RI 2015 A-Level H2 Chem Solution

This document is copyrighted, please do not reproduce it without permission © Raffles Institution Changes to 2015 H2 Chemistry A Level Question Paper Dear students, The TYS you have purchased is based on the 9647 (old) syllabus. You will be sitting for the 9729 (new) syllabus papers. This document will instruct you on the changes you need to make to the TYS questions. The Planning question for the 9729 syllabus will be in the Paper 4 (Practical). Some concepts are no longer tested in the 9729 syllabus and the values used for some calculations are now different (e.g. molar volume at s.t.p.) which will affect your choice of the answers. You are advised to make the changes on your question papers before you attempt it . Do inform your tutors if you notice any differences which were not highlighted in this document. ---------------------------------------------------------------------------------------------------------------------------------------------- Paper 1 15 Amend question “Which property generally increases down Group 2?” Amend Option D thermal stability of the carbonate Paper 2 3(d) Not in syllabus Paper 3 1(a)(ii) Amend question “How many chiral centres are there in the menthol molecule, and how many enantiomers are possible?” 2(b) Not in syllabus 3(a)(i) Not in syllabus 3(a)(ii) Not in syllabus - Reaction of calcium oxide with water 3(c)(i) - (ii) Not in syllabus 4(b) Bond energy of C=O in CO2 in the Data Booklet (for new syllabus) is the same as that given by the question 5(c)(ii) Amend question “Unlike other Group 1 carbonates, ….”

This document is copyrighted, please do not reproduce it without permission © Raffles Institution 2015 A - Level H2 Chemistry Paper 1 Suggested Solutions 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 B B C C B D B C A B D C D B D D A C B A 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 B D B A D D D C B A D A C A B A C C A B Q1 ( B) A 1s2 2s1 2p1 Excited state of Group 2 B 2s2 2p2 Ground state of Group 14 C 2s1 2p3 Excited state of Group 14 D 3p4 Excited state Q2 ( B) σ bonds formed by head-on overlap of either s or p orbitals. π bonds formed by side-to-side overlap of p-orbitals. s-or bitals are spherical and thus cannot overlap in a way to form π bonds. Q3 (C) Q4 (C) Calcium and sodium are metals. Their melting points depend on the strength of their metallic bonds. Strength of metallic bonds depend on: • charge of metal ion • size of metal ion • no. of valence electrons which can be delocalised (more delocalised electrons ⇒ greater attraction between metal cations and delocalised electrons) A true, but irrelevant B true, but irrelevant C Ca2+ has a higher charge and hence higher charge density than Na+. D true, but irrelevant. This option discusses the total no. of electrons in each cation and not the no. of delocalised electrons. Q5 (B) N C C O O H H H H H 120o109o107o N C C O O H H H H 120o109o H 109o zwitterion non-zwitterion 90° bond angle absent 90° bond angle absent –NH2 → –NH3+ 107° 109° 90° bond angle absent in both structures 107° bond angle absent only in zwitterion. 16 1 81OH − 8 electrons 8 neutrons 1 electron 0 neutron 1 additional electron no. of electrons = 8+1+1=10 no. of neutrons = 8+0=8 charge density (high charge & small size ⇒ high charge density ⇒ stronger attraction for delocalised electrons.)

This document is copyrighted, please do not reproduce it without permission © Raffles Institution Q6 (D) Lattice energy is the energy released when one mole of the solid ionic compound [ i.e. Na 2O(s)] is formed from its constituent gaseous ions [i.e. Na +(g) and O 2–(g)] under standard conditions of 298 K and 1 bar. D 2Na+(g) + O2–(g) → Na2O(s) (from gaseous ions) (to 1 mol of solid ionic compound) Q7 (B) Amt of NaOH used = (12.5 / 1000)(0.0500) = 0.000625 mol = Amt of HCl reacted Amt of HCl remaining = (25/1000)(0.100) – 0.000625 = 0.001875 mol [HCl] remaining = 0.001875 (12.5 25.0) 1000 + = 0.0500 mol dm–3 Q8 (C) “…methane reacted with oxygen to form a mixture of carbon dioxide and carbon monoxide in a mole ratio of 9:1…” CH4 + O2 → 9CO2 + 1CO + H2O Balance the equation: 10CH4 + 19.5O2 → 9CO2 + 1CO + 20H2O At constant pressure and temperature, volume of gas ∝ amt of gas in mol volume of O2 = ( )19.5 110 = 1.95 dm3 Q9 (A) CO2 forms hydrogen bonds with H2O molecules ⇒ ∆H is negative (bond formation is exothermic) Reaction causes decrease in amount of gaseous particles (1 mol to 0 mol of gaseous particles) ⇒ ∆S is negative (increase in order) Q10 (B) Standard electrode potential , E, is the electromotive force, • measured at 298 K • in which the concentration of any reacting species in solution is 1 mol dm–3, and • any gaseous species is at a pressure of 1 bar. Hence, [HOCl] = [H+] = [Cl2] = 1 mol dm–3 Q11 (D) CH3CO2H ⇌ CH3CO2– + H+ Initial C 0 0 Change –αC +αC +αC Eqm (1 – α)C αC αC 22 c () (1 ) (1 ) CCK C αα αα= =−− Q12 (C) To ↑ amt of methanol ⇒ to favour forward reaction Exothermic forward reaction is favoured by decrease in temperature. An increased pressure would favour the forward reaction, which reduces amt of gaseous particles. Q13 (D) Let x be the eqm amt of H2 and CO2. H2O(g) + CO(g) ⇌ H2(g) + CO2(g) Initial / mol 1.0 1.0 0 0 Change / mol –x –x +x +x Eqm /mol 1.0 – x 1.0 – x x x Total amt of gases at eqm = 2(1.0 – x) + 2(x) = 2 mol 2amt of H 0.333 0.666total amt of gases 2 x x== ⇒= 2 c 2 (0.666) 3.97 4(1 0.666)K = = ≈−

This document is copyrighted, please do not reproduce it without permission © Raffles Institution Q14 (B) Given: rate = k[H2][NO]2 From expt 1 to 2, [H2] remains the same when [NO] x ½, initial rate x (½)2 = ¼ ⇒ x = ¼(6.0) = 1.5 From expt 2 to 3, [NO] remains the same when [H2] x 2, initial rate x 2 ⇒ y = 2(1.5) = 3.0 From expt 3 to 4, [H 2] remains the same and initial rate x ¼ (from 3.0 to 0.75). Since rate ∝ [NO]2, [NO] must have halved from 1.0 to 0.5 mol dm–3 ⇒ z = 0.5 Q15 (D) A chargecharge density radius∝ Down Group 2, charge of M2+ remains the same, but ionic radius increases. ⇒ magnitude of charge density decreases down the group. B Electronegativity decreases as the number of shells of electrons increases down the group, reducing the attraction by the nucleus for the valence electrons. C Elements of Group 2 have the same number of valence electrons i.e. 2. D Thermal stability of Group 2 carbonates increase down the group. Down Group 2, • ionic radii of Group 2 cations increase • charge density, and hence polarising power of cations decrease, • covalent bonds in carbonate anion are less polarised / weakened to smaller extent, • more thermal energy required to break covalent bonds in carbonate anion, • thermal stability increases. Q16 (D) Period 3 elements: Na Mg Al Si P S Cl Ar A False. Sulfur also forms 2 acidic oxides, i.e. SO2 and SO3. B False. First ionisation energy (IE) generally increases across the period. Hence, Ar has the highest first IE. (or refer to IE data from Data Booklet) C False. Si forms SiCl4, while P forms PCl3 and PCl5, all of which dissolve in water to form acidic solutions. (Reaction of PCl3 with water in not in syllabus) D True. P, S and Cl ex ist as simple molecules with formula P4, S8 and Cl2 respectively. Q17 (A) A [Cu(NH3)4(H2O)2]2+SO42– dark blue solution B [Cu(NH3)6]2+SO42– dark blue prec ipitate solution C Cu(OH)2)(H2O)4 pale blue solution precipitate • essentially Cu (OH)2 which is a precipitate D