GCE Chemistry A. Mark Scheme for June Unit H032/02: Depth in Chemistry. Advanced Subsidiary GCE. Oxford Cambridge and RSA Examinations

Transcription

1 GE hemistry A Unit 032/02: Depth in hemistry Advanced Subsidiary GE Mark Scheme for June 206 Oxford ambridge and RSA Examinations

2 (Oxford ambridge and RSA) is a leading UK awarding body, providing a wide range of qualifications to meet the needs of candidates of all ages and abilities. qualifications include AS/A Levels, Diplomas, GSEs, ambridge Nationals, ambridge Technicals, Functional Skills, Key Skills, Entry Level qualifications, NVQs and vocational qualifications in areas such as IT, business, languages, teaching/training, administration and secretarial skills. It is also responsible for developing new specifications to meet national requirements and the needs of students and teachers. is a not-for-profit organisation; any surplus made is invested back into the establishment to help towards the development of qualifications and support, which keep pace with the changing needs of today s society. This mark scheme is published as an aid to teachers and students, to indicate the requirements of the examination. It shows the basis on which marks were awarded by examiners. It does not indicate the details of the discussions which took place at an examiners meeting before marking commenced. All examiners are instructed that alternative correct answers and unexpected approaches in candidates scripts must be given marks that fairly reflect the relevant knowledge and skills demonstrated. Mark schemes should be read in conjunction with the published question papers and the report on the examination. will not enter into any discussion or correspondence in connection with this mark scheme. 206

3 032/02 Mark schemes June 206 Annotations Annotation Meaning orrect response Incorrect response Omission mark Benefit of doubt given ontradiction Rounding error Error in number of significant figures Error carried forward Level Level 2 Level 3 Benefit of doubt not given Noted but no credit given Ignore

4 032/02 Mark schemes June 206 Abbreviations, annotations and conventions. Annotation Meaning / alternative and acceptable answers for the same marking point DO NOT ALLOW IGNE ALLOW Separates marking points Answers which are not worthy of credit Statements which are irrelevant Answers that can be accepted ( ) Words which are not essential to gain credit EF AW A Underlined words must be present in answer to score a mark Error carried forward Alternative wording Or reverse argument

5 032/02 Mark Scheme June 206 (a) (i) s 2 2s 2 2p 6 3s 2. ALLOW upper case S and P, and subscripts, e.g....2s 2 3P 6 (ii) (Mg) loses/transfers/donates two electrons. ALLOW Mg loses the 3s electrons provided 2 electronic configuration in (a)(i) is 3 S ALLOW Mg Mg e IGNE reference to oxidation numbers / states (b) (i) Sr + (g) Sr 2+ (g) + e 2.5 ALLOW Sr + (g) e Sr 2+ (g) ALLOW e for electron (i.e. charge omitted) (ii) Atomic radius larger atomic radius more shells IGNE states on the electron FULL ANNOTATIONS MUST BE USED ALLOW A: comparison needed for each mark. ALLOW more/higher energy levels ALLOW electrons further from nucleus ALLOW extra/new shell Effect of nuclear charge/shielding Increased nuclear charge outweighed by increased distance/shielding more/increased shielding IGNE more orbitals more sub-shells different shell ALLOW more electron repulsion from inner shells IGNE responses with no comparison Nuclear attraction less nuclear attraction less attraction on electrons IGNE nuclear charge/effective nuclear charge ALLOW less nuclear pull electrons held less tightly 5

6 032/02 Mark schemes June 206 (c) (i) Diagram of labelled reaction vessel for reaction 3.3 ALLOW (conical) flask, test-tube or boiling tube. DO NOT ALLOW volumetric flask, beaker, measuring cylinder DO NOT ALLOW delivery tube below reacting solution ALLOW any of these diagrams. ALLOW a single line for the tube IGNE Sealed end of delivery tube Labelled (gas) syringe diagram of gas collection over water in a labelled measuring cylinder / inverted burette. AND closed system with a tube connecting reaction vessel to gas collection apparatus DO NOT ALLOW measuring tube 6

7 032/02 Mark schemes June 206 (ii) FIRST EK ALULATED VALUE F MOLAR / ATOMI MASS OF ALIUM IF answer = is seen anywhere award first two marks DO NOT ALLOW pv = nrt for the calculation of the amount in moles for marking point. n( 2 ) n(group 2 metal) = = (mol) 2.8 ALLOW 3 SF up to calculator value correctly rounded ( ) molar mass/atomic mass of Group 2 metal = = 40. (g mol ) 2.8 ALLOW 3 SF up to calculator value correctly rounded ( ) ALLOW EF from incorrectly calculated amount in moles Group 2 metal: calcium/a 3.2 DO NOT ALLOW alcium if no working ALLOW EF as in Group 2 closest to the value calculated (d) Less (volume/products) AND Smaller amount/fewer moles/fewer atoms of the metal reacting 3.2 IGNE higher relative atomic mass/molar mass ALLOW a calculation showing that moles and volume are less n( 2 ) = 0.62/87.6 = Volume = = 44(.4) cm 3 Total 2 7

8 032/02 Mark scheme June (a) Phosphorus has more electrons. ALLOW A but comparison should be used for the all marks DO NOT ALLOW Phosphorus has more electrons in the outer shell or larger electron cloud. Stronger London forces Stronger induced dipole(-dipole) interactions IGNE Phosphorus molecules are bigger or have greater M r. ALLOW more for stronger ALLOW stronger van der Waals /vdw forces More energy required to break the intermolecular forces/bonds London forces 2. DO NOT ALLOW attraction between atoms or that covalent bonds are broken (b) Magnesium metallic (bonds) cations/positive ions/mg 2+ AND delocalised electrons. 4 ALLOW the (electrostatic) attraction between cations/positive ions and delocalised electrons for both Mg marks DO NOT ALLOW molecules for second mark IGNE sea of electrons Silicon covalent ALLOW the attraction between a shared pair of electrons and the nuclei of the (bonded) atoms for both marks between atoms DO NOT ALLOW any intermolecular forces in marking points 2 and 4 or silicon molecules (c) Al 2 S O 2Al(O) S 2.5 IGNE state symbols ALLOW correct multiples Total 8 8

9 032/02 Mark scheme June (a) (i) Any one from: bond is between bonding atoms/nuclei AND bond is above and below the bonding atoms/nuclei bond has direct/head-on overlap of orbitals AND bond has sideways overlap bond has a lower bond enthalpy / is weaker than a bond bond has electron density between bonding atoms AND bond has electron density above and below bonding atoms. IGNE the length of the bond and bond IGNE the type of orbital for bond (ii) One carbon atom (in double bond) is attached to two groups which are identical/the same. ALLOW One carbon atom in (double bond) is not attached to (two) different groups / groups of atoms Right-hand carbon is attached to two groups that are the same/two methyl groups. Two groups are the same on right-hand side Three groups are the same (on the double bond) DO NOT ALLOW Two groups on the same side of the double bond Must be right-hand side; Same side could be top or bottom) Functional groups molecules for groups 9

10 032/02 Mark scheme June 206 (iii) Mark Independently ALLOW correct structural displayed skeletal formulae a combination of above as long as unambiguous ALLOW 2 5 for 2 IGNE connectivity of alkyl groups BUT.DO NOT ALLOW 2 (Z-)pent-2-ene 2.5 DO NOT ALLOW trans-pent-2-ene (b) 2 3. ALLOW correct structural displayed skeletal formulae a combination of above as long as unambiguous O O ALLOW any vertical bond to O, e.g. ALLOW O O DO NOT ALLOW O 0

11 032/02 Mark scheme June 206 (c) Please refer to marking instructions on page 4 of mark scheme for guidance on how to mark this question. Level 3 (5 6 marks) A comprehensive description with all three scientific points explained thoroughly. There is a well-developed and detailed description of the mechanism, including correct structures, accurately drawn curly arrows and using charges and dipoles consistently. andidates compare tertiary and secondary carbocation stability to justify major product. Level 2 (3 4 marks) Attempts to describe all three scientific points but explanations may be incomplete. Explains two scientific points thoroughly with no omissions. The description has some structures with reasonably accurate curly arrows and some charges and dipoles identified Throughout: ALLOW correct structural displayed skeletal formulae a combination of above if unambiguous Indicative scientific points. Two possible products of reaction ( )Br 2 Br( ) IGNE names where correct structures are present 2. Mechanism for formation of either product. urly arrow from = to attack the atom of the Br orrect dipole on Br urly arrow from Br bond to Br arbocation with full positive charge on carbon atom urly arrow from negative charge on Br or lone pair on Br - to carbon atom with positive charge 3 Br Br Level ( 2 marks) A simple description based on at least two of the main scientific points Explains one scientific point thoroughly with few omissions. 3 Br The description is communicated in an unstructured way, including some use of curly arrows, charges or dipoles. Br Br Br

12 032/02 Mark scheme June marks No response worthy of credit Major organic product Major product: 2-bromo-2-methylbutane Total 2 Br Major product is formed from the most stable carbocation intermediate Br is attached to carbon atom with the least hydrogens attached the carbon with the most groups attached the is attached to the carbon atom with most hydrogens attached 2

13 032/02 Mark scheme June (a) (i) More energy is required for bond breaking than is released by bond making 2. (ii) Enthalpy profile diagram labelled 82 on vertical arrow Products above reactants (either chemical symbols or the words products and reactants) Arrow upwards 2.2 enthalpy N 2(g) + / 2O 2(g) N 2O(g) progress of reaction Formulae AND state symbols N 2 (g) + / 2 O 2 (g) N 2 O(g) IGNE activation energy DO NOT ALLOW multiples of equation: mole of N 2 O is formed 3

14 032/02 Mark scheme June 206 (b) FIRST EK ANSWER ON TE ANSWER LINE IF answer = (Pa) award 4 marks If there is an alternative answer, check to see if there is any EF credit possible Amount of N 2 O n(n 2 O) = Unit conversion 4.25 (mol) 2.2 ALLOW EF from incorrect amount of N 2 O e.g. use of incorrect M r for N 2 O could still score 3 marks Volume conversion to m 3 = (m 3 ) Ideal gas equation / temperature conversion p = nrt V p = AND Use of T = 293 K Final answer ommon Errors (3 marks) No temperature conversion p = 3 = Incorrect volume conversion p = = p = (Pa) Must be calculated in standard form AND to 3 SF No volume conversion p = 2.32 = (c) Propagation step NO + O 3 NO 2 + O 2 Propagation step 2 NO 2 + O NO + O 2 Total 9.2 No standard form = ALLOW one mark for both correct symbol equations with (any or all) dots missing or extra dots e.g. NO + O 3 NO 2 + O 2 NO 2 + O NO + O 2 4

15 032/02 Mark scheme June (a) (i) carbon dioxide lost/evolved/given off/or produced as a 3. DO NOT ALLOW water or steam or O 2 evaporates gas (ii) FIRST EK ANSWER ON TE ANSWER LINE IF answer = g award 3 marks n(no 3 ) = = mol If there is an alternative answer, check to see if there is any EF credit possible n(sro 3 ) = = mol ALLOW EF from incorrect n(no 3 ) m(sro 3 ) = =.845 g.85 g molar mass of SrO 3 = 47.6 (g mol ) ALLOW EF from incorrect n(sro 3 ) (b) (i) rate of reaction decreases AND concentration decreases/reactants are used up 2.7 ALLOW reaction slows down ALLOW concentration of reactants decreases. less frequent collisions 2.3 ALLOW fewer collisions per unit time collisions less often decreased rate of collision IGNE less successful collisions/ less collisions less chance of collisions 5

16 032/02 Mark scheme June 206 (ii) Attempted tangent on graph drawn to line at approximately t = 200 s 3. Gradient (y/x) e.g = ALLOW SF up to calculator value, in range to IGNE units IGNE sign (c) Flask beaker AND balance AND stopwatch stop clock other timing device Records mass at time intervals 3.3 DO NOT ALLOW round-bottomed flask. IGNE weighing scales ALLOW weigh at time intervals Time interval quoted between 0-50s Total 6

17 032/02 Mark scheme June (a) The O group is attached to a carbon that is attached to one hydrogen atom The O group is attached to a carbon that is attached to two atoms/ alkyl groups/r groups. ALLOW alcohol/ hydroxyl/functional group for O (b) 04.5º. ALLOW (oxygen atom) has two bond pairs and two lone pairs Bonded pairs/lone pairs/electron pairs repel Lone pairs repel more than bonding pairs NOTE: Lone pairs repel more than bonding pairs would gain the last two marking points ALLOW lp and bp ALLOW bonding regions for bond pairs IGNE bonds repel / electrons repel DO NOT ALLOW atoms repel ALLOW alternative phrases/words to repel e.g. push apart (c) (i) Equation (O) 2 + [O] O O Structure of product could be allowed from equation O 2 (ii) Butan-2-ol/butanone is flammable Butan-2-ol/butanone is volatile/low boiling point Butan-2-ol /butanone will evaporate/boil away (eat under) reflux a description of reflux with vertical condenser and a round bottomed or pear shaped flask with source of heat ALLOW molecular formulae: 4 0 O and 4 8 O ALLOW 4 9 O ALLOW 2 5 for 2 ALLOW correct structural displayed skeletal formulae a combination of above as long as unambiguous IGNE vague answers about health and safety ALLOW alcohol for butan-2-ol ALLOW ketone for butanone DO NOT ALLOW the product or reactant. DO NOT ALLOW distillation DO NOT ALLOW any reference to closed system. 7

18 032/02 Mark scheme June 206 (d) FIRST EK ANSWER ON TE ANSWER LINE IF answer = 67.4% award all 3 marks for calculation If there is an alternative answer, check to see if there is any EF credit possible n(butan-2-ol) (m/m) = 20.2/74 = mol. ALLOW 3 SF: up to calculator value of correctly rounded n(2-bromobutane (m/m) = 25.2/36.9 = 0.84 mol % yield = (0.84/0.273) 00) = 67.4% ALLOW 3 SF: 0.84 up to calculator value of correctly rounded ALLOW (25.2/37) = ALLOW 67% if evidence shows 67.4% in answer ALLOW 67.4% up to calculator value correctly rounded. ommon EFs (2 marks) Incorrect M r resulting in incorrect moles of butan-2-ol or 2-bromobutane ALLOW calculation in mass for 2nd and 3rd marks m( Br 2 ) = = 37.4 g % yield = (25.2/37.4) 00 = 67.4% Total 2 8

19 032/02 Mark scheme June Please refer to marking instructions on page 4 of mark scheme for guidance on how to mark this question. Level 3 (5 6 marks) A comprehensive description with all three scientific points explained thoroughly. identified as a carboxylic acid containing four carbon atoms linked to the peak in the mass spectrum at 43. The explanation makes use of all the evidence including the secondary carbocation in justifying the correct structure of. Level 2 (3 4 marks) Attempts all three scientific points but explanations may be incomplete. Explains two scientific points thoroughly with few omissions. The analysis is clear and includes some interpretation of IR and MS peaks. Level ( 2 marks) A simple explanation based on at least two of the main scientific points. Explains one scientific point thoroughly with few errors. The analysis is communicated in an unstructured way and includes interpretation of peaks from IR MS spectrum LOOK ON TE SPETRA for labelled peaks. Indicative scientific points may include:. Molecular formula Element % mass Ar moles ratio O empirical formula = 2 4 O molecular ion peak m/z or Mr = 88 molecular formula = 4 8 O 2 2. Infrared spectrum peak at (cm ) is O peak at (cm ) is =O is a carboxylic acid ALLOW stated values within the ranges above IGNE references to O peaks 3. Identifying the carboxylic acid ( 2 2 OO ( ) 2 OO) Mass spectrum peak at m/z = 43 = 3 7 ( + ) secondary carbocation: + compound : ( ) 2 OO O O IGNE name of carboxylic acid if structure given 0 marks No response worthy of credit. Total 6 9

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