GCE AS Chemistry Summer 2008 Mark Schemes Issued: October 2008
NORTHERN IRELAND GENERAL CERTIFICATE OF SECONDARY EDUCATION (GCSE) AND NORTHERN IRELAND GENERAL CERTIFICATE OF EDUCATION (GCE) Introduction MARK SCHEMES (2008) Foreword Mark Schemes are published to assist teachers and students in their preparation for examinations. Through the mark schemes teachers and students will be able to see what examiners are looking for in response to questions and exactly where the marks have been awarded. The publishing of the mark schemes may help to show that examiners are not concerned about finding out what a student does not know but rather with rewarding students for what they do know. The Purpose of Mark Schemes Examination papers are set and revised by teams of examiners and revisers appointed by the Council. The teams of examiners and revisers include experienced teachers who are familiar with the level and standards expected of 16- and 18-year-old students in schools and colleges. The job of the examiners is to set the questions and the mark schemes; and the job of the revisers is to review the questions and mark schemes commenting on a large range of issues about which they must be satisfied before the question papers and mark schemes are finalised. The questions and the mark schemes are developed in association with each other so that the issues of differentiation and positive achievement can be addressed right from the start. Mark schemes therefore are regarded as a part of an integral process which begins with the setting of questions and ends with the marking of the examination. The main purpose of the mark scheme is to provide a uniform basis for the marking process so that all the markers are following exactly the same instructions and making the same judgements in so far as this is possible. Before marking begins a standardising meeting is held where all the markers are briefed using the mark scheme and samples of the students work in the form of scripts. Consideration is also given at this stage to any comments on the operational papers received from teachers and their organisations. During this meeting, and up to and including the end of the marking, there is provision for amendments to be made to the mark scheme. What is published represents this final form of the mark scheme. It is important to recognise that in some cases there may well be other correct responses which are equally acceptable to those published: the mark scheme can only cover those responses which emerged in the examination. There may also be instances where certain judgements may have to be left to the experience of the examiner, for example, where there is no absolute correct response all teachers will be familiar with making such judgements. The Council hopes that the mark schemes will be viewed and used in a constructive way as a further support to the teaching and learning processes. iii
CONTENTS Page AS 1: Module 1 1 AS 2: Module 2 7 AS 3: Module 3 Practical Examination 1 15 AS 3: Module 3 Practical Examination 2 21 v
ADVANCED SUBSIDIARY (AS) General Certificate of Education 2008 Chemistry Assessment Unit AS 1 assessing Module 1: General Chemistry [ASC11] WEDNESDAY 4 JUNE, MORNING MARK SCHEME 1
Quality of Written Communication: 2 marks The candidate expresses ideas clearly and fluently through well-linked sentences and paragraphs. Arguments are generally relevant and well-structured. There are few errors of grammar, punctuation and spelling. 1 mark The candidate expresses ideas clearly, if not always fluently. Arguments may sometimes stray from the point. There may be some errors of grammar, punctuation and spelling, but not such as to suggest a weakness in these areas. 0 marks The candidate expresses ideas satisfactorily, but without precision. Arguments may be of doubtful relevance or obscurely presented. Errors in grammar, punctuation and spelling are sufficiently intrusive to disrupt the understanding of the passage. 2
1 C Section A 2 A 3 C 4 A 5 C 6 C 7 B 8 C 9 A 10 D [2] for each correct answer [20] 20 Section A 20 3
Section B 11 (a) s, d and p in correct order [2] (b) d [1] p [1] s [1] [3] (c) in sodium electron further from nucleus/in chlorine closer or Cl radius smaller than Na [1] conditional on comment on size less attraction/more attraction (of nucleus for electrons) [1] [2] (d) (i) [1] (ii) or or [1] 9 12 (a) (i) three (bonding/shared) pairs of electrons between the nitrogens [2] [2] (ii) a lot of energy needed to break the bonds [1] [1] (iii) H C C H [1] (b) (i) It is negative hence exothermic [1] (ii) enthalpy reactants products [3] (c) (i) NH 3 + H + NH 4 + [1] (ii) (iii) H H N H H [2] H H N H H each error [ 1] [1] 12 4
13 (a) (i) electron excited to n = 2 [1] electron falls to n = 1 [1] (u.v.) radiation emitted [1] [3] (ii) H(g) H + (g) + e [2] (b) (i) ability of an atom to attract electrons [1] in a covalent bond [1] [2] (ii) ammonia has hydrogen bonding and phosphine has van der Waals [1] comment on strength of hydrogen bonds [1] [2] (iii) pyramidal [1] electron pairs repel [1] move as far apart as possible [1] [3] (iv) Ca 3 P 2 + 6H 2 O 3Ca(OH) 2 + 2PH 3 [2] unbalanced = [1] (c) (i) HI, HBr, HCl, HF [1] (ii) HF [1] (d) (i) Na + H x [Na] + [H ] x [2] (ii) hydride ion is larger [1] electrons repel [1] [2] (iii) NaH(s) + H 2 O(l) NaOH(aq) + H 2 (g) [2] (iv) ions [1] are able to move [1] [2] 24 14 (a) +6 [1] chlorine is an oxidising agent [1] [2] (b) add nitric acid [1] add silver nitrate solution [1] white precipitate [1] add NH 3 (aq) [1] ppt. dissolves [1] to a maximum of [3] Ag + + Cl AgCl [1] essential [4] Quality of written communication [2] (c) green [1] (d) glowing splint [1] relights [1] [2] (e) (i) solution changes from colourless [1] to brown (not clear)/black solid [1] [2] 5
(ii) starch [1] blue/black [1] to colourless [1] [3] (iii) moles of thiosulphate = 23.45 0.1/1000 = 2.345 10 3 [1] moles of ferrate = 2.345 10 3 /3 = 7.82 10 4 [1] concentration = 7.82 10 4 40 = 3.12 10 2 M = 0.0312 M [1] [3] 19 each error [ 1], carry error through 15 (a) red/brown solid forms [1] blue solution [1] fades/goes colourless [1] zinc dissolves [1] Any two from four [2] (b) 13.2 C 13.2 75 4.2 = 4.158 kj 0.00656 mole Cu 633.8 kj mol 1 [4] 6 16 (a) (i) prevents moisture entering the apparatus (and reacting with PCl 5 ) [1] (ii) fume cupboard [1] (iii) prevent loss/sublimation of product [1] (iv) PCl 3 + Cl 2 PCl 5 [1] (b) (i) 8 electrons in the outer shell [1] (ii) 10 electrons (in the outer shell around phosphorus atom) [1] (c) (i) other products are gases [1] (ii) moles POCl 3 = 1.53/153.5 = 0.01 moles HCl = 0.03 volume HCl = 0.03 24 = 0.72 dm 3 error [ 1] [3] 10 Section B 80 Total 100 6
ADVANCED SUBSIDIARY (AS) General Certificate of Education 2008 Chemistry Assessment Unit AS 2 assessing Module 2: Organic, Physical and Inorganic Chemistry [ASC21] THURSDAY 12 JUNE, AFTERNOON MARK SCHEME 7
Quality of written communication: 2 marks The candidate expresses ideas clearly and fluently through well-linked sentences and paragraphs. Arguments are generally relevant and well-structured. There are few errors of grammar, punctuation and spelling. 1 mark The candidate expresses ideas clearly, if not always fluently. Arguments may sometimes stray from the point. There may be some errors of grammar, punctuation and spelling, but not such as to suggest a weakness in these areas. 0 marks The candidate expresses ideas satisfactorily, but without precision. Arguments may be of doubtful relevance or obscurely presented. Errors in grammar, punctuation and spelling are sufficiently intrusive to disrupt the understanding of the passage. 8
1 B 2 B 3 C 4 A 5 B 6 D 7 A 8 B 9 C 10 B Section A [2] for each correct answer [20] 20 Section A 20 9
Section B 11 Hydrogen: burning splint [1] (squeaky) pop [1] Sulphur dioxide: (acidified) potassium permanganate [1] decolourised [1] or changes (acidified) potassium dichromate [1] (from orange) to green [1] Hydrogen chloride: with ammonia [1] white fumes [1] [6] 6 12 (a) An atom or molecule with an unpaired electron [1] (b) CH 4 + Cl 2 CH 3 Cl + HCl [1] (c) (i) Cl 2 2Cl. [1] (ii) CH 4 + Cl. CH. 3 + HCl. CH 3 + Cl 2 CH 3 Cl + Cl. CH 3 Cl + Cl. CH 2 Cl. + HCl (or any correct alternative, any two [1] each) [2] (iii) Cl. + Cl. Cl 2 CH. 3 + CH. 3 CH 3 CH 3 CH. 3 + Cl. CH 3 Cl (or any correct alternative) [1] 6 13 (a) Calcium ions: Description of a flame test [1] gives a brick red flame [1] Carbonate ions: Fizzing with hydrochloric acid [1] gas turns lime water milky [1] [4] (b) (i) CaCO 3 CaO + CO 2 [1] (ii) Calcium carbonate is more stable than magnesium carbonate but less stable than strontium carbonate [1] As the Group is descended the cation size increases [1] changing the lattice enthalpy [1] [3] (c) (i) Indicator: Phenolphthalein [1] or methyl orange From: colourless [1] to: pink [1] or red to orange [3] (ii) (0.1 11.6)/1000 = 11.6 10 4 = 1.16 10 3 mol 11.6 10 3 = 1.16 10 2 mol [1] (1 30)/1000 = 0.030 mol 0.030 0.0116 = 0.0184 mol [1] 0.0184/2 = 0.0092 mol 0.92g (0.92/1.05) 100 = 87.6% ([ 1] for each mistake, carry error through) [5] 16 10
14 (a) (i) Cracking [1] (ii) C 12 H 26 C 2 H 4 + C 10 H 22 [1] (iii) Add bromine water [1] decolourised/from orange to colourless [1] [2] (b) C 2 H 5 Br + NaOH C 2 H 4 + NaBr + H 2 O [1] Alcoholic OH [1] [2] (c) (i) Carbons in ethane held by sigma bond only [1] Carbons in ethene held by sigma and pi bonds [1] Bonding in ethene stronger making the bond shorter and increasing the bond energy [1] [3] (ii) Pi bond has overlapping p-orbitals exposing the electrons/high electron density [1] Attacked by electrophiles/pi bonds more easily broken [1] [2] (d) (i) H H H H H H C H C C C + H C C H H H H H H Br H Br Br ([ 1] for each mistake) [3] (ii) Electrophilic [1] addition [1] [2] (e) (i) About 60 C [1] and atmospheric pressure [1] Zeigler-Natla catalyst [1] All three [2] (ii) HD little or no branching/extensive crystalline regions [1] Making it rigid and giving it a higher melting point [1] LD lots of branching/little crystalline structure [1] Making it flexible and giving it a lower melting point [1] [4] Quality of written communication [2] (iii) Landfill Advantages cheap land reclaimed no CO 2 /greenhouse gases no toxic fumes Disadvantages non-biodegradable eyesore destroys habitat waste not recycled 11
Incineration Advantages Heat reused Removes waste quicker Small amount of residue Disadvantages Produces greenhouse gases Waste not recycled Toxic fumes Expensive Cannot use same reason for both disposal methods ([1] each, must have both landfill and incineration for full marks) [4] 28 15 (a) Beneficial: socialisation/relaxation/small amounts prevent heart disease Harmful: damage to liver/depression (Must have one beneficial and one harmful for comparison and both marks) [2] (b) (i) C 2 H 5 OH + 3O 2 2CO 2 + 3H 2 O [2] (ii) Reduces the use of fossil fuels/less CO 2 emissions/ reduces SO 2 /CO 2 emissions/renewable [2] (c) Na CH 3 CH 2 ONa [1] CH 3 COCl CH 3 COOCH 2 CH 3 [1] PCl 5 CH 3 CH 2 Cl [1] H + 2 /Cr 2 O 7 CH 3 COOH or CH 3 CHO [1] [4] 10 16 (a) (i) 2NO + 2CO N 2 + 2CO 2 [2] (ii) E A (cat) E A Curve: starting at 0/0 [1] correct shape not touching x-axis [1] E A (cat) to left of E A [3] (iii) Reactants adsorbed (onto the surface) [1] Molecules closer together/orientated [1] Bonds in reactants weakened/broken [1] [3] 12
(b) (i) Reaction moves to the left [1] To absorb heat/endothermic process [1] [2] (ii) Reaction moves to the right [1] Fewer molecules/less volume [1] [2] (iii) No effect [1] Less time to reach equilibrium [1] [2] 14 Section B 80 Total 100 13
ADVANCED SUBSIDIARY (AS) General Certificate of Education 2008 Chemistry Assessment Unit AS 3 assessing Module 3: Practical Examination 1 [ASC31] MONDAY 12 MAY, AFTERNOON MARK SCHEME 15
1 Observation/deduction Safety goggles must be worn at all times during this practical examination. Care should be exercised, especially when using sodium hydroxide solution and concentrated hydrochloric acid. (a) You are provided with 2 g of a mixture, labelled A, which contains two salts with a common anion, but different cations. Carry out the following experiments on the mixture. Record your observations and deductions in the spaces below and identify the two salts in A. Experiment Observations Deductions 1 Describe the appearance of A. White powder/crystals/ solid [1] Not a transition metal salt/cation Gp I or II compound/ NH 4 + compound [1] 2 (a) Add half a spatulameasure of A to a test tube half-full of sodium hydroxide solution and warm gently. (b) Test the gas evolved with damp Universal Indicator paper pungent smell [1] dissolves [1] Goes blue or green-blue [1] Alkaline/[1] ph (linked to colour) (c) Test the gas evolved by dipping a glass rod into concentrated hydrochloric acid and holding it over the mouth of the test tube. 3 Dip a nichrome wire loop into concentrated hydrochloric acid; touch sample A with the wire, then hold it in a blue Bunsen flame. 4 Dissolve half a spatulameasure of A in about 2cm 3 of dilute nitric acid. Add 2cm 3 of barium chloride solution. White fumes/smoke [1] + NH 3 /cation is NH 4 [1] Yellow flame [1] Sodium [1] White ppt [1] Sulphate [1] Name the two salts present in A Ammonium sulphate [1] Sodium sulphate [1] 16
(b) You are provided with an organic liquid labelled B which has the formula C 3 H 8 O. Carry out the following experiments. Record your observations and deductions in the spaces below. Suggest a structure for B. On completion of your experiments, dispose of B in the container provided. Experiment Observations Deductions 1 Describe the liquid B. Colourless/smell [1] 2 Add 2 cm 3 of B to 2 cm 3 of water. 3 Place 5 drops of B on a watch glass placed on a heatproof mat and ignite using a splint. 4 Add 1 cm 3 of dilute sulphuric acid to 2 cm 3 of potassium dichromate solution. Add 10 drops of B and warm gently. 5 Add 1 cm 3 of a solution of iodine in potassium iodide. Add 10 drops of B followed by 1 cm 3 of sodium hydroxide solution. Add 1 cm 3 of sodium hypochlorite solution. Miscible [1] Clean/non-sooty Blue/yellow flame [1] Orange green [1] reference to smell [1] Yellow ppt [1] H-bonds formed/polar/hydroxygroup/alcohol [1] Saturated/low carbon content [1] Prim. or sec. alcohol/not tertiary [1] Dichromate reduced/alcohol oxidised or reducing agent aldehyde/ketone formed [1] Iodoform/ contains structural feature CH CH 3 ΟΗ Sec. alcohol [1] Functional group in B Class of compounds OH, hydroxy [1] (Secondary) Alcohol [1] OH Structure of B CH 3 CH CH 3 [1] [1] 25 17
2 (a) Titration Consistency: Decimal places: this does not apply to the averaging of titres. All to one decimal place, except 0.05 or 0.00, two or more decimal places penalised [ 1] each time. The use of 0 is penalised if used more than once [ 1] Numerical error in calculating total value [ 1] [2] Titration consistency: This is the difference between the first and second accurate readings Difference Mark +/ 0.1 [5] +/ 0.2 [4] +/ 0.3 [3] +/ 0.4 [2] +/ 0.5 [1] Titration agreement with supervisor: Difference Mark +/ 0.2 [6] +/ 0.3 [5] +/ 0.4 [4] +/ 0.5 [3] +/ 0.6 [2] +/ 0.7 [1] Average value: The average can be two decimal places, e.g. 0.33. The use of the rough value is [ 1]. An incorrect calculation is [ 1] [2] [15] (b) red/pink to orange/yellow [1] [1] (c) (i) Na 2 CO 3 + 2HCl 2NaCl + H 2 O + CO 2 [2] (ii) calculation using vc/1000 [1] (iii) calculation using 1:2 ratio [2] (iv) calculation using dilution factor [1] (v) calculation using 250/1000 [2] (vi) calculation multiplying by Mr [1] 25 18
3 (a) (i) burner for alcohol [1] (ii) fewer losses by evaporation/less heat loss [1] (iii) less heat loss from surface in B II [1] (iv) heat shield prevents loss of heat to surroundings [1] (b) (i) 0.2 g [1] (ii) 0.2/46 = 0.0043 moles [1] (iii) 11 degrees [1] (iv) Q = mct = 0.1 4.18 10 3 11 = 4598 J H c = 4.6 103 1/0.0043 = 1070 10 3 = 1.07 10 6 J mol 1 = 1.07 10 3 kj mol 1 [3] (v) loss of heat to surroundings [1] (c) (i) CH 3 CH 2 OH + 3O 2 2CO 2 + 3H 2 O [2] (ii) carbon dioxide [1] (iii) lime water [1] milky [1] [2] (d) (i) CH 3 CH 2 OH + 1.5O 2 C + CO + 3H 2 O or CH 3 CH 2 OH + O 2 2C + CO + 3H 2 O [2] (ii) C+H 2 O CO + H 2 [2] 20 4 (a) (i) H 2 SO 4 + 2NaBr Na 2 HSO 4 + HBr [2] H 2 SO 4 + 2NaBr Na 2 SO 4 + 2HBr [1] [2] (ii) in the reaction mixture [1] (b) to promote smooth boiling [1] (c) (i) water out water in heat 19 [3]
(ii) a still head [1] with a thermometer [1] is placed in the flask the condenser is set to collect and condense the gaseous product [1] a container placed at the end of the condenser [1] To a maximum of [3] [3] (d) drying agent [1] (e) (i) impurities in a dirty flask will contaminate the product [1] product has been dried, so should not contact water again [1] [2] (f) (i) 3 0.8/74 = 0.032 moles [2] (ii) 3.55/137 = 0.026 moles [1] (iii) 81.7% [1] (g) Practical: loss during transfer loss during distillation Theoretical: side reactions does not go to completion Must be in correct category Any three from four [3] 20 Total 90 20
ADVANCED SUBSIDIARY (AS) General Certificate of Education 2008 Chemistry Assessment Unit AS 3 assessing Module 3: Practical Examination 2 [ASC32] FRIDAY 16 MAY, MORNING MARK SCHEME 21
1 Observation/deduction Safety goggles must be worn at all times during this practical examination. Care should be exercised, especially when using sodium hydroxide solution and concentrated hydrochloric acid. (a) You are provided with 2 g of a mixture, labelled X, which contains two salts with a common anion but different cations. Carry out the following experiments on the mixture. Record your observations and deductions in the spaces below and identify the two salts in X. Experiment Observations Deductions 1 Describe the appearance of X. white powder crystals solid [1] 2 (a) Add half a spatula-measure of X to a test tube half-full of sodium hydroxide solution and warm gently. pungent smell [1] dissolves [1] not a transition metal salt/cation/gpi or II compound/ NH 4 + compound [1] (b) Test the gas evolved with damp Universal Indicator paper. (c) Test the gas evolved by dipping a glass rod into concentrated hydrochloric acid and holding it over the mouth of the test tube. goes blue or green-blue [1] white fumes/smoke [1] alkaline/ph linked to colour [1] NH 3 /NH 4 + [1] 3 Dip a nichrome wire loop into concentrated hydrochloric acid; touch sample X with the wire, then hold it in a blue Bunsen flame. 4 Dissolve half a spatula-measure of X in about 2 cm 3 of dilute nitric acid. Add 2 cm 3 of silver nitrate solution. lilac flame [1] potassium [1] white ppt [1] chloride [1] Name the two salts present in X ammonium chloride [1] potassium chloride [1] 22
(b) You are provided with an organic liquid labelled Y which has the formula C 3 H 8 O. Carry out the following experiments. Record your observations and deductions in the spaces below. Suggest a structure for Y. On completion of your experiments, dispose of Y in the container provided. Experiment Observations Deductions 1 Describe the liquid Y. colourless/smell [1] 2 Add 2 cm 3 of Y to 2 cm 3 of water. 3 Place 5 drops of Y on a watch glass placed on a heatproof mat and ignite using a splint. miscible [1] clean/non-sooty blue/yellow flame } [1] H-bonds formed/hydroxy-group/ polar/alcohol [1] saturated/low carbon content [1] 4 Add 1 cm 3 of dilute sulphuric acid to 2 cm 3 of potassium dichromate solution. Add 10 drops of Y and warm gently. 5 Add 1 cm 3 of a solution of iodine in potassium iodide. Add 10 drops of Y followed by 1 cm 3 of sodium hydroxide solution. Add 1 cm 3 of sodium hypochlorite solution. orange green [1] reference to smell [1] No yellow solid/solution turns yellow [1] prim. or sec. alcohol [1]/ not tertiary [1] Dichromate reduced/aldehyde ketone formed/alcohol oxidised or reducing agent [1] primary alcohol/no iodoform/ [1] not CH CH 3 OH Functional group in Y OH, hydroxy [1] Class of compounds (primary) alcohol [1] Structure of Y CH 3 CH 2 CH 2 OH [1] 25 23
2 (a) Titration Consistency: Decimal places: this does not apply to the averaging of titres All to one decimal place, except 0.05 or 0.00, two or more decimal places penalised [ 1] each time The use of 0 is penalised if used more than once [ 1] [2] Titration consistency: This is the difference between the first and second accurate readings Difference Mark +/ 0.1 [5] +/ 0.2 [4] +/ 0.3 [3] +/ 0.4 [2] +/ 0.5 [1] Titration agreement with supervisor: Difference Mark +/ 0.2 [6] +/ 0.3 [5] +/ 0.4 [4] +/ 0.5 [3] +/ 0.6 [2] +/ 0.7 [1] Average value: The average can be two decimal places, e.g. 0.33 The use of the rough value is [ 1] An incorrect calculation is [ 1] [2] [15] (b) red/pink to orange/yellow [1] (c) (i) 2NaOH + H 2 SO 4 Na 2 SO 4 + 2H 2 O [2] (ii) calculation using vc/1000 [1] (iii) calculation using 2:1 ratio [2] (iv) calculation using dilution factor [2] (v) calculation using 250/1000 [1] (vi) calculation multiplying by Mr [1] 25 24
3 (a) (i) burner for decane [1] (ii) avoids loss of heat [1] to surroundings [1] [2] (b) (i) 0.2 g [1] (ii) 0.2/142 = 0.0014 moles [1] (iii) 6 degrees [1] (iv) Q = mct = 0.15 4.18 10 3 6 = 3.762 10 3 J H c = 3.762 10 3 /0.0014 = 2.69 10 6 J = 2690 kj mol 1 [3] (v) loss of heat to surroundings [1] (c) (i) C 10 H 22 + 15.5O 2 10CO 2 + 11H 2 O [2] (ii) carbon dioxide [1] (iii) lime water [1] milky [1] [2] (iv) 106 142 = 7042 1:10 ratio i.e. 70 420 70 420 44 = 3.098 10 6 g 3.1 10 3 kg/3.1 tonnes [3] (d) C 10 H 22 + 14.5O 2 C + 9CO 2 + 11H 2 O or C 10 H 22 + 5.5O 2 10C + 11H 2 O or correctly balanced equation [2] 20 4 (a) (i) H 2 SO 4 + NaBr NaHSO 4 + HBr [1] H 2 SO 4 + 2NaBr Na 2 SO 4 + 2HBr [1] [2] (ii) in the reaction mixture [1] (b) to promote smooth boiling [1] (c) (i) water out water in heat [3] 25
(ii) a still head [1] with a thermometer [1] is placed in the flask the condenser is set to collect and condense the gaseous product [1] a condenser placed at the end of the condenser [1] To a maximum of [3] [3] (d) drying agent [1] (e) impurities in a dirty flask will contaminate the product [1] product has been dried, so should not contact water again [1] [2] (f) (i) 3 0.8/60 = 0.04 moles [2] (ii) 4.0/123 = 0.032 moles/0.0325 [1] (iii) 0.032/0.04 100 = 80%/81.3% [1] (g) Practical: loss during transfer loss during distillation Theoretical: side reactions does not go to completion Must be in correct category Any three from four [3] 20 Total 90 26