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1 | A d d i t i o n a l Q u e s t i o n s C h a p t e r 9 T h e r m o c h e m i s t r y S K 0 2 6 1 a) i. Define standard enthalpy of formation, H° f. ii. Calculate the standard enthalpy of formation of ethanol if, ∆Hºc [C (s)] = 393 kJ mol 1 ∆Hºc
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1 | Additional Questions Chapter 9 Thermochemistry SK026   1 a) i. Define standard enthalpy of formation,  H° f  . ii. Calculate the standard enthalpy of formation of ethanol if, ∆ Hºc [C (s)] =  393 kJ mol  1   ∆ Hºc [H 2 (g)] =  293 kJ mol  1   ∆ Hºc [C 2 H 5 OH (l)] =  1376 kJ mol  1 [5 marks] b) Construct a Born-Haber cycle for magnesium chloride compound and determine the second ionization energy of magnesium by using the data given below: Enthalpy of sublimation = +149 kJ mol  1  First ionization energy of magnesium = +740 kJ mol  1  Enthalpy of atomization of chlorine = +121 kJ mol  1  Electron affinity of chlorine =  364 kJ mol  1  Lattice energy of magnesium chloride =  2773 kJ mol  1  Enthalpy of formation of magnesium chloride =  870 kJ mol  1 [5 marks] 2 a) Define the enthalpy of combustion. [1 mark] b) In a calorimetry study, powdered xylose, C 5 H 10 O 5  combusted in a bomb calorimeter. The following data was obtained: mass of xylose burned = 1.183 g heat capacity of calorimeter = 4.728 kJ  C  1  initial calorimeter temperature = 23.29  C final calorimeter temperature = 27.19  C i. Calculate the heat of combustion of xylose in kilojoule per mole. ii. Write the thermochemical equation for the combustion of xylose. [7 marks] c) Which substance in the following pair has a larger lattice energy? Explain. KCl or RbCl [2 marks] 3 a) A 0.1375 g of magnesium is burned in a constant volume bomb calorimeter that has a heat capacity of 1769 J  C  1 . If the calorimeter contains 300 g of water and the temperature increases by 1.126  C, calculate the heat of combustion of magnesium in kJ mol  1 . [5 marks] b) Based on the data given below: ∆ H  hydration  Na +  =  390 kJ mol  1   ∆ H  hydration  Cl   =  380 kJ mol  1   ∆ H  solution  NaCl = +6 kJ mol  1  i. Construct an energy cycle diagram to show the dissolution of solid NaCl. ii. Calculate the lattice energy of NaCl. [5 marks]  2 | Additional Questions Chapter 9 Thermochemistry SK026   4 a) Combustion of C 8 H 8  ( l ) forms water and carbon dioxide. Using the data given, calculate the amount of heat released if 100 g of C 8 H 8  ( l ) is completely burnt. H 2  (g) + ½O 2 (g) → H 2 O ( l )  H 1  = -286 kJ / mol C (s) + O 2 (g) → CO 2  ( g )  H 2  = -392 kJ / mol 8C(s) + 9H 2 (g) → C 8 H 18 ( l )  H 3  = -250 kJ / mol [5 marks] The Born-Haber Cycle for magnesium oxide is shown below : i. Name the enthalpy change for  H 1 ,  H 3 and    H 7 [3 marks] ii. Compare the lattice energy of magnesium oxide, MgO and barium oxide, BaO. Give a reason. [2 marks] 5 a) A calorimeter contains 400 mL of water at 25 o C. If 600 mL of water at 60 o C is added to it, determine the final temperature. Assume that the heat absorbed by the calorimeter is negligible. [4 marks] b) The combustion of a sample of aluminium produces 0.25 mol of aluminium oxide and releases 419 kJ of heat at standard conditions. Al(s) + ¾ O 2  (g) ½ Al 2 O 3  (s) i. Define the standard enthalpy of combustion. ii. Calculate the standard enthalpy of combustion of aluminium. iii. Determine the enthalpy of formation of Al 2 O 3  and write its thermochemical equation. [6 marks]  H 7  Mg 2+ (g) + O 2- (g) MgO(s) Mg + (g) + O - (g) Mg (g) + O (g) Mg (s) + ½O 2  (g)  H 3    H 6    H 2    H 1    H 5    H 4    H 8    3 | Additional Questions Chapter 9 Thermochemistry SK026   6 a) Define standard enthalpy of combustion. [1 mark] b) Propane, C 3 H 8 , is a hydrocarbon gas that is commonly used as a fuel for cooking. The standard enthalpy of combustion is -220.1 kJ mol -1 . Write a balanced equation for the complete combustion of propane gas. Assuming all of the heat evolved in burning 10.0 g of propane is transferred to 8.0 kg of water; calculate the increase in temperature of the water. [5 marks] c) When phosphorus is reacted with an excess of oxygen, tetraphosphorus decaoxide, P 4 O 10 , is formed as shown in the following reaction: 4P (s) + 5O 2  (g) P 4 O 10  (s) Using the information below, calculate the heat of formation, ∆H Øf   , of P 4 O 10 . 2PCl 3  (l) + O 2  (g) 2POCl 3  (l) ∆ H o  = −  587 kJ P 4 O 10  (s) + 6PCl 5  (s) 10POCl 3  (l) ∆ H o  = −  419 kJ 2P (s) + 3Cl 2  (g) 2PCl 3  (l) ∆ H o  = −  686 kJ 2P (s) + 5Cl 2  (g) 6PCl 5  (s) ∆ H o  = −  892 kJ [4 marks] 7 a) A matriculation student carried out an experiment involving a reaction between an acid and a base as shown in the following reaction: CH 3 COOH(aq) + NaOH(aq) CH 3 COONa(aq) + H 2 O(l) In this experiment, 20.0 cm 3  of 2.0 M of NaOH solution was added to 30.0 cm 3  of 1.0 M of CH 3 COOH solution in a plastic cup. A maximum rise of 5.8 o C in temperature was recorded. [Heat capacity of the plastic cup = 37.30 J o C -1 . Assume density of solution = density of water] i) Name the enthalpy for this reaction. ii) Calculate the standard enthalpy for this reaction. [6 marks] b) i) Construct a Born-Haber cycle for the formation of magnesium fluoride, MgF 2 , by using the data below: Enthalpy of atomization of magnesium + 148 kJ mol -  Enthalpy of atomization of fluorine + 159 kJ mol -  First ionisation energy of magnesium + 738 kJ mol -  Second ionisation energy of magnesium +1450 kJ mol -  Electron affinity of fluorine −  328 kJ mol -  Standard enthalpy of formation of magnesium fluoride −  1123 kJ mol -  ii) Based on the Born-Haber cycle, determine the lattice energy of magnesium fluoride. [4 marks]  4 | Additional Questions Chapter 9 Thermochemistry SK026   8 (a) Combustion of 1 mole of butane gas, C 4 H 10  involves 2874 kJ of heat. (i)   Write a thermochemical equation for the combustion of butane. (ii)   Calculate the mass of butane that must be burn in order to increase 1.56°C of 1000 g water in a bomb calorimeter with heat capacity of 958 J°C -1 . [5 marks]  (b) Based on the data given in TABLE 1 , construct an energy cycle diagram for dissolution of lithium chloride , LiCl. TABLE 1 Process Enthalpy of reaction (kJ mol -1 ) Lattice energy of LiCl ∆ H = -846 Dissolution of LiCl ∆ H = -77 Hydration of Li +   ∆ H = -510 Hydration of Cl -   ∆ H = -413 9 (a) In an experiment, 2.00 g of benzene, C6H6, were burned in a bomb calorimeter. A temperature change of 9.62 o C was recorded. If the heat of combustion of benzene is  –  3268 kJ mol -1 , calculate the heat capacity of the bomb calorimeter. [4 marks ] (b) The Born-Haber cycle for the formation of calcium chloride, CaCl 2 , is shown in FIGURE 1. Enthalpy Ca 2+ (g) + 2Cl(g) +242 kJ − 708 kJ Ca 2+ (g) + 2Cl - (g) Ca 2+ (g) + Cl 2 (g) +1730 kJ − 2249 kJ +190 kJ Ca(s) + Cl 2 (g) ∆ H formation  CaCl 2 (s) FIGURE 1  5 | Additional Questions Chapter 9 Thermochemistry SK026   (i) Complete the empty box  in FIGURE 1 and calculate the enthalpy of formation of calcium chloride.   (ii) The enthalpy of solution for calcium chloride crystal is  –  81.3 kJmol -1 . Based on the data from the above Born-Haber cycle, calculate the enthalpy change for the reaction below:   Ca 2+ (g) + 2Cl - (g) Ca 2+ (aq) + 2Cl - (aq) [6 marks ] 10 (a) A chemical engineer studying the properties of hydrocarbon placed 10.0 g of propane, C 3 H 8 , in a bomb calorimeter. The temperature of the bomb calorimeter rose from 20.0 o C to 36.2 o C. If the heat capacity of the bomb calorimeter is 31.4 kJ o C -1 , (i)   determine the heat of combustion of propane. (ii)   write the thermochemical equation for combustion of propane. [5 marks] (b) (i) Define lattice energy. (ii) Based on the data in TABLE 2 , construct the Born-Haber cycle for lithium chloride and calculate its lattice energy. TABLE 2 Enthalpy ∆ H (kJ mol -  Li(s) Li(g) +159 Li(g) Li(g) + e -  +520 ½Cl 2 (g) Cl(g) +121 Cl(g) + e -  Cl - (g) − 364 Li(s) + ½Cl 2 (g) LiCl(s) − 409 [5 marks] 11 (a) (i) Define enthalpy of formation. (ii) Given CaCO 3 (s) = −1206.9 kJ mol -1 , write a thermochemical equation for the formation of CaCO 3 . [2 marks] (b) Two pollutants gaseous formed in an auto exhaust are CO and NO. These gases can be converted into less harmful gases through the following reaction: CO(g) + NO(g) CO 2 (g) + ½ N 2 (g) Based on the following information, determine ∆H for the above reaction. CO(g) + ½ O 2 (g) CO 2 (g) ∆H = −283.0 kJ  N 2 (g) + O 2 (g) 2NO(g) ∆ H = +180.6 kJ [3 marks] (c) Given that: 2CH 3 OH(l) + 3O 2 (g) 2CO 2 (g) + 4H 2 O(l) ∆H = −1452 kJ  A 2.00 g sample of methanol, CH 3 OH, is burned in a constant volume bomb calorimeter that has a heat capacity of 1340 J o C -1 . If the calorimeter contains 500 g of water and the initial temperature of the calorimeter is 25 o C, calculate the final temperature of the calorimeter. [5 marks] of  H 

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