ALS3Q - Section 3 Energetics 1 SKH Bishop Baker Secondary...

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Section 3/ Energetics / SKH Bishop Baker Secondary School AL Chemistry Section 3 Energetics 80 IB Q.6 (a) M + (g) + Cl - (g) → MCl(s) H ° 298 = - 697.8 kJ mol - 1 (M is a metal) (i) What is the term given to the enthalpy change associated with this reaction ? With reference to the equation, explain the meaning of this term. (3 marks) (ii) The experimental value of the above enthalpy change ( - 697.8 kJ mol - 1 ) is determined indirectly using a Born-Haber cycle. Draw and label the Born-Haber cycle used. (2 marks) (iii) Using the principles of electrostatics, the theoretical value of H ° 298 for M + Cl - (s) is - 697.8 kJ mol - 1 . The corresponding values for silver chloride are also known. The two sets of values are given in the following table : H ° 298 / kJ mol - 1 Compound Theoretical Experimental M + Cl - (s) - 697.8 - 697.8 Ag + Cl - (s) - 768.6 - 916.3 (I) What basic assumptions are made in determining the theoretical value ? (II) Why is there no difference between the theoretical and experimental value in the case of M + Cl - (s) and a large difference in the case of Ag + Cl - (s) ? (III) To which group in the Periodic Table do you think does metal M belong? Explain. (5 marks) (b) Two experiments were carried out to determine an approximate value for the enthalpy change ( H 3 ) for the reaction : X (s) + yH 2 O(l) → X . yH 2 O(s) reaction I The results of these experiments and some relevant data are given below : Experiment 1 When 8.0 g of an anhydrous salt X were added to 100 cm 3 of water in a thin plastic container, there was a rise in temperature of the solution by 8 o C. Experiment 2 When 13.0 g of the hydrated salt were added to 100 cm 3 of water in a thin plastic container, there was a fall in temperature of the solution by 1 o C. Molar heat capacity of water = 75.6 J K - 1 mol - 1 Molar mass of the anhydrous salt = 160 g Molar mass of the hydrated salt = 250 g. 1
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Section 3/ Energetics / (i) From experiment 1, calculate the enthalpy change ( H 1 ) for dissolving one mole of the anhydrous salt X in water. (1 mark) (ii) From experiment 2, calculate the enthalpy change ( H 2 ) for dissolving one mole of the hydrated salt X . yH 2 O in water. (1 mark) (i) Draw an energy cycle linking the equations for the reactions which take place in experiments 1 and 2 with the above reaction I . (1 mark) (iii) Calculate the enthalpy change for reaction I . (1 mark) (iv) Indicate three assumptions which are made in calculating H 1 and H 2 . (3 marks) (v) Indicate three sources of error in experiments 1 and 2. How could these errors be minimized ? (3 marks) 82 IB Q.7 (b) Determine the enthalpy of formation of magnesium carbonate at room temperature by using the results from the two simple experiments below. Experiment A
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This note was uploaded on 08/17/2011 for the course CHEM 100 taught by Professor Fleet during the Fall '09 term at CUHK.

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ALS3Q - Section 3 Energetics 1 SKH Bishop Baker Secondary...

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