OCR A Jun 2011 Paper 5 Q5 (with explained solutions) |

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OCR A Jun 2011 Paper 5 Q5

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5 Chemists use three energy terms, enthalpy, entropy and free energy, to help them make predictions about whether reactions may take place. 12 (a) The table below shows five processes. Each process has either an increase in entropy or a decrease in entropy. For each process, tick () the appropriate box. process increase in entropy decrease in entropy C2H5OH(g) C2H5OH(l) C2H2(g) + 2H2(g) NH4Cl (s) + aq 4Na(s) + O2(g) 2CH3OH(l) + 3O2(g) C2H6(g) NH4Cl (aq) 2Na2O(s) 2CO2(g) + 4H2O(l) [2] (b) At 1 atm (101 kPa) pressure, ice melts into water at 0 C. Complete the table below using the symbols +,or 0 to show the sign of H and S for the melting of ice at 0 C and 1 atm. For each sign, explain your reasoning. energy change sign +,or 0 reasoning H SOCR 2011 [2]<br />
(c) Much of the hydrogen required by industry is produced by reacting natural gas with steam: 13 CH4(g) + H2O(g) 3H2(g) + CO(g) Standard entropies are given in the table below. substance S o / J K1 mol1 CH4(g) 186 H2O(g) 189 H2(g) 131 CO(g) 198 (i) Calculate the standard entropy change, in J K1 mol1, for this reaction of natural gas with steam. S o =J K1 mol1 [2] (ii) State two large-scale uses for the hydrogen produced. 1.2.[1]OCR 2011 Turn over<br />
(d) Ammonium chloride, NH4Cl, can dissociate to form ammonia, NH3, and hydrogen chloride, HCl. 14 NH4Cl (s) NH3(g) + HCl (g) At 298 K, H = +176 kJ mol1 and G = +91.2 kJ mol1.Calculate G for this reaction at 1000 K.Hence show whether this reaction takes place spontaneously at 1000 K. Show all your working. G =kJ mol1 [4] [Total: 11]OCR 2011<br />
15 BLANK PAGE PLEASE DO NOT WRITE ON THIS PAGE TURN OVER FOR QUESTIONS 6, 7 AND 8OCR 2011 Turn over<br />

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Mark Scheme increase decrease Mark Guidance June 2011 (cid:57) (cid:57) (a) F325 Question Answer 5 process C2H5OH(l)C2H5OH(g) C2H2(g) + 2H2(g)C2H6(g) NH4Cl(s) + aqNH4Cl(aq) 4Na(s) + O2(g)2Na2O(s) 2CH3OH(l) + 3O2(g)2CO2(g) + 4H2O(l) All 5 correct2 marks 4 correct1 mark H: + AND bonds broken (cid:57) S: + AND more random/more disorder/more ways of arranging energy (cid:57) (b) (cid:57) (cid:57) (cid:57) (c) (i) S = (3 x 131 + 198)(186 + 189) (cid:57) S = (+)216 (J K1 mol1) (cid:57) 13 Sign and reason required for each mark ALLOW forces of attraction/hydrogen bonds are overcome DO NOT ALLOW response in terms of bonds breaking AND bond making (for melting bonds are just broken) DO NOT ALLOW responses implying that bonds within H2O molecules are broken IGNORE comments related to G IGNORE comments related to G ALLOW 1 mark for 216 (wrong sign) ALLOW 1 mark for 46 (131 instead of 3 x 131) ALLOW 1 mark for 594 (sign of 189)<br />
F325 Question Answer 5 (c) (ii) Two from points below: fuel OR fuel cells Mark Scheme June 2011 Mark Guidance 1. 2. manufacture of margarine OR hydrogenation of alkenes/unsaturated fats 3. manufacture of ammonia OR Haber process (cid:57) 4. manufacture of HCl/hydrochloric acid 5. reduction of metal ores/metal oxides FIRST, CHECK THE ANSWER ON ANSWER LINE IF answer = 109, award first 3 marks for calculation -------------------------------------------------------------------- At 298 K, 91.2 = 176TS (cid:57) S (= 17691.2 OR S (= 17600091200 subsumes 1st marking point At 1000 K, G = 1761000 x 0.285 = 109 (kJ mol1) (cid:57) Reaction does take place (spontaneously) because Note: If no value of G, this mark cannot be awarded. G < 0 OR G is -ve (cid:57) ) = 0.285 (kJ K1 mol1) ) = 285 (J K1 mol1) (cid:57) 298 298 1 4 2 uses for one mark IGNORE hydrogenation of margarine IF there is an alternative answer, check to see if there is any ECF credit possible using working below ------------------------------------------------------------- ANNOTATE WITH TICKS AND CROSSES, etc ALLOW 0.285 (3 SF) up to calculator value of 0.284563758 ALLOW 285 (3 SF) up to calculator value of 284.563758 ALLOW 109 up to calculator value correctly rounded, i.e.108.6, 108.56, etc ALLOW ECF from incorrect S, ie calculated value of G from G = 1761000 x calculated value of S Answer and reason BOTH needed for mark ALLOW reaction is feasible for reaction does take place Note: If candidate has a + G value, mark ECF, ie reaction does not take place because G > 0 OR G is +ve (d) Total 11 14<br />

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