A chemical reaction often indicated by a transfer of energy measured in heat. By measuring this heat transfer in a constant pressurized environment, the enthalpy of the reaction can be used to infer certain information about a specific reactions reactants and products. The transfer of heat from outside sources in would be described as an endothermic reaction. Contrary, when a reaction releases heat out to its surroundings it is described as an exothermic reaction. Method:
Small strips of Magnesium (0.2480g) were added into a precise measurement of 5mL of 1M Sulfuric Acid (measured with a transfer pipet) within a calorimeter. Precautions were taken to ensure the most accurate reading of the transfer of heat and the complete isolation of the reaction’s heat exchange. To do this first the Sulfuric Acid was chilled. Secondly, the calorimeter was covered in ice and tested for the amount of heat captured in the calorimeter (Calorimeter Data Table 1.) Lastly, the heat change could have been measured with a thermometer, but the thermometer would’ve added heat to the solution, so this was not used; instead, with a known initial volume of the calorimeter, the change in volume was measured at different time intervals using an inverted volumetric pipet to determine the heat captured within the calorimeter. The same measurements were used at later time intervals to determine the volume change during and after the reactions.
Linear values were shown for before the reaction and after. The line represents the change in volume between the two trend lines. The two R values are slightly different either suggesting that the reaction didn’t fully take place or that the calorimeter was still coming to a steady temperature. Calculations:
Ice has a larger volume than liquid water, so it is vital that this variable be taken to account during calculations. By using the density of ice and water, the volume of dissolving ice in water can be calculate. Density of ice= 1mL/0.9157g=1.0920mL/1g H2O
Density of water= 1 mL/0.9998 g = 1.0002 mL/1 g
Density of ice- Density of water= volume of dissolving ice in water= 0.0918 mL/g.
Due to the differences in R, the change in volume min, avg, and max are used to more accurately determine the energy transfer that will be used to determine the enthalpy of the reaction.
Change in volume min =0.8957-0.2908+(690*(-0.0003+0.0002))=0.5359mL Change in volume avg =0.8957-0.2908+(765*(-0.0003+0.0002))=0.5284mL Change in volume max =0.8957-0.2908+(840*(-0.0003+0.0002))=0.5209mL
Mass of ice min=0.5359mL/0.0918 mL/g=5.837691g
Mass of ice avg=0.5284mL/0.0918 mL/g=5.755991g
Mass of ice max=0.5209mL/0.0918 mL/g=5.674292g
Energy transfer min=5.837691g *334/1000=1.949789KJ
Energy transfer avg=5.755991g*334/1000=1.922501KJ
Energy transfer max =5.674292g *334/1000=1.895214KJ
Enthalpy min=1.949789KJ/.005mol H2SO4=389.96KJ/mol
Enthalpy avg=1.922501KJ/.005mol H2SO4=384.50KJ/mol
Enthalpy max=1.895214KJ/.005mol H2SO4=379.04KJ/mol
The experimental change in enthalpy was found to be exothermic -384 +/- 5 KJ/mol at 0°C. This leaves a 17.6% error with the theoretical value -466.9KJ/mol. Discussion:
The experiment yielded an exothermic reaction. The calculated enthalpy was far less than the theoretical. The cause of this is a lower heat transfer than the theoretical transfer. This can be caused by heat loss from the calorimeter outwards or not letting the reaction to fully take place.
Brown, LeMay, Bursten, General Chemistry, 10th ed., Houghton Mifflin, Boston, 2006 Thermochemistry lab document, Figure 1. Ice calorimeter apparatus, Mesa Community College CHM152LL website, www.physci.mc.maricopa.edu/Chemistry/CHM152, accessed 09/11/2014.