OCR A Jan 2013 Paper 2 Q2 (with explained solutions) |

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OCR A Jan 2013 Paper 2 Q2

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2 Phosphoric acid, H3PO4, can be manufactured by a two step process involving the reaction of phosphorus with oxygen, followed by a reaction with water. P4(s) + 5O2(g) P4O10(s) H1 = 2984 kJ mol1 P4O10(s) + 6H2O(l) 4H3PO4(l) H2 (a) Explain why H1 represents the enthalpy change of formation of P4O10.[2] (b) Enthalpy changes of formation are shown in the table below. Substance Enthalpy change of formation, Hf / kJ mol1 P4O10(s) H2O(l) H3PO4(l) 2984 286 1267 Calculate the enthalpy change of reaction, H2. H2 =kJ mol1 [3] (c) Write the overall equation for the manufacture of H3PO4 from P4. Use this equation to explain why this process has a 100% atom economy.[2]OCR 2013 [Total: 7] Turn over<br />

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F322/01 Question 2 (a) (b) Mark Scheme January 2013 Answer (enthalpy change for the ) formation of one mole (of P4O10) (cid:57) from (constituent) elements OR from P4/phosphorus and O2/oxygen (cid:57) FIRST, CHECK THE ANSWER ON ANSWER LINE IF answer = 368 (kJ mol1) award 3 marks (+)2984 +(+)6286 OR (+)2984 +(+)1716 OR (+)4700 (cid:57) ()12674 OR ()5068 (cid:57) 368 (cid:57) Marks Guidance ALLOW energy required OR energy released ALLOW makes one mole of product/substance/molecule/ compound ALLOW made from P and O2 OR made from two elements IGNORE comments related to standard conditions IF there is an alternative answer, check to see if there is any ECF credit possible using working below. See list below for marking of answers from common errors. IGNORE sign IGNORE sign ALLOW ECF for enthalpy change of productsenthalpy change of reactants ALLOW for 2 marks: +368 cycle wrong way around OR 1798 no6 OR (+)3433 no x 4 OR 3352 missing 2984 OR (+) 9768 product the wrong sign around OR () 9768 reactants the wrong sign ALLOW for 1 mark: (+)1798 no x 6 and cycle wrong way around OR 3433 cycle wrong way around and not4 OR (+)3352 missing 2984 and cycle wrong way around OR (+)2003 no x 6 or x 4 OR (+)449 missing 2984 and x 4 OR 4782 missing 2984 and x 6 Note: There may be other possibilities.<br />
F322/01 Question (c) Mark Scheme January 2013 Answer P4 + 5O2 + 6H2O (cid:198) 4H3PO4 (cid:57) Only the desired product is made (cid:57) Second marking point can only be awarded if the equation is correct. Marks Guidance ALLOW there are no waste products OR there are no by-products OR there is only one product. DO NOT ALLOW it is an addition reaction Total<br />

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