Heat of Reaction Practice

1. The heat of reaction using the heat of formation method is -137 kJ while the heat of reaction using the bond enthalpy method is -127 kJ. The results are similar but not exactly the same. This is mainly due to the error inherent to the bond enthalpy method. BE data are average bond strengths rather than specific to this reaction. 2. The standard heat of reaction is -354.8 kJ which is exothermic. 3. The standard heat of formation for Al2O3 is -1675.7 kJ/mol. So the formation of Al2O3 from Al (s) and O2 (g) is an exothermic process. 4. The heat of reaction for the Hall Process is given as endothermic at +2170.9 kJ for every 4 moles of aluminum refined. This is 2170.9 kJ/4 mol Al. If you convert 4 mol Al to grams of Al using molar mass, you get 20.12 kJ/g. So for 5.000 g, the endothermic reaction involves 100.6 kJ of energy. Much less energy is involved for 5 g Al than for 4 moles of Al!