OCR A Jun 2016 Paper 2 Q3 (with explained solutions) |

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OCR A Jun 2016 Paper 2 Q3

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This question is about different ways of determining enthalpy changes. (a) A student carries out an experiment to determine directly the enthalpy change of reaction, Hr , shown below. Mg(s) + CuSO4(aq) MgSO4(aq) + Cu(s)The student measures the initial temperature of 25.0 cm3 of 0.500 mol dm3 CuSO4(aq). The student adds an excess of magnesium powder and stirs the mixture. The student measures the maximum temperature of the solution. Results = 21.5 C Initial temperature of solution Maximum temperature of solution = 63.0 C Density of the solution = 1.00 g cm3; specific heat capacity of the solution = 4.18 J g1 K1. (i) Calculate the enthalpy change of reaction, Hr , in kJ mol1. Give your answer to three significant figures. Hr =kJ mol1 [4] (ii) The student weighed out enough magnesium so that it would be in excess by at least 25%. The student had access to a two decimal-place balance. Calculate the minimum mass of magnesium that the student would need to weigh out on this balance.OCR 2016 mass =g [1] Turn over<br />
(b) Enthalpy changes of formation can be determined indirectly from standard enthalpy changes of combustion, Hc o. Three enthalpy changes of combustion are shown below. Substance Hc o / kJ mol1 C(s) H2(g) C9H20(l) 394 286 6125 (i) Define standard enthalpy change of combustion. Include the standard conditions that are used.[3] (ii) The equation that represents the enthalpy change of formation, Hf , of nonane is shown below. Calculate the standard enthalpy change of formation of nonane. 9C(s) + 10H2(g) C9H20(l) Hf =kJ mol1 [2]OCR 2016<br />
(c) The bond enthalpy for the bond in carbon monoxide can be calculated from the information below. CH4(g) + H2O(g) CO(g) + 3H2(g) H = +210 kJ mol1 Bond CH OH HH Average bond enthalpy / kJ mol1 413 464 436 (i) What is meant by the term average bond enthalpy?[2] (ii) Calculate the bond enthalpy for the bond in carbon monoxide. Show your working. bond enthalpy =kJ mol1 [3] [Total: 15]OCR 2016 Turn over<br />

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F322 Question 3 (i) (a) Mark scheme June 2016 Marks Answer IF Hr = 347 (kJ mol1) award 4 marks IF Hr = (+)347 (kJ mol1) award 3 marks (incorrect sign) -------------------------------------------------------------------------------- Moles Amount, n(CuSO4), calculated correctly = 0.0125 (mol) (cid:57) Energy q calculated correctly = 4336.75 (J) OR 4.33675 (kJ) (cid:57) Calculating (cid:39)H correctly calculates (cid:39)H in kJ mol1 to 3 or more sig figs (cid:57) Rounding and Sign calculated value of (cid:39)H rounded to 3 sig. fig. with minus sign (cid:57) Guidance ANNOTATE ANSWER WITH TICKS AND CROSSES Note: q = 25.04.1841.5 ALLOW 3 SF up to calculator value of 4336.75 J IGNORE sign IGNORE working Note: from 4336.75 J and 0.0125 mol (cid:39)H = (-)346.940 kJ mol1 IGNORE sign at this intermediate stage ALLOW ECF from n(CuSO4) and/or energy released Final answer must have correct sign and three sig figs Answer is still 347 from rounding of q to 4340 J ALLOW ECF for mass correctly rounded to 2 dp from incorrect moles of CuSO4 in 3(a)(i) (a) (ii) Minimum mass = 0.0125 (cid:117) 24.3 (cid:117) 1.25 = 0.38(0) g (cid:57) 14<br />
F322 Question 3 (i) (b) (b) (ii) Answer (enthalpy change that occurs) when one mole of a substance (cid:57) completely combusts OR reacts fully with oxygen (cid:57) 298 K / 25 C AND 1 atm / 100 kPa / 101 kPa / 105 Pa / 1 bar (cid:57) IF answer = 281 (kJ mol1), award 2 marks IF answer = (+)281 (kJ mol1), award 1 mark --------------------------------------------------------------------------------- Working for C AND H2 seen anywhere 9 (cid:117) ()394 AND 10 (cid:117) ()286 OR ()3546 AND ()2860 OR ()6406 (cid:57) Calculates Hc correctly 64066125 = 281 kJ mol1 (cid:57) (c) (i) (Average enthalpy change) when one mole of bonds (cid:57)(cid:3) (cid:3)(cid:3) of (gaseous covalent) bonds is broken (cid:57) 15 Mark scheme June 2016 Marks Guidance ALLOW energy required OR energy released ALLOW one mole of a compound OR one mole of an element ALLOW combusts in excess oxygen ALLOW burns in excess oxygen Combusts in excess air is not sufficient IGNORE reference to concentration ANNOTATE ANSWER WITH TICKS AND CROSSES IF there is an alternative answer, check to see if there is any ECF credit possible Common incorrect answers are shown below Award 1 mark for 5445 (not used9 and10) 2871 (not used9) 2293 (not used10) IGNORE energy required OR energy released DO NOT ALLOW bonds formed IGNORE heterolytic/homolytic<br />
Mark scheme June 2016 Guidance ANNOTATE ANSWER WITH TICKS AND CROSSES IGNORE sign IGNORE sign ALLOW ECF IGNORE rounding of 1062 to 1060 and credit 1062 from working Award 2 marks for 1272 (from (25801308)) 1482 (from (25801308 + 210)) F322 Question 3 (ii) (c) Marks Answer IF answer = (+)1062 (kJ mol1), award 3 marks IF answer = 1062 (kJ mol1), award 2 marks --------------------------------------------------------------------------------- ((cid:39)H for bonds broken =) 2580 (kJ mol1) OR 1652 AND 928 (kJ mol1) (cid:57) ((cid:39)H for bonds formed =) 1308 (kJ mol1) (cid:57) (bond enthalpy CO = 25801308210) = (+)1062 (kJ mol 1) (cid:57) Total 15 16<br />

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