Materials and Procedure
Refer to the lab manual pg. 50-52. experiment #4 (Department of Chemistry, 2009). All materials and procedures were performed with no deviations.
Experimental Observations
Part A: Neutralization of NaOH with HCl
Concentration of NaOH = 2.335M, Temperature of NaOH = 23.5°C
Concentration of HCl = 1.9027M, Temperature of HCl = 23.5 °C
Table1: Temperature Change for Neutralization of NaOH and HCl (First Trial)
Figure 1: Temperature Change for Neutralization of NaOH and HCl (First Trial)
Table2: Temperature Change for Neutralization of NaOH and HCl (second trial)
Figure 2: Temperature Change for Neutralization of NaOH and HCl (Second Trial)
Part B: Neutralization of NaOH with HNO3
Concentration of NaOH = 2.335M, Temperature of NaOH = 23.5 °C
Concentration of HNO3 = 1.749M, Temperature of HNO3 = 23.5 °C
Table 3: Temperature Change for Neutralization of NaOH and HNO3 (First Trial)
Figure 3: Temperature Change for Neutralization of NaOH and HNO3 (First Trial)
Table 4: Temperature Change for Neutralization of NaOH and HNO3 (Second Trial)
Figure 4: Temperature Change for Neutralization of NaOH and HNO3 (Second Trial)
Part C: Neutralization of NaOH with phenol
Concentration of NaOH = 2.335M, Temperature of NaOH = 23.5 °C
Concentration of phenol = 0.5000M, Temperature of phenol = 23.5 °C
Table 5: Temperature Change for Neutralization of NaOH and phenol (First Trial)
Figure 5: Temperature Change for Neutralization of NaOH and phenol (First Trial)
Table 6: Temperature Change for Neutralization of NaOH and phenol (Second Trial)
Figure 6: Temperature Change for Neutralization of NaOH and phenol (Second trial)
Part D: Determination of the Molarity of an HCl solution of Unknown Concentration
Concentration of NaOH = 2.335M, Temperature of NaOH = 23.5°C
Concentration of HCl = ? M, Temperature of HCl = 23.5 °C
Table 7: Temperature Change for Neutralization of NaOH and HCl (First Trial)
Figure 7: Temperature Change for Neutralization of NaOH and HCl (First Trial)
Table 8: Temperature Change for Neutralization of NaOH and HCl (SecondTrial)
Figure 8: Temperature Change for Neutralization of NaOH and HCl (Second Trial)
Discussion
In this experiment, three different neutralization reacts with known concentrations of electrolytes were observed, and one neutralization reaction with unknown electrolyte is performed. The temperature changes of each reaction was measure using a simple coffee cup calorimeter.
The first part of the lab consisted of neutralizing NaOH with HCl. HCl of a known molarity was added to the coffee cup calorimeter which already had excess NaOH in it, and as the reaction occurred, the temperature was measured in certain time interval. This was repeated once again, giving two trials. The average enthalpy was 41.3kJ/mol, and a percentage error of 26.1%. The second part of the lab consisted of neutralizing NaOH with HNO3. The same procedure was repeated as part one, however using HNO3 instead of HCl. The average enthalpy was 43.1 kJ/mol, and a percentage error of 22.9%. The third part of the lab consisted of neutralizing NaOH with phenol. the same procedure was repeated as in part one and two, using phenol instead. The average enthalpy was 37.7kJ/mol, and a percentage error of 23.2%. With around 24% error for the first three parts of the lab, it clearly shows that this experiment was performed wrongly and was not completely successful. Factors that contribute to this are the possible sources of errors.
The final part of the lab was to determine the molarity of an HCl solution. The same procedure was performed to measure the change in temperature when it was added to the NaOH. Then, heat of the reaction is calculated using q = mc∆T. Because the enthalpy of the reaction is already given (∆H = 55.90kJ/mol), the concentration can be calculated using two formulas: n= heat / heat per mol, and C = n/V. By using these formulas, the unknown concentration was calculated to be 1.68M.
The possible sources of errors are the following. First, some of the heat is absorbed by the cup calorimeter, and it is neglected in the lab which could increase the percentage error. The other factors could be such as contamination of solution and accuracy of measurement. Since the temperature had to be recorded down by visually watching the thermometer, the temperature changes that occur may have been too fast to record, or so insignificant that it was not noticed. Also, heat could have been lost through the top of the cup since the lid was not always working.
Conclusion
The purpose of the experiment was to calculate the record the molar heat of each reaction and to identify the concentration of the unknown acid using calorimetry. Each neutralization reaction of HCl, HNO3, and phenol with NaOH were observed with the coffee cup calorimeter and the enthalpy of each reaction was found. These results were compared to the actual value given in the lab manual. The unknown concentration of HCl was calculated to be 1.68M.
In the experiment, NaOH was always in excess in the cup calorimeter, and acids with similar temperature were added to the NaOH. The temperature change was measured and recorded in certain time intervals with a thermometer.
The molar heat of each reaction for part A, B, and C is 41.3 kJ/mol, 43.1 kJ/mol, and 37.7kJ/mol respectively, which gives the percentage error of 26.1%, 22.9%, and 23.2% error. The unknown concentration of HCl was calculated to be 1.68M.
Overall, the experiment had a significant amount of errors based on the percentage error. If the possible sources of errors were to be improved, perhaps the experiment would be more successful.
References
Department of Chemistry 2009 “Chemistry 120L Laboratory Manual” University of Waterloo, Waterloo. pp. 50-55
Petrucci, R.H., Harwood, W.S., Herring, F.G., Madura, J.D. (2007) General Chemistry: Principles & Modern Applications, 9th edition. New Jersey: Pearson Education, Inc.
