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Confirming Hess's Law Experiment

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LAB Hess' Law Lab HYPOTHESIS The heat of combustion for magnesium can be calculated by determining the heat of reaction for the following reactions: 1) Mg(s) + 2HCl(aq) --> MgCl2(aq) + H2(g) 2) MgO(s) + 2HCl(aq) --> MgCl2(aq) + H2O(l) 3) H2(g) + 1/2 O2(g) --> H2O(l) Once these values have been obtained, one can add or subtract these values in order to achieve the heat of combustion for magnesium. VARIABLES Independent: � Type of chemicals used � Amount of chemicals used Dependant: � Change in enthalpy Controlled: � Elevation � Air pressure � Room temperature � Equipment � Humidity OBSERVATIONS Qualitative: Hydrochloric acid solution: - transparent, water-like liquid Magnesium metal strip: - dull silver, grey strip - extremely ductile Magnesium oxide: - fine, white powder - flour-like appearance Reaction #1 between hydrochloric acid solution and magnesium metal strip: - soft fizzing noise coming from the calorimeter - calorimeter is warm to touch Reaction #1 upon removing the Styrofoam cup cover: - clear solution - magnesium metal strip has fully dissolved in the hydrochloric acid solution - pungent smell - bubbles are apparent at the side of the cup Reaction #2 between hydrochloric acid solution and magnesium oxide: - calorimeter is slightly warm to touch - no apparent noise Reaction #2 upon removing the Styrofoam cup cover: - clear solution ...read more.


0.05 J -1158 ? 0.05 J 0.0248 ? 0.001 mol 0.0248 ? 0.001 mol - 53.59 ? 0.040 kJ - 46.69 ? 0.040 kJ Trial #1: 1) Mg(s) + 2HCl(aq) --> MgCl2(aq) + H2(g) ?H = - 468.5 kJ/mol 2) MgO(s) + 2HCl(aq) --> MgCl2(aq) + H2O(l) ?H = - 53.59 kJ/mol 3) H2(g) + 1/2 O2(g) --> H2O(l) ?H = -285.8 kJ/mol (www.chm.davidson.edu/ChemistryApplets/Calorimetry/HeatOfCombustionofMethane.html) (1) + (-2) + (3) = Mg(s) + 1/2 O2(g) --> MgO(s) ?Hcomb = (- 468.5 kJ/mol) + (46.69 kJ/mol) + (-285.8 kJ/mol) = - 700.71 kJ/mol Trial #2: ?Hcomb = (- 468.5 kJ/mol) + (53.59 kJ/mol) + (-285.8 kJ/mol) = - 707.61 kJ/mol CONCLUSION Theoretical value of heat of combustion of magnesium = 601.8 kJ/mol (cstl-csm.semo.edu/Hathaway/CH085/Heat%20of%20Reaction.doc) Trial #1: Trial #2: The heat of combustion for magnesium was determined algebraically by adding the molar enthalpies of three intermediate chemical reactions. For two out of the three reactions, the molar enthallpies were determined experimentally. A temperature probe was used to calculate the temperature change in both of the reactions. This value was then used in the equation Q = -mc?T. Since both of these reactions are exothermic, the negative sign reflects this nature. Also the specific heat capacity value used in the equation was that of water (4.18 J/g?C) ...read more.


Finally, due to the time restrictions and chemical limitations implemented during this lab, further trials were unable to be completed to ensure the accuracy of this lab. However, these would have aided in determining a more accurate heat of combustion of magnesium, as an average of all results would be obtained that best reflected the actual result. Also, these values would be more accurate with more practise. All these errors could be prevented or eliminated in order to improve this experiment. To ensure that the temperature probe is within the hydrochloric acid solution, a small cup should have been used. Therefore, the rubber stopper could have blocked the hole which was escaping air, and this would not have prevented the temperature probe from reaching the solution. Also, any gaps between the paper cups could have been blocked using tape to guarantee a tight fit, however, this would have been tedious to perform during the lab. To ensure the even distribution of magnesium, magnesium powder should have been used instead of the magnesium strip, which would allow for a better reaction due to the larger surface area. During calculations, the density of the hydrochloric acid should also have been determined, instead of assuming it was equal to water. Finally, through the benefit of more trials, the heat of combustion could have been calculated more accurately. ...read more.

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