2811_Jun_03MS

2811_Jun_03MS - 2811 Foundation Chemistry June 2003 Mark...

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Unformatted text preview: 2811 Foundation Chemistry June 2003 Mark Scheme 2811 Mark Scheme June 2003 The following annotations may be used when marking: X ^ bod ecf con = = = = = sf = incorrect response (errors may also be underlined) omission mark benefit of the doubt (where professional judgement has been used) error carried forward (in consequential marking) contradiction (in cases where candidates contradict themselves in the same response) error in the number of significant figures Abbreviations, annotations and conventions used in the Mark Scheme: / ; NOT () ___ (underlining) ecf AW ora = = = = = = = = alternative and acceptable answers for the same marking point separates marking points answers not worthy of credit words which are not essential to gain credit key words which must be used allow error carried forward in consequential marking alternative wording or reverse argument 2811 1. (a) Mark Scheme June 2003 (i) P ! [1] (ii) 1s22s22p63s23p3 ! [1] (iii) charge on ion: 3− ! [1] electronic configuration of ion of A: 1s22s22p63s23p6 ! [1] (b) (i) different number of neutrons ! [1] (ii) Ni ! [1] (c) (i) average atomic mass/weighted mean/average mass ! compared with carbon-12 ! 1/12th of mass of carbon-12/on a scale where carbon-12 is 12 ! OR The mass of 1 mole of atoms of an element ! compared with 12 g ! of carbon-12 ! [3] (ii) 58.0x68.2/100 + 60.0x27.3/100 + 62.0x4.5/100 / 58.726 ! = 58.7 ! (to 3 sig figs: allow full marks for answer. 58.726 (calc) gets 1 mark only) [2] [Total: 11 marks] 2811 2. (a) Mark Scheme June 2003 correct dot and cross ! [1] (b) number of electrons increases down group ! → greater van der Waals’ forces/intermolecular forces ! more energy/higher temperature needed to break these intermolecular forces ! [3] (c) (i) brown/orange/yellow colour ! [1] (ii) 2NaBr + Cl2 → Br2 + 2NaCl balanced equation ! or ionic equation: 2Br− + Cl2 → Br2 + 2Cl− [1] (iii) Cl/Cl2 gains electron(s) ! Br− loses an electron ! [2] (iv) Cl is more reactive/more powerful oxidising agent than Br ! [1] (v) add AgNO3 / Ag+ ! yellow precipitate ! OR add Cl2 / Br2 ! purple in hexane / blue-black in starch ! [2] [Total: 11 marks] 2811 3. Mark Scheme (a) correct dot and cross ! June 2003 correct charges ! [2] (b) (i) Ca: (+)2 ! [1] (ii) C: (+)4 ! [1] (c) moles CaCO3 = 20 x 106/100 = 200 000 mol ! mass CaO = 200 000 x 56 = 11 200 000 g / 1.12 x 107 g / 11.2 tonnes ! use of 56 x 20/100 OR 56/5 is worth 1 mark decimal point in wrong place i.e. 1.12 x 10x is worth 1 mark. units needed for 2nd mark. [2] (d) CaO + H2O → Ca(OH)2 ! [1] (e) (i) molar mass = 40.1 + (16 + 1) x 2 = 74.1 (g mol−1) ! [1] (ii) moles HCl = 0.200 x 25.0/1000 = 0.005 mol ! [1] (iii) moles Ca(OH)2 = 0.5 x 0.005 = 0.0025 mol ! mass Ca(OH)2 = 0.0025 x 74.1 = 0.185 g ! accept from 0.19 g to 0.18525 g i.e. 0.0025 x answer to (i) candidate who does not use 0.5 will get 0.37 g – worth 1 mark [2] (iv) 1 mol NaOH reacts with 1 mol HCl / Ca(OH)2 Has more OHs / OHs needed to neutralise ! Therefore twice the number of moles of NaOH are needed / twice number of OHs in Ca(OH)2 ! [2] (f) solution reacted with CO2 ! forming CaCO3 ! [2] [Total: 15 marks] 2811 4. Mark Scheme June 2003 (a) 4 valid examples !!!! Can be names or formulae. If a formula is used, it must be correct for the structure: i.e. for simple molecular, H2, P4, S8, etc is feuqired. [4] (b) (i) positive ions/metal ions/cations ! surrounded by free/delocalised/sea of electrons ! [2] (ii) electrons move ! [1] (c) solid lattice, ions are in fixed positions ! molten, ions are free to move and conduct ! [2] (d) giant has stronger forces/simple has weaker forces ! (i.e. comparison of forces) giant: covalent bonds break ! simple: molecules/intermolecular forces break / van der Waals break ! [3] [Total: 12 marks] 2811 5. Mark Scheme June 2003 electron pairs repel ! as far apart as possible ! lone pair repulsion > bonded pair repulsion / lone pair reduces bond angle ! [3] 4 examples, for each: shape !!!! either a named molecule that matches a correct shape or correct number and type of electron pairs to match shape 3 correct bond angles ! !! [7] [Sub-total: 10] qowc: technical words of three shapes: i.e. linear non-linear / V-shaped / angular / bent trigonal planar / planar triangle tetrahedral / tetrahedron pyramid(al) / trigonal pyramid(al) [Sub-total: 1] [Total: 11 marks] 2811 Mark Scheme June 2003 Further notes on Question 5 1. Three marks that explain the theory: electron pairs repel ! • This could simply be within ‘electron pair repulsion theory’ or ‘lone pairs repel’ lone pairs repel more ! repelled as far apart as possible ! 2. Seven marks for shapes and bond angles Bond angles • Credit up to three correct bond angles for chosen examples. i.e. 3 max Shapes (a) If a candidate has drawn shapes of molecules, • mark the shapes irrespective of any words that describe them. • only look at the words (pyramidal, etc) for the QoWC mark (see below) • do not use an incorrect name as a ‘con’ or we will be looking for both the shape and the name for the mark. e.g. O H H 104.5o ! ! ignore except for QoWC trigonal planar (b) If a candidate has drawn a ‘correct’ shape but for a molecule that does not exist, then the shape mark cannot be awarded. e.g. BeCl3 shown as a trigonal planar molecule would not score the shape (but could score an angle mark of 120 ° if ‘trigonal planar’ is used as a fall back). The example below is certainly worth something! e.g. Cl " Be Cl Cl 120o ! trigonal planar (c) This rescues the angle mark If a candidate has not drawn a diagram, • the shape mark is still possible if the correct technical word is used. If this tactic has been used then you can still award the technical words as part of the QoWC mark (see below). e.g. CH4 has a tetrahedral shape ! with a bond angle of 109.5 ° ! QoWC One mark Use of any three of the ‘shape technical words’ with correct shapes. i.e. testing ‘correct usage’ of technical words. ...
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This note was uploaded on 10/20/2011 for the course ECON 03 taught by Professor Mark during the Winter '08 term at UCLA.

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