Chapter

CHAPTER 9: THEMOCHEMISTRY 1. State Hess Law [2 marks] Determine the enthalpy change, ΔHf for the reaction C(graphite) + 2H2(g)  CH4(g) Given: C(graphite) + O2(g)  CO2(g) 2H2(g) + O2(g)  2H2O(l) CH4(g) + 2O2(g)  2H2O(l) + CO2(g) ΔH1 = - 393.5 kJ ΔH2 = - 571.6 kJ ΔH3 = - 890.4 kJ [3 marks] 2. Draw the Born-Haber cycle for the formation of magnesium chloride, MgCl2 from magnesium metal and chlorine gas. Calculate the enthalpy of formation of MgCl2. Given: Heat of sublimation of magnesium, ΔH1 = 149 kJ mol-1 First ionization energy of magnesium, ΔH2 = 740 kJ mol-1 Second ionization energy of magnesium, ΔH3 = 1456 kJ mol-1 Heat of atomization of chlorine, ΔH4 = 240 kJ mol-1 Electron affinity of chlorine, ΔH5 = -369 kJ mol-1 Lattice energy of MgCl2, ΔH6 = -3933 kJ mol-1 [10 marks] 3. FIGURE 1 shows an energy cycle for the dissolution of halogen, X2. X represents bromine or iodine. H1 1/2 X2 (standard X -(aq) state) H2 X(g) H4 H3 X -(g) FIGURE 1 a) i. ii. What is the phase for the bromine and iodine at their standard states? Name the enthalpy change for ΔH3 and ΔH4. [5 marks] b) TABLE 1 shows values of ΔH2, ΔH3 and ΔH4 of the above processes for bromine and iodine. TABLE 1 Bromine (kJ mol-1) Iodine (kJ mol-1) ΔH2 112 107 ΔH3 -342 -295 ΔH4 -351 -307 i. ii. Calculate ΔH1 values for bromine and iodine. What can be deduced from b (i)? [5 marks] 1 CHAPTER 9: THEMOCHEMISTRY 4. a) Define standard enthalphy of formation, ΔH of . The combustion of 4.6 g of ethanol has evolved 136.7 kJ heat at 25oC and 1 atm. Calculate the standard enthalphy of formation of ethanol. [ΔH of CO2(g) = -393.5 kJ mol-1 and ΔH of H2O(l) = -285.8 kJ mol-1] [9 marks] b) The following equations show the combustion reactions of ethyne and propane. 2C2H2(g) + 5O2(g)  4CO2(g) + 2H2O(l) C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) ΔHo = -2598.8 kJ ΔHo = -2220.0 kJ A mixture of 5 mol gases consisting of ethyne and propane is burnt and 7647.8 kJ heat is evolved. What is the mole ratio of ethyne to propane in the mixture? [5 marks] 5. a) i. ii. Define standard enthalphy of formation. Calculate the heat of combustion of methane using standard heats of formation below: ΔHf (CO2(g)) = -394 kJ mol-1 ΔHf (H2O(l)) = -286 kJ mol-1 ΔHf (CH4(g)) = -75 kJ mol-1 [5 marks] b) i. Name the enthalpies ΔH2, ΔH4 and ΔH5 in the following Born Haber cycle for sodium chloride, NaCl. Calculate the lattice energy, ΔH6 for NaCl. ii. + Na (g) + Cl(g) H5 = -348.8 kJ H4 = 495.4 kJ Na(g) + Cl(g) + - Na (g) + Cl (g) H3 = 121.3 kJ Na(g) + 1/2 Cl2(g) H6 H2 = 107.8 kJ Na(s) + 1/2 Cl2(g) H1 = -411.3 kJ NaCl(s) [5 marks] 2 CHAPTER 9: THEMOCHEMISTRY 6. a) The enthalpies of formation of Fe3O4(s) and H2O(g) are -1120 kJmol-1 and -242 kJmol-1 respectively. i. ii. iii. 7. Write the termochemical equations for the formation of both Fe3O4(s) and H2O(g). Calculate the enthalpy of the following reaction: 3Fe(s) + 4H2O(g)  Fe3O4(s) + 4H2(g) Which of the reaction involved in (i) and (ii) is most easily be made reversible? Give your reason. [6 marks] b) Calculate the heat released when 0.75 g of nitrogen dioxide (NO2), is produced in the thermochemical reaction below. 2NO(g) + O2(g)  2NO2(g) ΔHo = -114.1 kJ [4 marks] a) The enthalpy of combustion of fructose, C6H12O6 is 21.2 kJ mol-1. An amount of 2.63 g of C6H12O6 was completely combusted in a bomb calorimeter at 25 oC. i. ii. b) Write the thermochemical equation for the reaction. Calculate the final temperature if the calorimeter contains 225.0ml of water. [5 marks] The industrial process for production of sulphuric acid has three steps. S(s) + O2(g)  SO2(g) 2SO3(g) →O2(g)  2SO2(g) SO3(g) + H2O(l)  H2SO4(l) ΔH = -296.83 kJ ΔH = -198.20 kJ ΔH = -227.72 kJ Using above data, calculate the enthalpy of the reaction, 2S(s) + 3O2(g) + 2H2O(l)  2H2SO4(l) [5 marks] 8. Calcium fluoride, CaF2, is a stable compound in the solid state. The enthalpy of solution is affected by the lattice energy and the hydration energy of the ionic solid. Using CaF2 as an example, define standard enthalpy of solution and lattice energy. Construct a Born-Haber cycle for CaF2 in the solid state by using the data below. Calculate the lattice energy of CaF2. First ionization energy of Ca Second ionization energy of Ca Standard enthalpy of atomization of Ca Bond energy of F-F bond Electron affinity of F Standard enthalpy of formation of CaF2 ΔHo (kJ mol-1) +590 +1150 +178 +158 -328 -1220 What would you expect the lattice energy of calcium chloride,CaCl2,compared to that of CaF2? Explain your answer. 3 CHAPTER 9: THEMOCHEMISTRY 9. In terms of solubility in water, CaF2 is less soluble than CaCl2. Explain the observation. [15 marks] At constant pressure, the decomposition of 1 mol of methanol,CH3OH, in the gaseous phase requires 90.7 kJ of heat to produce carbon monoxide and hydrogen gases. a) Write the thermochemical equation for the reaction. b) Calculate the amount of heat required when 1.6 kg of gaseous methanol decomposes. c) The enthalpy change for a given sample of methanol for the above reaction is 64.7 kJ. Calculate the mass of hydrogen gas produced. d) Calculate the amount of heat released when 32.0 g of carbon monoxide reacts completely with hydrogen to form methanol under constant pressure. [10 marks] 10 a) The thermochemical processes involved in the formation of lithium fluoride, LiF, are given in TABLE 1. Write a complete thermochemical equation for each process. TABLE 1 Process Atomisation of litium Atomisation of fluorine gas Ionisation of lithium atom Electron affinity for fluorine atom Formation of lithium fluoride Enthalpy, ΔH/kJmol-1 155 150 520 -382 -594 [5 marks] b) 100.00 cm3 of 2.0 mol dm-3 hydrochloric acid and 100.00 cm3 potassium hydroxide solution, both at initial temperature of 30.0°C are mixed in a calorimeter. The maximum temperature of the solution is 41.0°C. Calculate the enthalpy of neutralization for the reaction. [5 marks] 4