Skip to content
OCR A AS Jun 2016 Paper 1 Q25
Answers available below
Show answer
Question
21
22
23
24
25
26
![20 25 Sulfur trioxide, SO3, is used for the industrial manufacture of sulfuric acid. SO3 is produced by reacting sulfur dioxide, SO2, and oxygen, O2, as shown in equilibrium 25.1 below. Equilibrium 25.1 2SO2(g) + O2(g) 2SO3(g) H = 197 kJ mol1 (a) Le Chateliers principle can be used to predict how different conditions affect the equilibrium position. (cid:129) (cid:129) Using le Chateliers principle, show that a low temperature and a high pressure should be used to obtain a maximum equilibrium yield of SO3. Explain why the actual conditions used in industry may be different from the conditions needed for a maximum equilibrium yield.[5]OCR 2016<br />
(b) Under certain conditions, Kc for equilibrium 25.1 is 0.160 dm3 mol1. 21 The equilibrium mixture under these conditions has the following concentrations of SO2 and O2. Species Equilibrium concentration / mol dm3 SO2 O2 2.00 1.20 (cid:129) Using the value of Kc, explain whether the equilibrium position will be towards the right or towards the left under these conditions. (cid:129) Calculate the concentration of SO3 in the equilibrium mixture.[4]OCR 2016 Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/June_2016_QP_-_Paper_1_OCR_Chemistry_A_AS-Level_25-0.jpg)
![H032/01 Mark Scheme June 2016 Question (a) 25 Answer EQUILIBRIUM CONDITIONS 4 marking points3 max3 MAX Mark first three CORRECT responses seen Temperature: (Forward) reaction is exothermic/H is negative OR (Forward) reaction gives out heatPressure: Right-hand side has fewer (gaseous) moles OR 3 (gaseous) moles form 2 (gaseous) molesEquilibrium shift Correct equilibrium shift in terms of temperatureCorrect equilibrium shift in terms of pressure----------------------------------------------------------------------- INDUSTRIAL CONDITIONS Low temperature gives a slow rate/slower reaction OR high temperatures needed to increase rate(High) pressure provides a safety risk OR (High) pressure is expensive (to generate) /uses a lot of energyMarks 15 AO element AO3.1 2 AO3.2 1 ----------- AO1.2 2 Guidance FULL ANNOTATIONS MUST BE USED ------------------------------------- ALLOW suitable alternatives for towards right, e.g.: towards SO3/products OR in forward direction OR favours the right ALLOW reverse reaction is endothermic /H is positive/takes in heat For moles, ALLOW molecules/particles ORA for reverse reaction IGNORE responses in terms of activation energy ALLOW high pressure is dangerous/explosive ALLOW These conditions are expensive Statement subsumes pressure as these will apply to pressure (required for this mark) and temperature ALLOW ORA e.g. Lower pressureless danger/uses less energy IGNORE Its expensive Link with pressure required<br />
H032/01 Question (b) Mark Scheme June 2016 AO element AO3.2 AO1.2 AO2.6 AO2.6 Marks Answer 1 mark Value of Kc Kc is small OR Kc < 1 AND equilibrium (position) is towards leftCalculation: FIRST CHECK ANSWER IF [SO3] = 0.876 OR 0.88 (mol dm3) award all 3 marks available for calculation ------------------------------------------------------------------------ Kc expression [SO3]2 1 mark 2.0021.20[SO3]2 [SO2]2[O2] OR Evaluation of Kc [SO2]2[O2] 1 mark Kc [SO2]2[O2] = 0.1602.0021.20 = 0.768Calculation of [SO3] ONLY available from correct evaluation for 2nd mark [SO3] = (0.1602.0021.20) = 0.876 (mol dm3)Total 16 Guidance FULL ANNOTATIONS MUST BE USED -------------------------------------- ALLOW suitable alternatives for towards left, e.g.: towards SO2/O2 OR towards reactants OR in reverse direction OR favours the left Square brackets required in Kc expression ALLOW ECF from ALLOW 0.77 (2 SF) ALLOW 0.88 (2 SF) up to calculator value of 0.876356092 correctly rounded IF Kc expression is inverted 2nd and 3rd marks are available by ECF: [SO2]2[O2] , i.e. no [SO3]2 [SO3] [SO3]2 = 2.0021.20 [SO3] = 30 = 5.48 OR 5.50.160 OR 30Any other Kc expressionNO MARKS, e.g. + [O2]0.8320.912 NO marks [SO3]2 [SO2]2<br />](https://learnah.org/wp-content/uploads/2018/08/June_2016_MS_-_Paper_1_OCR_Chemistry_A_AS-Level_25-0.jpg)
