OCR A Jun 2010 Paper 2 Q1 (with explained solutions) |

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OCR A Jun 2010 Paper 2 Q1

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Answer all the questions. The alkanes are an homologous series of hydrocarbons. The table shows information about some straight chain alkanes. alkane molecular formula boiling point / C methane ethane propane butane CH4 C2H6 C3H8 C4H10 1648942 1 (a) (i) What is meant by an homologous series?[1] (ii) Explain why the boiling points increase down the alkane homologous series.[2]OCR 2010<br />
(b) Alkynes are another homologous series of hydrocarbons. The table gives the molecular formulae of the first five straight chain alkynes. alkyne molecular formula ethyne propyne but-1-yne hex-1-yne C2H2 C3H4 C4H6 C5H8 C6H10 (i) Suggest the name of a straight chain alkyne with the molecular formula C5H8.[1] (ii) Deduce the general formula for an alkyne.[1] (iii) The alkynes contain the C C functional group. Suggest the displayed formula for propyne. (iv) Hex-1-yne has many cyclic structural isomers. Draw the skeletal structure of one of these cyclic structural isomers. [1] [1]OCR 2010 Turn over<br />
(c) Ethyne is commonly called acetylene. It is used in an oxy-acetylene flame which is hot enough to cut through steel. Ethyne completely combusts as shown in the equation below. 2 O O2 Calculate the enthalpy change of combustion of ethyne using the average bond enthalpies in the table below. bond CH C C O=O C=O OH average bond enthalpy / kJ mol1 +415 +837 +498 +805 +464 enthalpy change of combustion =kJ mol1 [3]OCR 2010<br />
(d) Ethyne is formed when water reacts with calcium carbide, CaC2. CaC2(s) + 2H2O(l) Ca(OH)2(s) + C2H2(g) The standard enthalpy change of this reaction can be determined indirectly using standard enthalpy changes of formation. (i) What is meant by the term standard enthalpy change of formation, Hf You should state the standard conditions in your answer. O ?[3] (ii) Standard enthalpy changes of formation are shown in the table below. substance CaC2(s) H2O(l) Ca(OH)2(s) C2H2(g) standard enthalpy change O / kJ mol1 of formation, Hf60 286 987 +227 Calculate the standard enthalpy change of the reaction: CaC2(s) + 2H2O(l) Ca(OH)2(s) + C2H2(g) standard enthalpy change of reaction = . kJ mol1 [3]OCR 2010 Turn over<br />
(e) A factory makes ethyne gas from calcium carbide, CaC2. One of the waste products is calcium hydroxide. CaC2 + 2H2O Ca(OH)2 + C2H2 Each day 1.00106 grams of calcium carbide are used and 3.60105 dm3 of ethyne gas, measured at room temperature and pressure, is manufactured. (i) Calculate the atom economy for this process using the relative formula masses in the table below. compound relative formula mass CaC2 H2O Ca(OH)2 C2H2 64.1 18.0 74.1 26.0 (ii) Calculate the amount, in moles, of CaC2 used each day. atom economy =% [2] (iii) Calculate the amount, in moles, of C2H2 made each day. amount of CaC2 = mol [1] (iv) Calculate the percentage yield of C2H2. amount of C2H2 = mol [1]OCR 2010 percentage yield = . % [1]<br />
(v) Comment on the percentage yield and the atom economy of this process in terms of sustainability.[2] [Total: 23]OCR 2010 Turn over<br />

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$F322 Question 1 i ii More surface contact OR bigger molecules (cid:57) More van der Waals forces (cid:57) Pent-1-yne OR pent-2-yne (cid:57) ii CnH2n2 (cid:57) Mark Scheme June 2010 Expected Answers Series having same functional group and a general formula (cid:57) Marks Additional Guidance ALLOW same functional group and members vary by CH2 ALLOW organic compounds with the same functional group that differ in length of their hydrocarbon chain BOTH answers need to be comparisons ALLOW higher relative formula mass OR has more electrons OR longer chain length OR more carbon atoms IGNORE surface area / bigger compounds ALLOW stronger van der Waals forces / stronger induced dipoles VDW forces is not sufficient More intermolecular forces is not sufficient DO NOT ALLOW breaking bonds within the chain / breaking covalent bonds IGNORE reference to bonds if not linked to covalent bonds ALLOW pentyne Look for answer in the table if not on answer line but answer line takes precedence ALLOW CnH2(n1)<br /> Mark Scheme June 2010 Correct displayed formula (cid:57) Expected Answers Marks Additional Guidance F322 Question 1 iii Correct skeletal formula of cyclic hydrocarbon with formula C6H10 (cid:57) Energy required to break bonds = (+) 2912 (cid:57) Energy released to make bonds = ()4148 (cid:57) Enthalpy of combustion = 1236 (cid:57) Examples of correct skeletal formulae include ALLOW full marks for correct answer with no working out ALLOW (2415) + (837) + (2.5498) ALLOW (4805) + (2464) OR (4805) + (2464) ALLOW ECF for calculation of enthalpy of combustion ALLOW 2 marks for +1236 with no working out<br /> F322 Mark Scheme June 2010 Marks Additional Guidance IGNORE energy required / energy released ALLOW (energy change) when one mole of a substance DO NOT ALLOW enthalpy change for one mole of products ALLOW 1 atmosphere pressure / 101 kPa / 105 Pa / 1.01105 Nm2 / 1000 millibars / 25 oC / any stated temperature in words IGNORE 1 mol dm3 for solutions ALLOW full marks for 128 with no working out ALLOW ECF from errors in calculation ALLOW two marks for answer of 414 / +128 / 1392 / +1392 ALLOW one mark for answer of +414 First mark for 100.1 OR (64.1 + 36.0) OR (74.1 + 26.0) at bottom of fraction with or without100 ALLOW full marks for 26.0 or 26% with no working out ALLOW from two significant figures up to calculator value ALLOW 25.97 / 26% NO ECF for this part from incorrect numbers in first expression Question i Expected Answers (Enthalpy change) when one mole of a compound (cid:57) is made from its elements (in their standard states) (cid:57) (Standard conditions are) 298 K and 100 kPa (cid:57) ii From energy cycle Enthalpy change to get elements = (60)(2 286) / (+) 632 (cid:57) Enthalpy change from elements = 987 + (+227) / ()760 (cid:57) Enthalpy change = 128 (cid:57) 26.0100 100.1 (cid:57) 26.0% (cid:57)<br /> Question ii 1.56104 OR 15600 OR 15601 (cid:57) Expected Answers Marks Mark Scheme June 2010 Additional Guidance ALLOW calculator value of 15600.62402 and any rounded value to a minimum of three significant figures ALLOW 1.50104 etc. ALLOW ECF from (iii)(ii) ALLOW calculator value 96.1538461 and any rounded value to a minimum of two significant figures ALLOW 96.14768284 if 15601 is used ALLOW any value between 88 to 89 if answer to (iii) was calculated by dividing by 26 ANNOTATE WITH TICKS AND CROSSES IGNORE comments about percentage yield ALLOW ECF from (i) e.g. high atom economy will have good sustainability ALLOW find a use for the waste to increase atom economy 23 F322 iii 1.5104 OR 15000 (cid:57) 96.2 (cid:57) Any two from: Low atom economy gives a poor sustainability OR low atom economy means lots of waste (cid:57) A use for the aqueous calcium hydroxide needs to be developed to increase atom economy (cid:57) Alternative process needs to be developed with high atom economy (cid:57) Total<br />$

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