paraphrase
MkhayatIntroduction:
The task presented in this lab was to determine the heat of combustion of Mg as described by the reaction: Mg(s) + 1/2O2(g) → MgO(s). In order to know the heat change of this reaction, other chemical reactions can be used. Hess’ law and the calorimetric technique can be used to measure the heat of the above reaction. Hess’ law states that the heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps. The conservation of energy states that energy may neither be created nor destroyed.
Procedures:
In order to determine the heat of combustion of Mg, first three empty Styrofoam cups were weighed with an electronic balance. The results were recorded. Second, a 25 ml of HCl was poured in the Styrofoam cup. This was in order to fully dissolve the Mg. Third, the temperature of the solution was measured (using a thermometer) in a covered Styrofoam cup until it reached the highest temperature. The initial temperature was recorded. After that, the magnesium was added to the covered Styrofoam cup. Finally, the temperatures of solutions were recorded when thermometer reached the highest number.
Possible Errors:
A Styrofoam cup with a plastic cover was used as the calorimeter in the experiment, so there is a very good chance that some of the heat may have escaped either through the Styrofoam, the plastic, or the small hole in the plastic cover in which the thermometer was piercing. Human error is always a factor when performing experiments, and it may have contributed to some inaccuracies in the experiment and procedures. These may include, but are not limited to: the amount of HCL measured in the graduated cylinder, and a mistake made pertaining to the weight of the cups. When experiment #1 was performed, three different cups were used. It was wrongly assumed that the weight of each cup would be the same, so only one of them was weighed, and the other cups were assumed to have the same weight. Because of this mistake, all of the values for the weight of the cup for experiment # 1 are the same. In experiment #2, the mistake was corrected, and the each of the three cups were weighed before being used. As seen in the table, the cup weights for experiment #2 have slightly different values from each other, and different values than the cups used in experiment #1.
Observations:
There were very few observations due to the fact of not being able to see the reaction take place. For experiment #1 when the metal strips of Mg were placed into the cup and into the clear aqueous solution of HCl, the reaction made a fizzing sound. This reaction exhibited heat, causing the temperature to rise an average of 39 degrees Celsius inside the zthe styrofoam cup. At the end of the reaction the metal Mg strips had completely dissolved into the aqueous solution of HCl. For experiment #2, the MgO which had an appearance of a thin white powder. When it was added to the clear solution of HCl, a reaction much less drastic than experiment #1 occurred and the fizzing sound was fainter. For the reaction of MgO and HCL, the heat of the cup rose an average of 8 degrees celsius. After the reaction had taken place, the white powder MgO had dissolved into the solution of HCl.
Conclusion:
The theoretical value of heat of combustion for Mg was -465 kJ/mol and in comparison to the, -462.5 kJ/mol, derived from the experiment the conclusion held true to the supposed value. Percent error is calculated by the theoretical value from the experimental value all divided by the theoretical value. This experiment resulted in the percent error being .5376%. It was fairly close meaning that though the styrofoam cup wasn’t the best insulator and errors could have been made in weighing substances, the results supported the theoretical value. If more precision and care was taken into consideration while undertaking the experiment the percent error would be lower.