Nadya Patrica E. Sauza, Jelica D. Estacio
Institute of Chemistry, University of the Philippines, Diliman, Quezon City 1101 Philippines Results and Discussion
Eight Styrofoam ball calorimeters were calibrated. Five milliliters of 1M hydrochloric acid (HCl) was reacted with 10 ml of 1M sodium hydroxide (NaOH) in each calorimeter. The temperature before and after the reaction were recorded; the change in temperature (∆T) was calculated by subtracting the initial temperature from the final temperature. The reaction was performed twice for every calorimeter. The heat capacity (Ccal) of each calorimeter was calculated using the formula,
where ∆Horxn is the total heat absorbed or evolved for every mole of reaction and nLR is the number of moles of the limiting reactant. The ∆Horxn used was -55.8kJ per mole of water while the nLR was 0.005 mole.
Table 1. Average Ccal from recorded ∆T values.
Trial∆T, (oC)Ccal, (J)Ave Ccal, (J)
Different heat capacities were calculated for each calorimeter (Table 1).
After calibration, a reaction was performed in a calorimeter by each pair. A total of eight reactions were observed by the whole class. The temperature before and after the reaction were recorded. Then the change in temperature was calculated. Each reaction was performed twice to produce two trials.
The experimental ∆Horxn for each reaction was solved using the formula,
〖∆H〗_rxn^o=(-C_cal ∆T)/n_LR 
where Ccal is the heat capacity previously calculated for each calorimeter.
The percent error for each reaction was computed by comparing the computed experimental ∆Horxn to the theoretical ∆Horxn using the formula,
% error=|(computed-theoretical)/theoretical|×100% 
Table 2. Comparison of calculated ∆Horxn and theoretical ∆Horxn. RxnLRTrial∆T, (oC)∆Horxn, (kJ/mol)Ave ∆Horxn, (kJ/mol)Theo ∆Horxn, (kJ/mol)% Error 1HCl13.5-142.04-131.89-132.510.47
There were differences in experimental and theoretical values of ∆Horxn as shown by the percent error for each reaction (table 2). The discrepancies were caused by many factors. One factor was the loss of heat. The heat may have been released when the thermometer was pushed or pulled during the reaction. The heat may also have been lost because the calorimeter is not totally isolated. Another factor was the dilution of the solution. The pipette or test tube may still have been wet when used. However, the concentration used in solving for values was the concentration of the undiluted solution. Another factor that may have contributed to the difference in the experimental and theoretical values was human error. It was manifested when reading the thermometer or measuring chemicals with different instruments. The factors aforementioned are the limitations of this experiment.
Petrucci, R.H.; Herring, F.G.; Madura, J.D.; Bissonnette, C. General Chemistry, 10th ed.; Pearson Education: Canada, 2011; Chapter 7.
Comparison of Observed and Theoretical Heats of Reactions
RxnLRTrial∆TnLRqrxn∆HorxnAve ∆HorxnTheo ∆Horxn% Error 1HCl13.500.00500-710.19-142.04-131.89-132.510.47