OCR A Jan 2013 Paper 5 Q2 (with explained solutions) |

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OCR A Jan 2013 Paper 5 Q2

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This question looks at different aspects of neutralisation. (a) Acidbase titration pH curves can be used to help choose suitable indicators for the following titrations:25.0 cm3 of 0.100 mol dm3 HCl(aq) with 0.100 mol dm3 NaOH(aq) 25.0 cm3 of 0.100 mol dm3 CH3COOH(aq) with 0.100 mol dm3 NaOH(aq). (i) On the axes below, sketch pH acidbase titration curves to show how the pH changes when the alkali is added from a burette until the alkali is in excess. 25.0 cm3 of 0.100 mol dm3 HCl (aq) with 0.100 mol dm3 NaOH(aq) 25.0 cm3 of 0.100 mol dm3 CH3COOH(aq) with 0.100 mol dm3 NaOH(aq) 14 pH 14 pH 25 volume of alkali added / cm3 50 25 volume of alkali added / cm3 50 [4] (ii) Explain how the choice of indicator is linked to the pH curve.[1]OCR 2013<br />
(b) A student carries out an experiment to measure the enthalpy change of neutralisation. (i) Define the term enthalpy change of neutralisation.[1] (ii) The student measures out 35.0 cm3 of 2.40 mol dm3 NaOH and 35.0 cm3 of 2.40 mol dm3 HCl. The temperature of each solution is the same. The student mixes the two solutions. The temperature rises by 16.5 C. The specific heat capacity of the mixture is 4.18 J g1 K1. Assume that the density of the mixture is 1.00 g cm3. Calculate the enthalpy change of neutralisation, in kJ mol1. enthalpy change of neutralisation =kJ mol1 [3] (iii) The student repeats the experiment using 70.0 cm3 of 1.20 mol dm3 HCl instead of 35.0 cm3 of 2.40 mol dm3 HCl. Explain why the temperature rise is less, and predict what the temperature rise will be.[2]OCR 2013 [Total: 11] Turn over<br />

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F325 Mark Scheme January 2013 Question 2 (a) (i) M1 Shape Answer On one graph (can be either), shape: slight rise/flat, then vertical, then slight rise/flat (cid:57) M2 pH at start for acid Weak acid pH curve starts at higher pH and below pH 7 (cid:57) M3 End point On both graphs, vertical section approximately 25 cm3 alkali have been added (cid:57) M4 pH when alkaline On both graphs, vertical section is still vertical through a ruler line aligned with the top of the pH axis label on left-hand axis (cid:57) (a) (ii) pH range (of the indicator) matches vertical section/rapid pH change OR end point/colour change matches vertical section/rapid pH change (cid:57) (enthalpy change for) the formation of 1 mole H2O from reaction of an acid/H+ with an alkali/base/OH (cid:57) (b) (i) Marks Guidance FULL ANNOTATIONS MUST BE USED --------------------------------------------------------------------------- Use ruler tool for 4th marking point, e.g. M4 M2 M1 M3 For M4, IGNORE final pH For M1 and M2, IGNORE small gap before curve starts Note: If pH curves wrong way round (i.e. adding acid to alkali), ONLY M3 (25 cm3) can be awarded ALLOW pH range (of the indicator) matches equivalence point ALLOW end point/colour change matches equivalence point IGNORE colour change matches end point Colour change is the same as end point ALLOW (enthalpy change for) the reaction of 1 mol H+ with 1 mol of OH DO NOT ALLOW formation of 1 mol H2O from 1 mole of acid and/or 1 mole of alkali DO NOT ALLOW formation of 1 mol H2O from an acid and its conjugate base<br />
F325 Mark Scheme January 2013 (ii) FIRST, CHECK THE ANSWER ON ANSWER LINE Answer Question 2 (b) IF answer = 57.5 (kJ mol1) award 3 marks -------------------------------------------------------------------- energy change = 70.0 x 4.18 x 16.5 = 4827.9 (J) OR 4.8279 (kJ) (cid:57) amount of H2O formed = 2.4(0) (cid:117) 35.0 1000 Hneut =4.8279 0.084(0) = 0.084(0) mol (cid:57) = 57.475 OR 57.48 OR 57.5 (kJ mol1) (cid:57) Marks Guidance FULL ANNOTATIONS MUST BE USED --------------------------------------------------------------------------- IF there is an alternative answer, check to see if there is any ECF credit possible using working below ------------------------------------------------------------- IGNORE any sign shown ALLOW 4830 AND 4828 (J) ALLOW amount of HCl OR amount of NaOH (same value)sign required ALLOW ECF from calculated energy change calculated moles H2O ALLOW 3 significant figures up to calculator value correctly rounded Common errors Use of 289.5 K can give up to 2 marks by ECF: = 70.0 x 4.18 x 289.5 = 84.71 x amount of H2O formed = 2.4(0) (cid:117) 35.0 1000 Hneut =84.71 0.084(0) Use of 35 can give up to 2 marks by ECF: = 35.0 x 4.18 x 16.5 = 2413.95 (J) x amount of H2O formed = 2.4(0) (cid:117) 35.0 1000 Hneut =2.41395 0.084(0) = 28.7375 OR 28.7 (kJ mol1) (cid:57) = 1008 OR 1010 (kJ mol1) (cid:57) = 0.084(0) mol (cid:57) = 0.084(0) mol (cid:57)<br />
F325 Mark Scheme January 2013 Marks Guidance ALLOW same energy heats greater volume /mass ALLOW the following alternatives for energy: Heat, q, mcT, enthalpy change, H ALLOW use to 105 cm3/105 g as evidence of greater volume/ mass ALLOW use of same energy value as in 2(b)(ii) as evidence for same energy May need to refer to previous part, 2(b)(ii) IGNORE more energy heats a greater volume ------------------------------------------------------ ASSUME units are C unless told otherwise Question 2 (b) (iii) Same energy is spread over larger volume (cid:57) Answer --------------------------------------------- 11 C (cid:57) Total 11<br />

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