OCR A Jun 2010 Paper 1 Q5 (with explained solutions) |

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OCR A Jun 2010 Paper 1 Q5

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This question is about elements in Period 2 of the Periodic Table. 10 (a) Lithium has a giant metallic structure and a boiling point of 1342 C. Describe, with the aid of a labelled diagram, the structure and bonding in lithium and explain why lithium has a high boiling point.[3] (b) Fluorine is a gas at room temperature and has a very low boiling point of 188 C. (i) Draw a dot-and-crossdiagram to show the bonding in a fluorine molecule. Show the outer electrons only. (ii) Explain why fluorine has a low boiling point.[2] [1]OCR 2010<br />
11 (c) Fluorine reacts with lithium at room temperature to form a white crystalline solid, lithium fluoride. Lithium fluoride is a good conductor of electricity when molten but not when solid. (i) Draw a dot-and-crossdiagram to show the bonding in lithium fluoride. Show the outer electrons only. (ii) Explain why lithium fluoride conducts electricity when molten but not when solid. [2][2]OCR 2010 Turn over<br />
(d) Fluorine reacts with boron, B, to form the fluoride BF3. 12 (i) Suggest an equation for this reaction.[1] (ii) Name the shape of, and state the bond angles in, a BF3 molecule. Explain why BF3 has this shape. In your answer, you should use appropriate technical terms spelt correctly.[4] (e) Nitrogen can also form a fluoride, NF3, which has a permanent dipole. Explain why NF3 has a permanent dipole.[2]OCR 2010<br />
13 (f) Describe and explain the trend in atomic radii of the elements Li to F across Period 2 of the Periodic Table. In your answer, you should use appropriate technical terms, spelt correctly.[4] [Total: 21] END OF QUESTION PAPEROCR 2010<br />

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$F321 Question 5 a Expected Answers++ Li ions Delocalised electrons Marks Diagram showing a regular arrangement of labelled Li+ or + ions with some attempt to show electrons (cid:57) Scattering of labelled electrons between other species OR a statement anywhere of delocalised electrons (can be in text or in diagram) (cid:57) The attraction between + ions and e is strong OR metallic bonding is strong (cid:57) Dot and cross bond + 6 matching electrons on each F atom (cid:57) ii F2 has induced dipoles OR temporary dipoles OR van der Waals forces (between the molecules) (cid:57) which are weak (cid:57) Mark Scheme June 2010 Additional Guidance Lattice diagram must have at least two rows of correctly charged ions and a minimum of 2 ions per row ALLOW as label: + ions, positive ions, cations If + is unlabelled in diagram, award label from a correct statement within the text below DO NOT ALLOW 2+, 3+ etc ions DO NOT ALLOW for label or in text: nuclei OR positive atom OR protons ALLOW e OR e as label for electron ALLOW a lot of energy is needed to break the (metallic) bond DO NOT ALLOW incorrect particles or incorrect attraction e.g. intermolecular attraction or nuclear attraction ALLOW diagram consisting of all dots OR all crosses Circles not essential ALLOW Fl for fluorine ALLOW little energy needed to overcome intermolecular bonding for second mark ALLOW weak intermolecular bonding for second mark ALLOW max 1 mark if structure is referred to as giant with first and second marking points correct Award no marks if weak is applied to incorrect bonding. E.g. ionic, covalent, metallic or unspecified bonding<br /> F321 Question 5 c i ii Li shown with either 2 or 0 electrons and F shown with 8 electrons with 7 crosses and one dot (or vice versa) (cid:57) correct charges on both ions (cid:57) Ions cannot move in a solid (cid:57) Ions can move OR are mobile when molten (cid:57) i 2B + 3F2 (cid:127)(cid:111) 2BF3 (cid:57) ii Shape: trigonal planar (cid:57) Bond angle: 120 (cid:57) Explanation: Pairs of electrons repel (one another equally) (cid:57) Boron has 3 bonded pairs (and 0 lone pairs) (cid:57) Expected Answers Marks Mark Scheme June 2010 Additional Guidance For first mark, if 2 electrons are shown in the cation then the extra electron in the anion must match symbol chosen for electrons in the cation IGNORE inner shell electrons ALLOW Fl for fluorine Circles not essential DO NOT ALLOW Li+ with 8 electrons Second mark is independent ALLOW ions are fixed in place IGNORE electrons IGNORE charge carriers or charged particles DO NOT ALLOW ions can move when in solution IGNORE charge carriers IGNORE delocalised ions or free ions ALLOW Ions can only move when molten for one mark Any mention of electrons moving when molten is a CON ALLOW B2 ALLOW multiples including fractions Trigonal planar must be seen and spelt correctly at least ONCE DO NOT ALLOW atoms repel or electrons repel ALLOW bonds repel ALLOW diagram showing B atom with three dot-and-cross pairs of electrons, but no lone pairs for 4th mark Must refer to boron / central atom ALLOW bonds for bonded pairs 10<br /> F321 Question 5 e Expected Answers F is more electronegative than N OR (cid:71)-FN(cid:71)+ (cid:57) Dipoles do not cancel OR NF3 is pyramidal (in words) / asymmetrical (cid:57) (As you go across the period) The atomic radii decreases (cid:57) The nuclear charge increases OR protons increase (cid:57) electrons are added to the same shell OR shielding remains the same (cid:57) greater (nuclear) attraction on (outer) electrons / (outer) shell(s) (cid:57) Total Mark Scheme June 2010 Marks 2 Additional Guidance ALLOW F attracts electrons more than N ALLOW N has a partial positive charge and F has a partial negative charge (partial must be seen) DO NOT ALLOW diagrams that contradict statements about polarity ALLOW unsymmetrical, non-symmetrical etc Use annotations with ticks, crosses ECF etc. for this part Assume across the period from Li to F ALLOW (outer shell) electrons get closer (to nucleus) IGNORE atomic number increases, but ALLOW proton number increases IGNORE nucleus gets bigger Charge increases is insufficient ALLOW effective nuclear charge increases OR shielded nuclear charge increases Nuclear OR proton(s) OR nucleus spelt correctly ONCE and used in context of 2nd marking point ALLOW shielding is similar ALLOW screening for shielding DO NOT ALLOW subshells DO NOT ALLOW distance is similar This will CON first marking point ALLOW greater (nuclear) pull for greater nuclear attraction DO NOT ALLOW pulled in more as this is a restatement of the first marking point 21 11<br />$

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