# SUGGESTION ANSWER SCHEME CHAPTER 8: THERMOCHEMISTRY. 1 (a) Use the data in the table below to answer the following questions:

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1 SUGGESTION ANSWER SCHEME CHAPTER 8: THERMOCHEMISTRY ANSWER SCHEME UPS 2004/2005 SK027 1 (a) Use the data in the table below to answer the following questions: Enthalpy change ΔH (kj/mol) Atomization energy of iron First ionization energy of iron Second ionization energy of iron Atomization energy of chlorine Electron affinity of chlorine 364 Lattice energy of iron (II) chloride 2493 i. Define enthalpy of formation. Heat change when 1 mole of a compound is formed from its elements in their stable states at standard condition (25 C and 1 atm). Sketch the Born-Haber cycle for iron (II) chloride H f Fe (s) + Cl 2 (g) FeCl 2 (s) Fe (g) Fe + (g) 2Cl (g) Fe 2+ (g) + 2Cl - (g) (+ 121 x 2) ( 364 x 2) 2493 i Hence, calculate the enthalpy of formation for iron (II) chloride. H f = (+ 150) + (+ 736) + (+ 1450) + (+ 121 x 2) + ( 364 x 2) + ( 2493) = 643 [8 marks] 1

2 ANSWER SCHEME UPS 2005/2006 SK027 2 (a) Define the enthalpy of combustion. Heat released when 1 mole of substance/compound is completely combusted in excess oxygen at standard condition (25 C and 1 atm). [1 mark] (b) A bomb calorimeter experiment is performed with powdered xylose, C 5 H 10 O 5 as the combustible substance. The data obtained are: Mass of xylose burned Heat capacity of calorimeter Initial calorimeter temperature Final calorimeter temperature = g = kj C -1 = C = C i. Calculate the heat of combustion of xylose, in kilojoule per mole? Heat given off by xylose = heat absorbed by the calorimeter Q = CΔT = kj o C -1 ( ) o C = kj Mole of xylose = / 150 = 7.89 x 10-3 mol Heat of combustion = kj / 7.89 x 10-3 = kj/mol = x 10 3 kj/mol H c xylose = x 10 3 Write the thermochemical equation for combustion of xylose. C 5 H 10 O 5 (s) + 5 O 2 (g) 5 CO 2 (g) + 5 H 2 O (l) ΔH = x 10 3 J mol -1 [7 marks] (c) Which substance in the following pairs has larger lattice energy? Explain. KCl or RbCl KCl. The size of K + is smaller than Rb +. 2

3 ANSWER SCHEME UPS 2006/2007 SK027 3 (a) A 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 g of water and the temperature increases by C, calculate the heat of combustion of magnesium in. [5 marks] Heat released by combustion = heat absorbed + heat absorbed by calorimeter by water q = C c ΔT + m w c w ΔT = [ 1769 J C -1 x C ] + [ 300 g x 4.18 J g -1 C -1 x C ] = x 10 3 J Mole of Mg = g / 24.3 g mol -1 = x 10-3 mol x x 10 3 J 1.0 mol = H c Mg = x 10 2 (b) Based on th data given below: ΔH hydration Na + ΔH hydration Cl - ΔH solution NaCl = 390 = 380 = + 6 i. Construct an energy cycle diagram to represent the dissolution of NaCl. H LE = x Na + (g) + Cl - (g) NaCl (s) H = 390 H = 380 H = + 6 Na (aq) + Cl - (aq) Calculatethe lattice energy of NaCl. Lattice energy = H hydration Na + + H hydration Cl - + H solution NaCl = [ ( 380) + ( 6)] = 776 [5 marks] 3

4 ANSWER SCHEME UPS2007/2008 SK027 4 (a) Combustion of C 8 H 18 (l) forms water and carbon dioxide. Using the data given, calculate the amount of heat released if g of C 8 H 18 (l) is completely burnt. H 2 (g) + ½ O 2 (g) H 2 O (l) ΔH 1 = 286 kj/mol --- (1) C (s) + O 2 (g) CO 2 (g) ΔH 2 = 392 kj/mol --- (2) 8 C (s) + 9 H 2 (g) C 8 H 18 (l) ΔH 3 = 250 kj/mol --- (3) [5 marks] Eq. (1) x 9 : 9 H 2 (g) + 9/2 O 2 (g) 9 H 2 O (l) Eq. (2) x 8 : 8 C (s) + 8 O 2 (g) 8 CO 2 (g) Rev Eq. (3) : C 8 H 18 (l) 8 C (s) + 9 H 2 (g) ΔH 1 = ( 286 x 9) kj ΔH 2 = ( 392 x 8) kj ΔH 3 = kj C 8 H 18 (l) + 25/2 O 2 (g) 8 CO 2 (g) + 9 H 2 O (l) H c = 5460 ΔH c = ΔH 1 + ΔH 2 + ΔH 3 = ( 3136) + (+ 250) = 5460 kj/mol No of mole of C 8 H 18 = 100 g / 114 g mol -1 = mol The amount of heat released = 5460 kj/mol x mol = kj = x 10 3 kj (b) The Born-Haber cycle for magnesium oxide is shown below: H 7 Mg 2+ (g) + O 2- (g) MgO (s) H 3 H 6 Mg + (g) + O - (g) H 2 H 5 Mg (g) + O (g) H 8 H 1 H 4 Mg (s) + ½ O 2 (g) 4

5 i. Name the enthalpy change for ΔH 1, ΔH 3 and ΔH 7. ΔH 1 enthalpy of sublimation of magnesium ΔH 3 second ionisation energy of magnesium ΔH 7 lattice energy of magnesium oxide [3 marks] Compare the lattice energy of magnesium oxide, MgO and barium oxide, BaO. Give reason. The lattice energy of MgO is higher/more electronegative than that of BaO. The size og Mg 2+ is smaller than Ba attractive forces between Mg 2+ and O 2- are stronger than those between Ba 2+ and O 2-. ANSWER SCHEME UPS 2008/2009 SK027 5 (a) A calorimeter contains ml of water at 25 C. If ml of water at 60 C is added to it, determine the final temperature. Assume that the heat absorbed by the calorimeter is negligible. Heat released by adding water = heat absorbed by adding water m 1 C 1 T 1 = m 2 C 2 T 2 T 1 = 60 C Y T 2 = Y 25 C 600 x 4.18 x (60 C Y) = 400 x 4.18 x (Y 25 C) Y = C [4 marks] (b) The combustion of a sample of aluminium produces 0.25 mol of aluminium oxide and releases 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. The amount of heat released when 1 mole of substance is completely burnt in excess of oxygen at standard conditions (25 C and 1 atm). Calculate the standard enthalpy of combustion of aluminium mol Al 2 O kj 0.5 mol Al 2 O 3? kj H c Al = 0.5 x 419 / 0.25 =

6 i Determine the enthalpy of formation of Al 2 O 3 and write its thermochemical equation. H f Al 2 O 3 = H c Al = 2 x ( 838) = Al (s) + 3/2 O 2 (g) Al 2 O 3 (s) H = 1676 [6 marks] UPS 2009/2010 SK027 6 (a) Define standard enthalpy of combustion. Heat released when 1 mole of substance/compound is completely combusted in excess oxygen at standard condition (25 C and 1 atm). [1 marks] (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 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. C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O (l) H c = Mr C 3 H 8 = 44 g mol of C 3 H 8 releases kj mol of C 3 H 8 releases kj Heat released = mc T x 10 3 J = 8000 g x 4.18 J g -1 C -1 x T T = C [5 marks] 6

7 (c) When phosphorus is reacted with an excess of oxygen, tetraphosphorus decaoxide, P 4 O 10, is formed as shown in the following reaction: 4 P (s) + 5 O 2 (g) P 4 O 10 (s) Using the information below, calculate the heat of formation, H f, of P 4 O PCl 3 (l) + O 2 (g) 2 POCl 3 (l) P 4 O 10 (s) 6 PCl 5 (s) 10 POCl 3 (l) 2 P (s) + 3 Cl 2 (g) 2 PCl 3 (l) 2 P (s) + 5 Cl 2 (g) 2 PCl 5 (s) Eq (1) x 5 : 10 PCl 3 (l) + 5 O 2 (g) 10 POCl 3 (l) Rev. eq (2) : 10 POCl 3 (l) P 4 O 10 (s) + 6 PCl 5 (s) Eq (3) x 5 : 10 P (s) + 15 Cl 2 (g) 10 PCl 3 (l) Rev. eq (4) x 3: 6 PCl 5 (s) 6 P (s) + 15 Cl 2 (g) 4 P (s) + 5 O 2 (g) P 4 O 10 (s) H = 587 kj --- (1) H = 419 kj --- (2) H = 686 kj --- (3) H = 892 kj --- (4) H = 2935 kj H = 419 kj H = 3430 kj H = kj H f = 3270 [4 marks] ANSWER SCHEME UPS 2009/2010 TS027 7 (a) Define standard enthalpy of formation. Heat change (absorbed or released) when 1 mole of compound is formed from its elements under standard conditions (25 C and 1 atm). [1 mark] (b) Elements X and Y form an ionic compound XY. The enthalpy changes involved in the formation of this compound under standard conditions are as follows: X (s) X (g) ½ Y 2 (g) Y (g) X (g) X + (g) + e Y (g) + e Y - (g) X (s) + ½ Y 2 (g) XY (s) H = + 82 kj H = kj H = kj H = 333 kj H = 441 kj i. Write an equation that represents the lattice energy of compound XY. X + (g) + Y - (g) XY (s) 7

8 Calculate the value of the lattice energy by using energy cycle method. [6 marks] 411 kj X (s) + ½ Y 2 (g) XY (s) + 82 kj kj X (g) kj 333 kj Y (g) H LE X + (g) + Y - (g) (+ 82) + (+ 426) + (+118) + ( 333) + H LE = 411 kj H LE = 734 (c) An amount of 1.23 kj of heat is supplied to 70.0 g of water at a temperature of 28.0 C. Calculate the final temperature of the water. Q = mc T 1.23 kj = 70.0 g x 4.18 J g -1 C -1 x (T final T initial ) C 1.23 kj = 70.0 g x 4.18 J g -1 C -1 x (T final 28) C T final = 32.2 C [3 marks] ANSWER SCHEME UPS 2010/2011 SK027 8 (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 C in temperature was recorded. [Heat capacity of the plastic cup = J C -1. Assume density of solution = density of water] i. Name the enthalpy for this reaction. Enthalpy of neutralization 8

9 Calculate the standard enthalpy for this reaction. Mol NaOH = 2.0 x 20.0 / 1000 = 0.04 mol Mol CH 3 COOH = 1.0 x 30.0 / 1000 = 0.03 mol Limiting reactant is CH 3 COOH Mol H 2 O = mol CH 3 COOH = 0.03 mol Assume volume of water = volume of solution Total volume = 20.0 cm cm 3 = 50.0 cm 3 Heat released = mc T + C T = (50.0 x 4.18 x 5.8) + (37.3 x 5.8) = kj H neutralisation = / 0.03 = [6 marks] (b) i. Construct a Born-Haber cycle for the formation of magnesium fluoride, MgF 2, by using the data below: Enthalpy of atomisation of magnesium Enthalpy of atomisation of fluorine First ionisation energy of magnesium Second ionisation energy of magnesium Electron affinity of fluorine 328 Standard enthalpy of formation of magnesium fluoride Mg (s) + F 2 (g) MgF 2 (s) Mg (g) Mg + (g) 2F (g) Mg 2+ (g) + 2F - (g) (+ 159 x 2) ( 328 x 2) H LE 9

10 Based on the Born-Haber cycle, determine the lattice energy of magnesium fluoride. H LE = [148 + (159 x 2) ( 328 x 2)] = 3121 [4 marks] ANSWER SCHEME UPS 2010/2011 TK025 9 (a) Use the data in the table below to answer the following questions: Enthalpy change H (kj/mol) Atomisation energy of iron First ionisation energy of iron Second ionisation energy of iron Atomisation energy of chlorine Electron affinity of chlorine 364 Lattice energy of iron (II) chlorine 2493 (i) (ii) Define enthalpy of formation. Heat change when 1 mole of a compound is formed from its elements in their stable states at standard conditions. [1 mark] Sketch the Born-Haber cycle with appropriate enthalpy values for iron (II) chloride. [6 marks] H f Fe (s) + Cl 2 (g) FeCl 2 (s) Fe (g) Fe + (g) Fe 2+ (g) + 2Cl (g) 2Cl - (g) (+ 121 x 2) ( 364 x 2) 2493 [6 marks] 10

11 (iii) Calculate the enthalpy of formation for iron (II) chloride. H f = (121 x 2) + ( 364 x 2) + ( 2493) = [1 mark] (b) Which compound between KCl or RbCl has larger lattice energy? Explain. KCl. The size of K + is smaller than Rb +. The electrostatic attraction between K + and Cl - is stronger than that of Rb + and Cl (a) Define Hess s Law. The total enthalpy change for a reaction is the same whether it occurs in one step or in many steps. [1 mark] (b) Lattice energy of potassium chloride, KCl is + 4 and its enthalpy of hydration is 686. i. Write the thermochemical equation for the hydration process of potassium chloride. K + (g) + Cl - (g) K + (aq) + Cl - (aq) H hyd = 686 Calculate the enthalpy of solution for the salt. Method 1: K + (g) + Cl - (g) K + (aq) + Cl - (aq) H = 686 KCl (s) + K + (g) + Cl - (g) H = + 4 KCl (s) K + (aq) + Cl - (aq) H = 682 [3 marks] Method 2: 686 K + (g) + Cl - (g) K + (aq) + Cl - (aq) KCl (s) +4 H soln =? H soln = H hyd + H LE = ( ) =

12 (c) Determine the enthalpy change for the oxidation of ammonia. 4 NH 3 (g) + 5 O 2 (g) 4 NO (g) + 6 H 2 O (g) Given; N 2 (g) + 3 H 2 (g) 2 NH 3 (g) N 2 (g) + O 2 (g) 2 NO (g) 2 H 2 (g) + O 2 (g) 2 H 2 O (g) Rev. Eq (1) x 2: 4 NH 3 (g) 2 N 2 (g) + 6 H 2 (g) Eq (2) x 2 : 2 N 2 (g) + 2 O 2 (g) 4 NO (g) Eq (3) x 3 : 6 H 2 (g) + 3 O 2 (g) 6 H 2 O (g) 4 NH 3 (g) + 5 O 2 (g) 4 NO (g) + 6 H 2 O (g) H = kj --- (1) H = kj --- (2) H = kj --- (3) H = kj H = kj H = kj H = kj [4 marks] ANSWER SCHEME UPS 2011/2012 TK (a) i. What is calorimetry? Calorimetry is the measurement of a quantity of heat i [1 marks] What is the basic principle of the simple calorimeter? All the heat lost by a system is gained by its surrounding (or vice heat released = heat absorbed. [1 marks] A 1.50 g sample of an alloy is heated to 99.0 C. It is then dropped into 28.0 g water in a calorimeter. The water temperature rises from C to C. Calculate the specific heat capacity of the alloy. heat released = heat absorbed (mc T) alloy = (mc T) water 1.5 g x c x C = 28.0 g x 4.18 J g -1 C -1 x 2.25 C c = 2.29 J g -1 C -1 [3 marks] 12

13 (b) FIGURE below is the Born-Haber cycle diagram for the formation of potassium bromide, KBr crystals. K + (g) + Br (g) H 4 H 5 X H 3 K (g) + ½Br 2 (l) H 2 K (s) + ½Br 2 (l) H 1 K + (g) + KBr (s) Br - (g) (i) Name H 2 and H 5. H 2 = enthalpy of atomization of potassium H 5 = electron affinity of bromine (ii) Write the appropriate products for X. X = K + (g) + ½ Br 2 (l) [1 marks] (iii) What will happen to value of H 3 if potassium (K) metal in the above cycle is replaced by sodium (Na) metal? Explain. H 3 for Na is larger. Na is a smaller atom with the valence shell closer to the nucleus and hence bound more tightly. 12 (a) Define (i) Standard enthalpy of formation. Heat change when 1 mole of compound is formed from its elements under standard condition (25 C and 1 atm). (ii) Standard enthalpy of combustion. Heat released when 1 mole of a substance is burnt in oxygen under standard condition (25 C and 1 atm). [2 mark] 13

14 (b) Standard enthalpy of combustion of ethanol, CH 3 CH 2 OH is Write the thermochemical equation to represent the statement. State whether the reaction is exothermic or endothermic. CH 3 CH 2 OH (l) + 3 O 2 (g) 2 CO 2 (g) + 3 H 2 O (g) H = 1386 Exothemic reaction. (c) Standard enthalpy of combustion for propane, CH 3 CH 2 CH 3 is (i) How much energy is released when 1.0 g of propene undergoes complete combustion? Mr propane, C 3 H 8 = 44.0 g mol -1 (ii) H = 1.0 x 2220 / 44.0 = kj How much energy is released when 33.6 dm 3 of propane at STP is completely burnt? At STP: 22.4 dm 3 is equivalent to 1 mol 33.6 dm 3 equal to 1.5 mol H = 1.5 x 2220 = 3330 kj (iii) How many grams of propane must be completely burnt in order to produce 100 kj of heat? 2220 kj 1 mol 100 kj (100/200) x 1.0 mol = mol Mass of propane = x 44 = 1.98 g ANSWER SCHEME UPS 2011/2012 SK (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. C 4 H 10 (g) + 13/2 O 2 (g) 4 CO 2 (g) + 5 H 2 O (l) H =

15 Calculate the mass of butane that must be burnt in order to increase 1.56 C of 1000 g water in a bomb calorimeter with a heat capacity of 958 J C -1. Q rxn = C c T + m w c w T = 958 J C (1.56 C) + (1000 g)(4.18 Jg -1 C -1 )(1.56 C) = J = 8.02 kj [5 marks] (b) i. 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 ( ) Lattice energy of LiCl H = 846 Dissolution of LiCl H = 77 Hydration of Li + H = 510 Hydration of Cl - H = 413 Li + (g) + Cl - (g) kj 413 kj LiCl (s) 77 kj kj mol - 1 Li + (aq) + Cl - (aq) Which of these salts, LiCl or KCl, would have larger lattice energy? Explain. LiCl because Li + has smaller ionic radius than K + [5 marks] ANSWER SCHEME UPS 2012/2013 TK (a) i. Define the standard enthalpy of combustion of ethanol, C 2 H 5 OH. The heat released when 1 mol of ethanol is burned in excess oxygen under standard condition. 15

16 Using the following standard enthalpies of formation, calculate the standard enthalpy of combustion for ethanol. Compound H f ( ) C 2 H 5 OH H 2 O (l) CO 2 (g) C 2 H 5 OH (l) + 3O 2 (g) 2CO 2 (g) + 3 H 2 O (l) C (s) + O 2 (g) CO 2 (g) H 2 (g) + ½ O 2 (g) H 2 O (l) 2 C (s) + 3 H 2 (g) + ½ O 2 (g) C 2 H 5 OH (l) Eq (1) x 2: 2 C (s) + 2 O 2 (g) 2 CO 2 (g) Eq (2) x 3: 3 H 2 (g) + 3/2 O 2 (g) 3 H 2 O (l) Reverse : C 2 H 5 OH (l) 2 C (s) + 3 H 2 (g) + ½ O 2 (g) C 2 H 5 OH (l) + 3O 2 (g) 2CO 2 (g) + 3 H 2 O (l) H f = x H = (1) H = (2) H = (3) H = kj H = kj H = kj H f = i Write a thermochemical equation for the combustion of ethanol. C 2 H 5 OH (l) + 3O 2 (g) 2CO 2 (g) + 3 H 2 O (l) H f = [6 marks] (b) The following data is obtained from an experiment using ethanol as fuel to heat a sample of water. Mass of spirit lamp (g) Mass of spirit lamp + ethanol Volume of water heated (cm 3 ) By using the enthalpy of combustion of ethanol obtained in (a) (ii), calculate the increase in temperature of the water. [4 marks] 16

17 = C 15 (a) State Hess s Law. [1 mark] Hess s Law states that when reactants are converted to products, the overall change in enthalpy is the same whether the reaction takes place in a single step or in a series of steps. (b) Phosphorus pentachloride, PCl 5 is prepared from PCl 3 and Cl 2 : PCl 3 (l) + Cl 2 (g) PCl 5 (s) (1) Use equation (2) and (3) to calculate Hrxn of equation (1): P 4 (s) + 6 Cl 2 (g) 4 PCl 3 (l) H = 1280 kj (2) P 4 (s) + 10 Cl 2 (g) 4 PCl 5 (l) H = 1774 kj (3) [4 marks] Rev Eq (2) x ¼ : PCl 3 (l) ¼ P 4 (s) + 3/2 Cl 2 (g) H = kj Eq (3) x ¼ : ¼ P 4 (s) + 5/2 Cl 2 (g) PCl 5 (l) H = kj PCl 3 (l) + Cl 2 (g) PCl 5 (s) H = kj (c) The Born-Haber cycle for calcium fluoride is shown below: Ca 2+ (g) + 2 F - (g) CaF 2 (s) H 3 Ca + (g) H 5 H 2 Ca (g) + 2 F (g) H 7 H 1 H 4 Ca (s) + F 2 (g) i. Name the enthalpy change for H 1, H 3 and H 5. H 1 = enthalpy of atomization of calcium H 3 = second ionization energy of calcium H 5 = electron affinity of fluorine 17

18 Compare the lattice energy of calcium chloride and calcium fluoride. Give your reason. Lattice energy of CaF 2 is larger than lattice energy of CaCl 2 because the ionic radius of F - is smaller than Cl -, so interionic distance of CaF 2 is smaller. [5 marks] 18

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