OCR A Jun 2015 Paper 1 Q3 (with explained solutions) |

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OCR A Jun 2015 Paper 1 Q3

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This question is about Group 7 elements. (a) Chlorine can be made by the redox reaction below. MnO2(s) + 4HCl(aq) MnCl2(aq) + 2H2O(l) + Cl2(g) Using oxidation numbers, show what has been oxidised and what has been reduced in this reaction. OxidisedReduced[2] (b) Complete the electron configuration of a manganese atom. 1s2[1] (c) Chlorine gas can be added to a cold, dilute alkaline solution to form bleach. Write the equation for this reaction.[1] (d) A student bubbles chlorine gas through aqueous potassium iodide. A reaction takes place. (i) State what the student would observe.[1] (ii) Write the ionic equation for this reaction. Include state symbols.[1]OCR 2015<br />
(e) Chlorine gas reacts with methane. One of the products is dichloromethane, CH2Cl2. (i) Chlorine is more electronegative than carbon and hydrogen, which have approximately equal electronegativity values. Explain what is meant by the term electronegativity.[2] (ii) Draw a 3-D diagram of a molecule of CH2Cl2. Use partial charges to indicate polar bonds. [2] (iii) Explain why a CH2Cl2 molecule is polar.[1] (f) Bromine has two isotopes, Br79 and Br81. The relative atomic mass of bromine is 79.9. Calculate the percentage of Br79 atoms in a sample of bromine.OCR 2015 Answer =% [1] [Total: 12] Turn over<br />

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F321 Mark Scheme June 2015 Question 3 (a) Answer Cl (has been oxidised) from Cl = 1 to Cl = 0 (cid:57) Mn (has been reduced) from Mn = +4 to Mn = +2 (cid:57) Mark Guidance ALLOW 4+ OR 4 OR 2+ OR 2 ALLOW oxidation numbers written above the equation but IGNORE these if oxidation numbers are given in the text ALLOW one mark for Cl is oxidised because the oxidation number increased by 1 AND Mn is reduced because the oxidation number decreased by 2 ALLOW one mark if all oxidation numbers are correct but redox is incorrect. IGNORE HCl is oxidised AND MnO2 is reduced IGNORE correct references to electron loss/gain DO NOT ALLOW incorrect references to electron loss/gain ALLOW 4s2 3d5 IGNORE 1s2 seen twice ALLOW multiples IGNORE state symbols ALLOW OH and ClO, i.e. Cl2 + 2OH (cid:198) ClO + Cl + H2O ALLOW NaOCl ALLOW shades and colours (eg dark yellow, yellow-orange) DO NOT ALLOW purple ALLOW multiples State symbols required ALLOW Cl2(aq) ALLOW Measure for ability ALLOW attraction for ability to attract ALLOW The ability of an atom to attract a shared pair of electrons for two marks (b) (c) 1s2 2s2 2p6 3s2 3p6 3d5 4s2 (cid:57) Cl2 + 2NaOH (cid:198) NaClO + NaCl + H2O (cid:57) (d) (i) (The solution would turn) yellow OR orange OR brown (cid:57) (ii) Cl2 (g) + 2I (aq) (cid:198) I2 (aq) + 2Cl (aq) (cid:57) (d) (e) (i) The ability of an atom to attract electrons (cid:57) (Electron pair) in a (covalent) bond (cid:57)<br />
F321 Question 3 (e) (ii) Answer Mark Scheme Mark Correct orientation of 3-D tetrahedral arrangement of bonds around C atom (cid:57) + on C atom ANDon both Cl atoms (cid:57) For a 3D structure, For bond in the plane of paper, a solid line is expected: For bond out of plane of paper, a solid wedge is expected: For bond into plane of paper, ALLOW: ALLOW a hollow wedge for in bond OR an out bond, provided it is different from the other in or out wedge e.g.: June 2015 Guidance ALLOW any 3D representation with a minimum of one bond into the plane of paper AND minimum of one out of plane of paper ALLOW 2 lines in the plane + 2 different bonds for M1 IGNORE dipole charges on H ALLOW partial charges do not cancel IGNORE charges do not cancel ALLOW (the more) electronegative atoms are on one side of the molecule (iii) The dipoles do not cancel out OR Because the molecule is non-symmetrical (cid:57) 55% (cid:57) (f) Total 12<br />

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