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OCR A Jun 2009 Paper 2 Q2
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![2 Enthalpy changes of combustion, Hc, are amongst the easiest enthalpy changes to determine directly. (a) Define the term enthalpy change of combustion.[2] (b) A student carried out an experiment to determine the enthalpy change of combustion of pentan-1-ol, CH3(CH2)4OH. In the experiment, 1.76 g of pentan-1-ol was burnt. The energy was used to heat 250 cm3 of water from 24.0 C to 78.0 C. (i) Calculate the energy released, in kJ, during combustion of 1.76 g pentan-1-ol. The specific heat capacity of water = 4.18 J g1 K1. Density of water = 1.00 g cm3. (ii) Calculate the amount, in moles, of pentan-1-ol that was burnt. energy =kJ [1] amount =mol [2] (iii) Calculate the enthalpy change of combustion of pentan-1-ol. Give your answer to three significant figures.OCR 2009 Hc =kJ mol1 [3]<br />
(c) The standard enthalpy change of formation of hexane can be defined as: The enthalpy change when 1 mol of hexane is formed from its constituent elements in their standard states under standard conditions. Hexane melts at 95 C and boils at 69 C. (i) What are standard conditions?[1] (ii) An incomplete equation is shown below for the chemical change that takes place to produce the standard enthalpy change of formation of hexane. Add state symbols to the equation to show each species in its standard state. 6C() + 7H2(.) C6H14(.) [1] (iii) It is very difficult to determine the standard enthalpy change of formation of hexane directly. Suggest a reason why.[1] (iv) The standard enthalpy change of formation of hexane can be determined indirectly. Calculate the standard enthalpy change of formation of hexane using the standard enthalpy changes of combustion below. substance Hc / kJ mol1 H2 C6H14 394 286 4163OCR 2009 answer =kJ mol1 [3] [Total: 14] Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_2_QP_JUN09_question_2-0.jpg)
![F322 Question 2 (a) (i) The enthalpy change for the complete combustion (cid:57) Expected Answers Mark Scheme June 2009 of 1 mol (of a substance) (cid:57) (i) 56.430 (kJ) (cid:57) (ii) Mr [CH3(CH2)4OH] = 88.0 (cid:57) n = 0.0200 mol (cid:57) ()2821.5 (cid:57) = ()2820 (3 SF) (cid:57) correct minus sign (cid:57) (b) (iii) (c) (i) pressure: 100 kPa OR 101 kPa AND temperature: 298 K OR 25 C (cid:57) (ii) 6C(s) + 7H2(g) (cid:198) C6H14(l) (cid:57) (iii) many different hydrocarbons would form OR activation energy too high OR reaction too slow OR they don't react together (cid:57) Marks Additional Guidance ALLOW energy change for combustion in excess oxygen OR energy released during complete combustion OR energy change for combustion in excess air NOT energy required This mark is not stand alone but must relate to statement about an enthalpy change even if the statement was not awarded a mark ALLOW 56.43 (kJ) OR 56.4 kJ (cid:57) OR 56 kJ ALLOW -56.43 i.e. ignore sign ALLOW 88 ALLOW 0.02 OR ecf from wrong Mr ALLOW full marks for 0.02 with no working out ALLOW correct substitution into formula(b)(i)(b)(ii) e.g. 56.40.02 this is essentially a mark for the working ALLOW ecf from i.e. answer from (b)(i)(b)(ii) The minus mark is stand alone and is independent of the numerical answer units needed ALLOW 1 bar OR 1 atm OR 760 mmHg ALLOW any stated temperature so for example 100kPa and 40oC would be credited with a mark IGNORE any reference to moles or concentration ALLOW graphite / gr ALLOW can form different isomers OR can form different structures IGNORE reaction may be reversible 12<br />
F322 Question (iv) 6394 + 7286 shown OR calculated as 4366 (cid:57) Expected Answers 4366 and 4163 added OR subtracted (cid:57) correct answer 4366(4163) = 203 (cid:57) Mark Scheme June 2009 Marks Additional Guidance ALLOW THREE marks for 203 on its own with no working out or written on the answer line ALLOW TWO marks for +203,+3483, +1513, +1767 or -8529 on its own with no working out ALLOW ONE mark for or 3483, 1513, 1767 or +8529 on its own with no working out units NOT needed Positive sign not needed for endothermic answers Total 14 13<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_2_QP_JUN09_answer_2-0.jpg)
