OCR A Jun 2017 Paper 1 Q18

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18 This question is about free energy changes, G, enthalpy changes, H, and temperature, T. 12 (a) The Gibbs equation is shown below. G = HTS A chemist investigates a reaction to determine how G varies with T. The results are shown in Fig. 18.1. G / kJ mol1 0,0 T/K Fig. 18.1 What is significant about the gradient of the line and the values P and Q shown in Fig. 18.1? Explain your reasoning.[4]<br /> (b) Iron can be extracted from its ore Fe3O4 using carbon. Several equilibria are involved including equilibrium 18.1, shown below. 13 equilibrium 18.1 Fe3O4(s) + 4C(s) 3Fe(s) + 4CO(g) H = +676.4 kJ mol1 S = +703.1 J K1 mol1 (i) Why is equilibrium 18.1 a heterogeneous equilibrium?[1] (ii) Write the expression for Kp for equilibrium 18.1. (iii) The forward reaction in equilibrium 18.1 is only feasible at high temperatures.Show that the forward reaction is not feasible at 25 C.Calculate the minimum temperature, in K, for the forward reaction to be feasible. [1] minimum temperature =K [3]<br /> (iv) Another equilibrium involved in the extraction of iron from Fe3O4 is shown below. H = 13.5 kJ mol1 Fe3O4(s) + 4CO(g) 3Fe(s) + 4CO2(g) 14 Enthalpy changes of formation, f H, for Fe3O4(s) and CO2(g) are shown in the table. Compound Fe3O4(s) CO2(g) f H / kJ mol1 1118.5 393.5 Calculate the enthalpy change of formation, f H, for CO(g). f H, for CO(g) =kJ mol1 [3]<br /> 15 BLANK PAGE PLEASE DO NOT WRITE ON THIS PAGE<br />

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