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Linda Henderson
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1 Atoms Mark Scheme Level Subject Exam Board Topic Booklet Pre U Chemistry Cambridge International Examinations Atoms-Physical Chemistry Mark Scheme Time Allowed: 47 minutes Score: /39 Percentage: /00 Grade Boundaries:

2 . a (i) Energy change to break one mole of bonds in the gas phase. mark for each underlined point. [3] (ii) r H o = 2 ( ) kj mol = 244kJ mol mark for bonds broken; mark for bonds made; mark for correct sign if the answer is correct [3] (b) Energy change = ( (2 480))cm = cm (ii) s 2 2s 2 2p 6 3s 2 3p 6 4s Superscripts must be used. (iii) At least one K 4s atomic orbital labelled () Labelled sigma bond below labelled sigma antibond () (A single electron (spinning in either sense) in each atomic orbital and) two spin-paired electrons in the sigma bond () Electrons must be shown with a single- or double-headed arrow. [3] (iv) The outer electron in K is closer to the nucleus than the outer electron in Rb () There is less shielding of the nucleus for the K outer electron than the Rb outer electron. () (Despite the extra nuclear charge in rubidium) there is a weaker attraction of the electron to the nucleus () Allow the opposite statements with respect to Rb. [3] (v) Labelled Rb 5s orbital shown higher in energy than labelled K 4s orbital () Sigma bond is lower in energy than K 4s orbital and the antibond is higher in energy than the Rb 5s orbital () The bonding and antibonding orbitals must be labelled for the second mark. (vi) E = cm h c N A 00 cm m / 000 J kj = 0.3 kj mol () Two marks for correct answer. Deduct one mark for each error. One mark if final answer is out by a factor of N A i.e Allow two or more sig figs. [2] [2] [Total: 8]

3 2. (a) = 70 (b) 9 (c) 8 ecf from part (b), i.e. the number of elements wide = twice the number of orbitals (d) 4p 5s 4d 5p 6s 4f 5d (e) 6d, 7p, 8s and 5g should be added to the diagram as below s 2s 8s 2p 3 7p 3d 4 6d 4f 5f 5g (f) two g electrons [Total: 6]

4 Question Number Expected Answer Max Mark Rationale 3 (a) A: Electrons B: Protons/Hydrogen ions/h + 3 (b) platinum (palladium/nickel/metal hydrides) 3 (c) Cathode: O 4e 2 + 4H + + 2H 2 O Anode: H 2 2H + + 2e 3 (d (i) Any two from: eliminates/reduces greenhouse gases if hydrogen comes from electrolysis of water have higher efficiency (than diesel or gas engines) much quieter operation (than internal combustion engines) maintenance simple/few moving parts 3 (d) (ii) Any two from: reforming is technically challenging and not environmentally friendly refuelling and starting times are longer driving range of cars is shorter fuel cells generally bigger/heavier than comparable batteries or engines Max 2 Max 2 ALLOW one mark if correct equations but wrong way round ALLOW multiples Advantages must relate directly to context of use in motor vehicles Disadvantages must relate directly to context of use in motor vehicles

5 Question Number Expected Answer Max Mark Rationale 3 (e (i) Zn(s) Zn 2+ (aq) State symbols required 3 (e) (ii) being oxidised = Zn being reduced = Ag 2 O (or Ag in the Ag 2 O) DO NOT ALLOW Ag + 3 (e) (iii) Zn + Ag 2 O + H 2 O Zn OH + 2Ag ALLOW Zn(OH) 2 state symbols NOT required 3 (e) (iv) ( 0.76) = (e) (v) ALLOW ecf from 3(e)(iv) G = nfe = = 22(.30) kjmol 3 (e) (vi) G = RTlnK = lnk lnk = 22300/( ) = K = e =.7() 0 37 ALLOW ecf from 3(e)(v) Total [7]

6 4. (a 78/( ) 00% = 84.8% (b) correct plotting of point in van Arkel triangle () the point has coordinates (2.39, 2.45) half a gradation of leeway either side, i.e on the scale, is acceptable it is an insulator () [2] (c) it is ionic (d) reaction : HfO 2 + 4HCl HfCl 4 + 2H 2 O () reaction 2: HfCl 4 + 2Mg Hf + 2MgCl 2 () ecf incorrect hafnium chloride formula in step 2 from step [2] [Total: 6]

Paper 2 Part A Written For Examination from 2016 SPECIMEN MARK SCHEME 2 hours 15 minutes MAXIMUM MARK: 100

Cambridge International Examinations Cambridge Pre-U Certificate www.xtremepapers.com CHEMISTRY (PRINCIPAL) 9791/02 Paper 2 Part A Written For Examination from 2016 SPECIMEN MARK SCHEME 2 hours 15 minutes