Investigation 5.5.1 ACTIVITY Calorimetry versus Enthalpies of Formation In this activity, you will use enthalpies of formation to determine the enthalpy change of a chemical reaction. Then, you will use the enthalpy values obtained from calorimetry experiments to calculate standard enthalpies of formation. Finally, you will compare the results of the two calculations. The calorimetry data is from an experiment involving a bomb calorimeter. During the experiment, the thermal energy released by the substance in the calorimeter is transferred directly to the calorimeter rather than to water. The quantity of energy transferred is determined using bomb Cbomb X AT where Chomb is the heat capacity of the calorimeter in J/°C. SKILLS Purpose To compare the experimental value of the enthalpy of combustion of ethanol with that derived from calculations based on enthalpies of formation for ethanol, water, and carbon dioxide. A2.4 Procedure 1. Determine the enthalpy of combustion per mole of ethanol using the experimental data in Table 1. Table 1 Data from Combustion of Ethanol in a Bomb Calorimeter Mass of ethanol 3.26 g Heat capacity of calorimeter 11.7 kJ/°C Initial temperature of calorimeter Final temperature of calorimeter 21.5°C 29.4 °C 2. Determine the enthalpy of combustion per mole of ethanol using the standard enthalpies of formation in Table 2. • Questioning • Researching • Hypothesizing • Predicting Table 2 Standard Enthalpies of Formation Substance • Planning ● • Controlling Variables • Performing • Observing H₂O(1) CO₂(g) C₂H5OH(1) SKILLS MENU • Analyzing • Evaluating • Communicating AH (KJ/mol) -285.8 -393.5 -235.2 Analyze and Evaluate (a) Calculate the percent difference between the values you calculated in Steps 1 and 2. Explain any discrepancy between the two methods. KU TA Apply and Extend (b) Ethanol is commonly used as a fuel for cars and for heating homes. Based on your results, explain why ethanol makes a good fuel. KUA (c) Calculate the enthalpy of combustion for ethanol using bond energies and compare it with the other values. K/U™A