OCR A Jun 2011 Paper 1 Q3 (with explained solutions) |

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OCR A Jun 2011 Paper 1 Q3

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The chlor-alkali industry is an important part of the UK chemical industry. The raw material is brine, a concentrated aqueous solution of sodium chloride, NaCl(aq). Two products that can be manufactured from brine are chlorine and sodium hydroxidehence the name chlor-alkali. (a) Bleach can be made by reacting chlorine with cold aqueous sodium hydroxide. A solution of bleach contains the chlorate compound NaClO. Write the equation for the reaction taking place.[1] (b) The systematic name for NaClO is sodium chlorate(I). Other chlorate compounds exist, such as NaClO3. (i) Give the systematic name for NaClO3.[1] (ii) When heated, NaClO3 disproportionates as shown in the equation below. 4NaClO3 3NaClO4 + NaCl Using oxidation numbers, explain why this is a disproportionation reaction.[3]OCR 2011<br />
(c) Chlorine has been added to drinking water for over a century. Recently, some scientists have put forward the case for not chlorinating drinking water. This is because chlorine may react with organic compounds in the water to form CH3Cl. (i) State one valid reason that supports the scientists case and state one reason why chlorine should be added to drinking water.[2] (ii) Draw a dot-and-cross diagram to show the bonding in a molecule of CH3Cl. Show outer electrons only. [1] (iii) Name the shape of a molecule of CH3Cl .[1] (d) A sample of brine is a concentrated aqueous solution of sodium chloride, NaCl(aq). Describe a simple chemical test that you could carry out to show that brine contains aqueous chloride ions. How would you confirm that no other halide ions are present? Include an ionic equation in your answer.[4]OCR 2011 [Total: 13] Turn over<br />

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F321 Question Answer 3 (a) 2NaOH + Cl2 (cid:198) NaClO + NaCl + H2O (cid:57) (b) (i) Sodium chlorate(V) (cid:57) (ii) Cl in NaClO3 is (+)5 AND Cl in NaClO4 is (+)7 AND Cl in NaCl is 1 (cid:57) Chlorine has been both oxidised and reduced OR The oxidation number of chlorine has increased AND decreased (cid:57) Chlorine has been oxidised from (+)5 to (+)7 AND chlorine has been reduced from (+)5 to 1 (cid:57) (These points would secure marking points 2 and 3) 4NaClO3 (cid:198) 3NaClO4 + NaCl +5 +7 1 This diagram gets all reduction oxidation 3 marks (c) (i) Chlorinated hydrocarbons are carcinogens OR toxic OR Chlorine is toxic OR poisonous (cid:57) (Chlorine) kills bacteria OR kills germs kills micro-organisms OR makes water safe to drink OR sterilises water OR disinfects (cid:57) 1 1 1 1 1 Mark Scheme June 2011 Mark Guidance ALLOW NaOCl IGNORE state symbols ALLOW sodium chlorate V DO NOT ALLOW sodium chlorate 5 USE annotations with ticks, crosses, con, ECF, etc for this part. ALLOW 5+, 7+ 1 Look for oxidation numbers seen above equation. DO NOT ALLOW Cl in NaCl The second and third marking points must refer to chlorine ALLOW it for chlorine if oxidation numbers of chlorine are given ALLOW Cl for chlorine DO NOT ALLOW Cl2 for chlorine ALLOW correct references to oxidation and reduction even if based on incorrect oxidation numbers of chlorine IGNORE references to electron loss / gain if correct. DO NOT ALLOW 3rd mark for reference to electron loss/gain If oxidation numbers are correct, ALLOW 1 mark for chlorine is oxidised to form NaClO4 ALLOW 1 mark for chlorine is reduced to form NaCl ALLOW one mark for disproportionation is when a species is both oxidised and reduced whether or not chlorine is mentioned ALLOW CH3Cl for chlorinated hydrocarbons IGNORE harmful IGNORE carcinogenic for chlorine DO NOT ALLOW antiseptic ALLOW to make water potable ALLOW removes for kills IGNORE virus IGNORE purifies water IGNORE cleans water<br />
F321 Question (ii) (c) Answer Electron pairs in covalent bonds shown correctly using dots and crosses in a molecule of CH3Cl AND lone pairs correct on Cl (cid:57) Cl Mark Scheme June 2011 Mark Guidance 1 1 1 1 1 Must be dot-and cross ALLOW different symbol for third type of electron Circles for outer shells not needed IGNORE inner shells Non-bonding electrons of chlorine do not need to be shown as pairs ALLOW Ag+(aq) seen in the ionic equation IGNORE references to nitric acid IGNORE references to adding water or dissolving the brine DO NOT ALLOW references to any other additional reagent as well as the silver nitrate for the first mark White AND precipitate required DO NOT ALLOW hint of any other colour IGNORE turns grey ALLOW solid as alternative for precipitate IGNORE states DO NOT ALLOW conc. NH3 DO NOT ALLOW any mention of incomplete dissolving ALLOW (for 4th mark) add Cl2(aq) AND no colouration would be seen OR no change OR no reaction (d) (iii) Tetrahedral OR tetrahedron (cid:57) Add AgNO3(aq) OR Ag+(aq) OR silver nitrate OR AgNO3 (cid:57) White precipitate (cid:57) Ag+ + Cl (cid:198) AgCl (cid:57) Add dilute NH3 and precipitate (completely) dissolves OR disappears (cid:57) Total 13<br />

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