpy change of combustion of ethane is 1561 kj mol 1. The values of some average bond enthalpies are shown in the table below. Bond Average bond enthalpy / kj mol 1 C C 348 O O 495 C O 799 O H 463 C 2 H O 2 2CO 2 + 3H 2 O Balance the equation for the combustion of ethane and use the information given to calculate the average bond enthalpy for a C H bond. [4] 2410U Bond enthalpy =... kj mol 1 (iii) Suggest a reason why bond enthalpies are described as being average bond enthalpies. [1] 09 Turn over.

10 10 (b) Charcoal consists mainly of carbon. It has been produced for many centuries by heating wood in the absence of oxygen. Natural gas consists mainly of methane and is obtained from underground sources. It was formed in a similar way to coal and oil. Examiner only The enthalpy changes of combustion of carbon and methane are in the table. Substance Enthalpy change of combustion / kj mol 1 carbon, C 394 methane, CH Two students were discussing the use of charcoal and methane as fuels. One said that methane produced more heat per gram when burned so that it was a better fuel. The other student said that the use of charcoal contributed less to the overall increase of carbon dioxide levels in the atmosphere. Discuss these two statements. [6 QER] 13 10

11 11 9. All hydrocarbons can be burned but, apart from in combustion reactions, alkenes are more reactive than alkanes. Examiner only (a) Describe the bonding in propene and use this to explain its reactivity. [5] 2410U (b) Draw the mechanism for the reaction between propene and bromine. You should show any relevant dipoles, lone pairs of electrons and curly arrows to indicate the movement of pairs of electrons. [4] 9 11 Turn over.

12 Halogenoalkanes are hydrolysed when heated with aqueous sodium hydroxide. Examiner only (a) (i) Write the equation to show the hydrolysis of 3-chloro-2-methylpentane with aqueous sodium hydroxide. You should show clearly the structure of the organic reagent and product. [1] (ii) State the name of the mechanism for the reaction taking place in part (i). [1]... (iii) Describe a chemical test to show that this hydrolysis reaction has occurred. Include the test and the result expected. [2] 12

13 13 (b) The rate of reaction between a halogenoalkane and aqueous sodium hydroxide was measured when different concentrations of reagents were used. The results are shown in the table. Examiner only Experiment Initial concentration OH (aq) / mol dm 3 Initial concentration Initial rate of reaction halogenoalkane / mol dm 3 / mol dm 3 s (i) Use these data to deduce how the concentration of each reagent affects the rate of reaction. Explain how you reached your conclusions. [2] (ii) What would be the effect on the initial rate of reaction if the halogen in the halogenoalkane were changed from chlorine to bromine? You should assume that this is the only change made. Explain your answer. [3] 9 13 Turn over.

14 (a) Esters are formed when carboxylic acids are heated with alcohols in the presence of concentrated sulfuric acid. This is a reversible reaction that is carried out by heating the reagents under reflux. Examiner only (i) Draw a labelled diagram of the apparatus you would use to carry out a reaction under reflux. [3] (ii) Explain how the apparatus that you have drawn in part (i) results in the process of reflux. [1] (iii) Why is it necessary to use a reflux technique in this type of reaction? [1] (iv) State the function of the concentrated sulfuric acid in this reaction. [1] 14

15 (b) The equation shows the reaction between ethanoic acid and methanol. 15 CH 3 COOH + CH 3 OH a CH 3 COOCH 3 + H 2 O When 3.00 g of ethanoic acid was heated under reflux with 1.28 g of methanol in the presence of concentrated sulfuric acid for 10 minutes it was found that 1.18 g of methyl ethanoate was formed. Examiner only (i) Suggest how the methyl ethanoate could be separated from the mixture formed in the reaction vessel. [1] (ii) Calculate the percentage of theoretical yield of methyl ethanoate obtained. Show your working. [3] Percentage yield =... % (iii) Suggest one change that could be made to this preparation to improve the percentage yield of methyl ethanoate. Explain your suggestion. [2] 15 Turn over.

16 (c) 16 Two reactions of organic compound A are shown below. conc H 2 SO 4 Cr 2 O 2 7 /H + B A C Examiner only Compound B is a straight-chain hydrocarbon with the formula C 4 H 8. (i) What type of reaction is taking place when compound A is heated with concentrated sulfuric acid to form compound B? [1] (ii) Draw the displayed formulae of two possible isomers of A. [2] (iii) What colour change is seen when compound A reacts with the Cr 2 O 7 2 /H + solution? [1] (iv) What type of reaction is taking place when compound A is heated with Cr 2 O 7 2 /H + to form compound C? [1] (v) Draw a displayed formula of compound C. [1] 18 16

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18 Chemists are investigating an unknown compound X. They obtain information from a variety of sources. Examiner only Compound X contains 61.2 % carbon, 6.1 % hydrogen and 32.7 % oxygen by mass. A simplified mass spectrum of compound X is shown below. 100 Relative abundance m/z When solid sodium carbonate is added to an aqueous solution of X effervescence is observed. There are 5 peaks in the 13 C NMR spectrum of X. 1 mol of X reacts completely with 320 g of bromine in the dark. 18

19 19 Use all the data given to find the structure of compound X. Explain what information can be found from each piece of data. [10] Examiner only 10 END OF PAPER 19 Turn over.

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21 21 Question number Additional page, if required. Write the question number(s) in the left-hand margin. Examiner only 21

22 22 Question number Additional page, if required. Write the question number(s) in the left-hand margin. Examiner only 22

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25 GCE AS MARKING SCHEME SUMMER 2016 CHEMISTRY - NEW AS UNIT U20-1

26 INTRODUCTION This marking scheme was used by WJEC for the 2016 examination. It was finalised after detailed discussion at examiners' conferences by all the examiners involved in the assessment. The conference was held shortly after the paper was taken so that reference could be made to the full range of candidates' responses, with photocopied scripts forming the basis of discussion. The aim of the conference was to ensure that the marking scheme was interpreted and applied in the same way by all examiners. It is hoped that this information will be of assistance to centres but it is recognised at the same time that, without the benefit of participation in the examiners' conference, teachers may have different views on certain matters of detail or interpretation. WJEC regrets that it cannot enter into any discussion or correspondence about this marking scheme. 1

27 GCE CHEMISTRY SUMMER 2016 MARK SCHEME AS UNIT 2 ENERGY, RATE AND CHEMISTRY OF CARBON COMPOUNDS MARK SCHEME GENERAL INSTRUCTIONS Recording of marks Examiners must mark in red ink. One tick must equate to one mark, apart from extended response questions where a level of response mark scheme is applied. Question totals should be written in the box at the end of the question. Question totals should be entered onto the grid on the front cover and these should be added to give the script total for each candidate. Extended response questions A level of response mark scheme is applied. The complete response should be read in order to establish the most appropriate band. Award the higher mark if there is a good match with content and communication criteria. Award the lower mark if either content or communication barely meets the criteria. Marking rules All work should be seen to have been marked. Marking schemes will indicate when explicit working is deemed to be a necessary part of a correct answer. Crossed out responses not replaced should be marked. Marking abbreviations The following may be used in marking schemes or in the marking of scripts to indicate reasons for the marks awarded. cao = correct answer only ecf = error carried forward bod = benefit of doubt Credit should be awarded for correct and relevant alternative responses which are not recorded in the mark scheme. 2

28 Section A Marks available Question Marking details AO1 AO2 AO3 Total Maths Prac 1. (a) 3-chloro-2,4-dimethylpentanoic acid 1 1 (b) Sketch graph to show products at higher energy than reactants (1) 3. E a and H correct (1)

29 Question 4. Any two of following for (1) each Marking details Marks available AO1 AO2 AO3 Total Maths Prac Use a colorimeter to measure colour changes with time Use a ph meter to measure ph changes with time Use a gas syringe / collect gas over water to measure changes in volume with time Use a balance to measure mass changes with time Award (1) for one correct method but no mention of time 5. (a) Presence of a (carbon to carbon) double bond 1 1 (b) No because two of the groups on one of the double bonded carbon atoms are the same CuO + 2HCOOH (HCOO) 2 Cu + H 2 O 1 1 (ignore state symbols) Section A total

30 Section B Question Marking details 7. (a) Diagram to show insulated / polystyrene container with reagents (1) Marks available AO1 AO2 AO3 Total Maths Prac Lid (1) Thermometer (1) (b) Energy = = 1515 J (1) Moles Mg = 0.1/24.3 = (1) H = 1515/ = 368 kjmol 1 (1) ecf posible (c) Moles Mg = (1) Requires moles HCl for all to react Moles HCl = i.e. not enough for a complete reaction (1) (d) Maximum error in the one reading taken = 0.05 g (1) (Allow 0.1 g if clearly states that mass was measured using weighing by difference) Maximum % error = = 50% (1) (e) Cannot measure H can only measure T/ no water / solution present to measure temperature change Question 7 total

31 Question Marking details 8. (a) (i) Enthalpy / energy change when 1 mol of substance is burned (1) Marks available AO1 AO2 AO3 Total Maths Prac Completely / in excess oxygen under standard conditions (1) 2 2 (ii) C 2 H 6 + 3½O 2 2CO 2 + 3H 2 O (1) Bonds broken 1(C C) + 3½(O=O) + 6(C H) = (C H) (1) Bonds formed 4(C=O) + 6(O H) = 5974 (1) (C H) 5974 = 1561 (C H) = 389 kjmol 1 (1) award (3) for cao ecf possible (iii) Average used since each individual bond will be in a different environment and therefore have a different strength (1) 1 1 6

32 Question 8. (b) Indicative content Marking details Marks available AO1 AO2 AO3 Total Maths Prac Correct in that energy produced per gram is 32.8 kj from charcoal and 55.6 kj from methane Both give CO 2 on burning 1 mol of each fuel produces 1 mol of CO 2 Wood for charcoal comes from (living) trees Methane comes from sources living millions of years ago / is a fossil fuel Charcoal is renewable / methane is non-renewable Trees take in CO 2 in photosynthesis Trees release the same amount of CO 2 on combustion that they took in during growth Charcoal burning is overall carbon neutral marks Must calculate energy per gram for both fuels. The candidate constructs a relevant, coherent and logically structured account including key elements of the indicative content. A sustained and substantiated line of reasoning is evident and scientific conventions and vocabulary are used accurately throughout. 3-4 marks Clear comparison of methane and charcoal. The candidate constructs a coherent account including many of the key elements of the indicative content. Some reasoning is evident in the linking of key points and use of scientific conventions and vocabulary is generally sound. 1-2 marks Main focus on only one of methane or charcoal. The candidate attempts to link at least two relevant points from the indicative content. Coherence is limited by omission and/or inclusion of irrelevant material. There is some evidence of appropriate use of scientific conventions and vocabulary. 0 marks The candidate does not make any attempt or give an answer worthy of credit. Question 8 total

33 Question 9. (a) Any five of following for (1) each Marking details Marks available AO1 AO2 AO3 Total Maths Prac Alkenes contain and bonds bonds are formed from s-s orbital overlap / end-on orbital overlap bonds are formed from sideways overlap of p orbitals / overlap above and below plane The bond gives a region of high electron density This is susceptible to electrophilic attack/ attack by an electron deficient species (This attack) leads to addition reactions 5 5 (b) Diagram to show Correct dipole on Br 2 (1) Two correct arrows (1) Formula of intermediate and arrow from lone pair or negative charge (1) Correct product (1) 4 4 Question 9 total

34 Question Marking details 10. (a) (i) Equation with displayed or structural formulae e.g. Marks available AO1 AO2 AO3 Total Maths Prac CH 3 CH(CH 3 )CHClCH 2 CH 3 + NaOH CH 3 CH(CH 3 )CHOHCH 2 CH 3 + NaCl CH 3 CH(CH 3 )CHClCH 2 CH 3 + OH CH 3 CH(CH 3 )CHOHCH 2 CH 3 + Cl 1 1 (ii) Nucleophilic substitution 1 1 (iii) Neutralise hydroxide with nitric acid and add aqueous silver nitrate (1) White precipitate forms (1) Accept heat with acidified dichromate (1) orange to green (1) (b) (i) Rate concentration of halogenoalkane with explanation e.g. concentration doubles, rate doubles (1) 1 Rate not affected by concentration of OH (1) 2 2 (ii) Rate would be faster because C X bond needs to be broken (1) C Br is weaker than C Cl / takes less energy to break (1) This outweighs effect of greater dipole in C Cl / chlorine being more electronegative (1) 3 3 Question 10 total

35 Question Marking details 11. (a) (i) Labels on diagram to show vertical condenser (unsealed at top) (1) Marks available AO1 AO2 AO3 Total Maths Prac Water in and out of condenser in correct direction (1) Heat source below flask with reagents (1) (ii) Liquid evaporates, vapour goes into condenser, cools and goes back to liquid / condenses (iii) Any of following for (1) The reaction is slow Allows time for equilibrium to be established Stops reactants / products boiling away (iv) Catalyst/ dehydrating agent 1 1 (b) (i) Fractional distillation (1) (ii) Moles of ethanoic acid = 0.05 and moles of methanol = 0.04 (1) Theoretical yield of methyl ethanoate = = 2.96 g (1) % of theoretical yield = = 40 % (1) ecf possible 10

36 Question (iii) Reflux for longer (1) Marking details Marks available AO1 AO2 AO3 Total Maths Prac Reaction is slow/ needs time to establish equilibrium (1) or Add extra methanol / ethanoic acid (1) To allow more of the acid to react/ push the equilibrium to RHS (1) (c) (i) Dehydration/ elimination 1 1 (ii) Displayed formulae of butan-1-ol and butan-2-ol for (1) each butan-1-ol butan-2-ol 2 2 (iii) Orange to green 1 1 (iv) Oxidation of alcohol / redox

37 Marks available Question Marking details AO1 AO2 AO3 Total Maths Prac (v) Displayed formula of butanoic acid / butanal / butanone 1 1 Question 11 total

38 Question 12. From % composition ratio C: H : O Marking details Marks available AO1 AO2 AO3 Total Maths Prac 61.2 : 6.1 : 32.7 = 5.1 : 6.04 : 2.04 (1) : 3 : 1 5 : 6 : 2 Empirical formula is C 5 H 6 O 2 (1) From mass spectrum M r is 98 (1) Molecular formula is C 5 H 6 O 2 (1) Identification of one fragment from m/z value (1) From reaction with sodium carbonate X is a carboxylic acid/ contains CO 2 H (1) 1 From 13 C spectrum There are 5 different carbon environments (1) 1 From reaction with bromine 320 g of bromine is 2 mol (1) X contains 2 (C to C) double bonds (1) 2 X is CH 2 =CHCH=CHCO 2 H (accept any isomer with 2 double bonds, 5 carbon environments and CO 2 H) (1) 1 10 Question 12 total

39 SUMMARY OF MARKS ALLOCATED TO ASSESSMENT OBJECTIVES Question AO1 AO2 AO3 Total Maths Prac 1. to Totals WJEC GCE Chemistry AS Unit 2 MS/Summer