The accepted value for the enthalpy change for the following reaction is -217kJ while the experimental value was -2.100 ± 0.100kJ. This assures the fact that the experiment was exothermic, but the percentage error is 99% ± 5%. This large margin of error can be accounted for in uncertainties and various rooms for improvement in the procedure.
Evaluation:
Overall, the design and method of the experiment could be greatly improved in order for an answer that is more similar to that of the accepted value. The most important weakness was the fact that the experiment was conducted in a polystyrene calorimeter, a closed system, where energy but not matter can be exchanged. This allows for energy released from the system to escape the surroundings, therefore yielding an imprecise measure of enthalpy change. Firstly, the hole made for the thermometer in the Styrofoam cup was too large, therefore leaving room for energy to be transferred between the calorimeter and the air in the room. Furthermore, the initial temperature could not be read on the thermometer as the Styrofoam cups were too large. Thus, the cups were cut smaller in order for accurate readings to be read from the thermometer. Nevertheless, the cups no longer fit perfectly together and there were gaps left between the cups leaving room for energy to be transferred between the calorimeter and the air in the room, despite taping the cups together.
Next, the precision of the measuring devices inhibits the results of the experiment to match the accepted value of enthalpy change. Because there are uncertainties in all the equipment used, there is a margin of error associated with each apparatus. Nevertheless, the margin of error increases with consecutive calculations regarding enthalpy change. Thus, all calculations will not be completely accurate leading to discrepancies between experimental and accepted values. Also, this weakness is insignificant in comparison to the inefficiency of the calorimeter used and is difficult to circumvent.
Moreover, a less significant weakness is involved with the transferring of solutions. When transporting the copper (II) sulfate solution from the pipette to the Styrofoam cup, some droplets of the solution were left behind in the pipette. Nevertheless, this does not make a large difference in the overall experiment, therefore making it a less significant weakness. In addition, when the zinc powder was transferred from the weigh boat to the calorimeter, many small pieces were left behind because the powder was very brittle. Thus, the initial measured quantity of zinc powder did not match the amount of zinc powder used in the reaction.
Lastly, another less significant weakness in the case of this experiment was human error. This could account for any spillage or human mistakes that may have been made throughout the course of the experiment. Nevertheless, no major errors were made during this experiment, thus the significance of this weakness is minute in comparison to others.
Improvements:
Because many weaknesses and limitations were evaluated above, there are also various suggestions for improvement in the future. Firstly, instead of using a closed system for the redox reaction, an isolated system should be used. In order for a more precise and accurate enthalpy change, the experiment should have been conducted in an isolated system, where neither energy nor matter can be exchanged with the surroundings. If a bomb calorimeter is available, it would be preferable to use as there is insulation that does not allow energy to be transferred with the air in the room. Nevertheless, if this is not available, smaller Styrofoam cups could be used in the polystyrene calorimeter therefore preventing one’s self from cutting large cups that do not fit together.
Next, even though uncertainties were associated with all measuring devices, the most precise equipment available was used to prevent large uncertainties. For example, a pipette was used to measure the copper (II) sulfate solution instead of a beaker, which has a larger uncertainty associated with it. Furthermore, a scale with a very small uncertainty was used rather than a scale with a larger uncertainty. In the future, the usage of the most precise equipment should be used, such as a more precise weighing scale and pipette.
In addition, in order to prevent some of the zinc powder from being left behind, a more solidified sample of zinc should be used. Therefore, the measured quantity of zinc would be the same as the amount of zinc actually transported to the calorimeter, even though zinc was added in excess to this experiment. Also, a stirring rod could be used in order to ensure that both reactants mix thoroughly.
Lastly, human error is unavoidable. Nevertheless, In order to minimize this, one should be more careful when handling equipment and measuring devices.
