in which m was calculated using stoichiometry and c is the specific heat of the product, water, which is equivalent to 4.18 Jg-1˚C-1. qrxn can then be determined from qcal, given by the formula
since the qsystem is equal to zero in an adiabatic system.
After the calibration, ∆T from different reaction systems have been measured to determine the ∆Hrxn. The number of moles of the limiting reactant is determined and is used in the equation
to calculate for heat of reaction. In reactions involving solids, the solid reactants or products may absorb the heat from the reaction. To account for that event, the following formula is used:
After computing for ∆Hrxn, the results show that all the reactions were exothermic. Percent error was calculated to determine whether the results were accurate or not. Percent errors were quite high which shows that the results lack accuracy.
References
(1) Petrucci, H., et.al., General Chemistry: Principles and Modern Applications, 10th edition. Pearson Education South Asia PTE. Ltd., 2010, 244-251
Appendices
Appendix A: Calculations for Calibration
c = 4.18 J g-1 -1
Group 1
Trial 1
Trial 2
Average Ccal = 203
Group 2
Trial 1
Trial 2
Average Ccal = 109
Group 3
Trial 1
Trial 2
Average Ccal = 558
Group 4
Trial 1
Trial 2
Average Ccal = 245
Group 5
Trial 1
Trial 2
Average Ccal = 81.4
Group 6
Trial 1
Trial 2
Average Ccal = 210
Group 7
Trial 1
Trial 2
Average Ccal = 112
Group 8
Trial 1
Trial 2
Average Ccal = 117
Appendix B: Calculations for Heats of Reaction
LR: HCl
Trial 1
Trial 2
Average = -132 kJ/mol
LR: HOAc
Trial 1
Trial 2
Ave = -99.6 kJ/mol
LR: HOAc
Trial 1
Trial 2
Ave = -204 kJ/mol
LR: HNO3
Trial 1
Trial 2
Ave = -71.1 kJ/mol
LR:
Trial 1
Trial 2
Ave = -139 kJ/mol
LR:
Trial 1
Trial 2
Ave = -638 kJ/mol
LR:
Trial 1
Trial 2
Ave = -44.2 kJ/mol
LR: CaCl2
Trial 1
Trial 2
Ave = -5.85 kJ/mol
Answers to Questions:
1. The percent error of the ∆Hrxn may be accounted to the following: (a) test tube not closed immediately, which can cause heat to be released, (b) considerable space inside the Styrofoam ball, (c) wrong concentration and/or amount of reagents, (d) water droplets inside the test tube, which can cause reactions or change in concentration.
2. a. To make sure that the reaction is complete, and there is no change in the number of moles.
b. The number of moles to be used in the formula is dependent on the concentration of the limiting reactant.
c. In reactions involving solids, the formula for the determination of ∆H puts the mass of the solid into consideration given the fact that it may absorb heat released in the reaction.
3. a.
b. HA is a strong acid.
c.
4. a.
b.
c.
d.
5.
