Quantitative:
Table 1: Data observed for Reaction #1 between hydrochloric acid solution and magnesium metal strip
Table 2: Data observed for Reaction #2 between hydrochloric acid solution and magnesium oxide
CALCULATIONS
Reaction #1: Mg(s) + HCl(aq) → MgCl2(aq) + H2(g)
ΔT = Tf – Ti
ΔT = (34.5 ± 0.1°C) – (21.3 ± 0.1°C)
= (13.2 ± )°C
= 13.2 ± 0.14 °C
Q = - mcΔT
mMg = 0.291 ± 0.001g
mHCl = (D) (V)
= (1.016g/mL)(100.0 ± 0.5mL)
= 101.6 ± 0.5g
mtotal = (0.291 ± 0.001g) + (101.6 ± 0.5g)
= (101.9 ± )g
= 101.9 ± 0.5g
c = 4.18 J/g°C
ΔT = 13.2 ± 0.14 °C
Q = - (101.9 ± 0.5g)(4.18 J/g°C)(13.2 ± 0.14 °C)
= - 5622.4344 ± J
= -5622.4 ± 0.01 J
MM = 24.3050 g/mol
Table 3: Data processing for Reaction #2: MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l)
Trial #1:
1) Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) ΔH = - 468.5 kJ/mol
2) MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) ΔH = - 53.59 kJ/mol
3) H2(g) + ½ O2(g) → H2O(l) ΔH = -285.8 kJ/mol
(www.chm.davidson.edu/ChemistryApplets/Calorimetry/HeatOfCombustionofMethane.html)
(1) + (-2) + (3) = Mg(s) + ½ O2(g) → MgO(s)
ΔHcomb = (- 468.5 kJ/mol) + (46.69 kJ/mol) + (-285.8 kJ/mol)
= - 700.71 kJ/mol
Trial #2:
ΔHcomb = (- 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 = -mcΔT. 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) since the solution was excessively diluted. Therefore, the molar enthalpies for both reactions were calculated. The reaction involving the magnesium oxide and hydrochloric acid was determined twice, one with a rubber stopper and one without. The trial with the rubber stopper, Trial #1, proved to be more accurate, with a final percent error of 16.4% relative to the percent error of Trial #2, which was 17.6%. The rubber stopper is able to trap the heat and pressure within the calorimeter which is essential when calculating molar enthalpy. The third and final intermediate reaction involving the formation of water could not be determined experimentally, due to the dangerous procedures that are required. Theoretical values were used in its place. Hence, with the molar enthalpies of the three intermediate chemical reactions, the heat of combustion for magnesium could be calculated algebraically. However, the values determined experimentally are slightly inconsistent with the theoretical values, as was previously discussed.
EVALUATION
Several discrepancies could have occurred to affect the accuracy during the performance of this lab. Foremost, during the temperature recording session, the temperature was not fully submerged in the hydrochloric acid solution when the rubber stopper was in place. Therefore, the highest temperature could not be accurately recorded, as the probe could not reach the solution. Also, heat could have been lost to its surroundings through the gap between the two Styrofoam cups as well as through the hole where the temperature probe was held. This would affect the temperature change value and in turn, the molar enthalpy value for that reaction. There was also concern regarding the even distribution of the reactants within the hydrochloric acid solution. This concern was less significant for the magnesium oxide, as the fine powder form had a greater surface area and could therefore react better with the acid. However, it was difficult to guarantee that a complete reaction occurred with the magnesium strip, which would affect the molar enthalpy of the reaction. Due to loss of heat or incomplete reaction, the molar enthalpy value obtained for each of these reactions would be slightly lower than expected. 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.
