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OCR A Jun 2014 Paper 2 Q3
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![3 Hydrogen iodide, HI, is a colourless gas that can be made from the reaction of hydrogen, H2, and iodine, I2. This reversible reaction is shown in equilibrium 3.1 below. H2(g) + I2(g) 2HI(g) H = 9 kJ mol1 equilibrium 3.1 The activation energy for the forward reaction is 173 kJ mol1. (a) Complete the enthalpy profile diagram below for the forward reaction in equilibrium 3.1. On your diagram:Label the activation energy, Ea Label the enthalpy change of reaction, H Include the formulae of the reactants and products. enthalpy progress of reaction [2] (b) Calculate the activation energy, Ea, for the reverse reaction. Ea (reverse reaction) =kJ mol1 [1] (c) When the reverse reaction takes place hydrogen iodide, HI, decomposes to form iodine and hydrogen. Calculate the enthalpy change when 336 dm3 of hydrogen iodide, measured at room temperature and pressure, decomposes. Include the sign for enthalpy change in your answer.OCR 2014 enthalpy changekJ [2]<br />
(d) A student mixes hydrogen and iodine at room temperature and pressure and allows the mixture to reach dynamic equilibrium. H2(g) + I2(g) 2HI(g) H = 9 kJ mol1 equilibrium 3.1 (i) A closed system is required for dynamic equilibrium to be established. State one other feature of this dynamic equilibrium.[1] (ii) The student heats the equilibrium mixture keeping the volume constant. Predict how the composition of the equilibrium mixture changes on heating. Explain your answer.[2] (iii) Predict and explain what effect, if any, an increase in the pressure would have on the position of the equilibrium. effectexplanation[1] (e) Calculate the bond enthalpy for the HI bond in equilibrium 3.1, given the following information. Bond HH II Bond Enthalpy / kJ mol1 436 151OCR 2014 bond enthalpykJ mol1 [2] [Total: 11] Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_2_QP_JUN14_question_3-0.jpg)
![F322 Question 3 (a) There are 3 marking points required for 2 marks Answer Mark Guidance Mark Scheme June 2014 ANNOTATE ANSWER WITH TICKS AND CROSSES ETC IGNORE state symbols. Ea: ALLOW (+)173 only as an alternative label for Ea ALLOW no arrowhead or arrowheads at both ends of activation energy line The Ea line must point to maximum (or near to the maximum) on the curve OR span approximately 80% of the distance between reactants and maximum regardless of position ALLOW AE or AE for Ea IF there is no H labelled ALLOW 9 as an alternative label for H. IF H is labelled IGNORE any numerical value. DO NOT ALLOW H. ALLOW this arrow even if it has a small gap at the top and bottom i.e. does not quite reach reactant or product line This is the ONLY acceptable answer Ea H2(g) + I2(g) (cid:39)(cid:43) 2HI(g) H: H2 and I2 on LHS AND 2HI on RHS AND correctly labelled Ea (cid:57) H labelled with product below reactant AND arrow downwards (cid:57) (b) (+)182 (cid:57) 11<br />
Answer Look at answer if +63 kJ AWARD 2 marks If 63 (no sign) OR-63 (incorrect sign) AWARD 1 mark No of moles of HI = 14 moles (cid:57) Enthalpy Change = +63 kJ (cid:57) (d) (i) Rate of the forward reaction is equal to the rate of the reverse F322 Question (c) (ii) reaction (cid:57) OR concentrations do not change(cid:57) More H2 and I2 OR less HI (cid:57) (equilibrium position shifts) to the left AND (Forward) reaction is exothermic OR reverse reaction is endothermic OR in the endothermic direction(cid:57) Mark Scheme June 2014 Mark Guidance ALLOW one mark for +126 kJ Sign and value required. ALLOW ECF from incorrect number of moles of HI ALLOW both reactions occur at same rate IGNORE conc. of reactants = conc. of products 2 Mark each point independently ALLOW more reactants OR less products Note: ALLOW suitable alternatives for to the left e.g. towards reactants OR towards H2 / I2 OR in reverse direction OR favours the left. ALLOW gives out heat for exothermic ALLOW takes in heat for endothermic IGNORE responses in terms of rate ALLOW same number of molecules on each side (iii) No effect AND Same number of (gaseous) moles on both sides (cid:57) 12<br />
F322 Question (e) Mark Scheme June 2014 Answer Look at answer if (+)298 AWARD 2 marks If answer is -298 AWARD 1 mark (incorrect sign) 2 x H-I bond enthalpy correctly calculated (436 +151-(-9) =) (+)596 (cid:57) H-I bond enthalpy correctly calculated (Bond energy for H-I (+)596 =) (+)298 kJ mol-1 (cid:57) 2 Mark Total 11 Guidance ALLOW 1 mark for (+)293.5 kJ mol-1 (bonds broken divided by 2) ALLOW 1 mark for (+)289 kJ mol-1 (incorrect expression i.e. [436 +151+(-9)]) 2 13<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_2_QP_JUN14_answer_3-0.jpg)
