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OCR A Jan 2012 Paper 5 Q7
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![7 Dinitrogen tetroxide, N2O4(g), and nitrogen dioxide, NO2(g), coexist in the following equilibrium. 18 2NO2(g) N2O4(g) H = 57 kJ mol1 A chemist adds 4.00 mol NO2 to a container with a volume of 2.00 dm3. The container is sealed, heated to a constant temperature and allowed to reach equilibrium. The equilibrium mixture contains 3.20 mol NO2. (a) Calculate the value for Kc under these conditions. [5]OCR 2012<br />
(b) The experiment is repeated but the pressure in the container is doubled. 19 Explain in terms of Kc the effect on the concentrations of NO2 and N2O4 when the mixture has reached equilibrium.[3] [Total: 8]OCR 2012 Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_5_QP_JAN12_question_7-0.jpg)
![F325 Mark Scheme January 2012 Question Expected answers 7 a FIRST, CHECK THE ANSWER ON ANSWER LINE IF numerical value = 7.81102 OR 0.0781 AND [N2O4] = 0.2(00 mol dm3 AND [NO2] =1.6(0), award 4 calculation marks and check for the mark for correct units (cid:57) -------------------------------------------------------------------- Equilibrium amount of N2O4 0.400 mol N2O4 Equilibrium concentrations [N2O4] = 0.200 mol dm3 AND [NO2] = 1.60 mol dm3 (cid:57) Kc expression Kc = [N2O4 ] [NO2]2 (Square brackets essential) OR 0.200 1.602 (cid:57) Calculation = 7.81102 (cid:57) Units dm3 mol1 (cid:57) Marks Additional guidance IF there is an alternative answer, check to see if there is any ECF credit possible using working below ----------------------------------------------------------- ANNOTATIONS MUST BE USED -------------------------------------------------------------- ALLOW ECF for equilibrium amounts2 ALLOW 3 SF up to calculator value of 0.078125 correctly rounded ALLOW ECF using calculated equilibrium concentrations For units, ALLOW mol1 dm3 ALLOW ECF from incorrect Kc expression Common errors for 4 calculation marksRemember there is another mark for units 7.81 x 102 from wrong concs 0.03906 0.01953 0.3125 12.8 0.125 0.15625 MARK BY ECF as there are many different routes to this answer (cid:57) (cid:57)(cid:57) + units (cid:57)(cid:57)(cid:57) + units (cid:57)(cid:57)(cid:57) + units: mol dm3 Kc expression upside down (cid:57)(cid:57)(cid:57) + units; none (cid:57)(cid:57) + units (cid:57)(cid:57)(cid:57) + units look for [N2O4] = 0.8 AND [NO2] = 3.2 no conversion of both moles to concentration no conversion of NO2 moles to concentration moles of N2O4 taken as 3.2/2 [NO2] instead of [NO2]2 No units MUST be stated 21<br />
F325 Mark Scheme January 2012 Marks Additional guidance ALLOW Kc only changes with temperature IGNORE Kc changes with temperature ALLOW [N2O4] [NO2]2 < Kc OR [N2O4] [NO2]2 decreases IGNORE Kc decreases ALLOW top of Kc expression increases and bottom decreases until Kc is reached ALLOW equilibrium shifts to right to maintain/restore Kc IGNORE just restores equilibrium Kc IS REQUIRED IGNORE just equilibrium shifts to right IGNORE le Chatelier response: equilibrium shifts to right because there are fewer moles of gas on right-hand side Question Expected answers 7 b Each marking point is independent Effect on Kc Kc does not change (with pressure) (cid:57) Comparison of conc terms after increase in pressure [NO2]2 increases more than [N2O4] OR concentration (term) on bottom (of Kc) increases more that concentration (term) on top (of Kc) (cid:57) Changes in concentrations linked to Kc (amount /concentration of) N2O4 increases AND (amount /concentration of) NO2 decreases AND to maintain/restore Kc (cid:57) Total 22<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_5_QP_JAN12_answer_7-0.jpg)
