OCR A AS Specimen Paper 2 Q1 (with explained solutions) |

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OCR A AS Specimen Paper 2 Q1

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Answer all the questions. 1 Bromine is a reactive element. It combines with other non-metals to form covalent compounds. Phosphorus tribromide, PBr3, and iodine monobromide, IBr, are examples of covalent compounds used in organic synthesis. (a) PBr3 can be prepared by heating bromine with phosphorus, P4. (i) Write an equation for this reaction. . (ii) How many molecules are present in 1.3535 g of PBr3? Specimen (iii) The dot-and-cross diagram of a molecule of PBr3 is given below. number of molecules =[1] [3] [3] Name the shape of this molecule and explain why the molecule has this shape. name:explanation: . . .OCR 2014 H032/02<br />
(b) Bromine reacts with iodine to form iodine monobromide, IBr. The table below lists some average bond enthalpies which are required in different parts of this question. Bond BrBr II IBr Average bond enthalpy / kJ mol1 +193 +151 +175 (i) Average bond enthalpy is the enthalpy change for the breaking of 1 mole of bonds in gaseous molecules. Why do Br2 and I2 not exist in the gaseous state under standard conditions? .(ii) Calculate the enthalpy change of formation, fH, for IBr. [1] Specimen (c) fH =kJ mol1 [2] Iodine monobromide, IBr, is a polar molecule. Heterolytic fission of the IBr bond forms an electrophile. State the meaning of the term electrophile and suggest the formula of the electrophile formed from IBr. .(d) Bromine disproportionates when it reacts with potassium hydroxide solution. Suggest an equation for this reaction. [2]OCR 2014[1] H032/02 Turn over<br />
BLANK PAGE SpecimenOCR 2014 H032/02<br />

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Mark Scheme June 20XX (i) P4 + 6Br24PBr3 (cid:57) Answer (ii) FIRST CHECK THE ANSWER ON THE ANSWER LINE IGNORE state symbols Guidance Marks If there is an alternative answer, check to see if there is any ECF credit possible using working below. ALLOW in working shown as 28.1 + 35.54 ALLOW ECF from incorrect molar mass of PBr3 ALLOW 0.005(00) (mol) for two marks ALLOW ECF for incorrect amount of PBr3 ALLOW calculator value or rounding to 3 significant figures or more BUT IGNORE trailing zeroes, e.g. 0.200 allowed as 0.2 DO NOT ALLOW any marks for: 1.3535 x 6.02 x 1023 = 8.15 x 1023 Specimen ALLOW forces of attraction between molecules OR van der Waals forces IGNORE reference to strong or weak H032/02 Question 1 (a) (b) (i) If answer = 3.011021 award 3 marks Mr(PBr3) = 270.7 (g mol1) (cid:57) n(PBr3) = 1.3535 / 270.7 = 5.000103 mol (cid:57) number of molecules = 5.0001036.021023 = 3.011021 molecules (cid:57) (iii) Pyramidal (cid:57) (because there are) 3 bonded pairs and 1 lone pair (around the central phosphorus atom) (cid:57) and electron pairs repel each other as far apart as possible so will take on a tetrahedral arrangement (giving a pyramidal shape overall) (cid:57) (because energy is needed to break) induced dipole dipole interactions / London forces between molecules (cid:57) (ii) Bond breaking (+193) + (+151) = (+)344 P M T<br />
Answer Mark Scheme June 20XX Marks Guidance Correct answer scores 2 marks H032/02 Question (c) (d) = 3 (kJ mol1) (cid:57) (cid:12) (cid:12) 344 350 (cid:11) (cid:14) 2 AND Bond making 2(175) = ()350 (cid:57) fH = (cid:11) (cid:14) Electron pair acceptor (cid:57) I+ (cid:57) Br2 + 2KOHKBr + KBrO + H2O (cid:57) ALLOW 3Br2 + 6KOH5KBr + KBrO3 + 3H2O ALLOW ionic equation 13 Total Specimen P M T<br />

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