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Hess' Law Lab

  • Date Submitted: 03/27/2013 02:21 PM
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LAB - Using Hess’ Law and calorimetery to determine the molar enthalpy of combustion of magnesium  
Theory:
Thermochemistry is the study of energy changes accompanying chemical transformations and the change in enthalpy is a useful figure used by thermo-chemists. Enthalpy is defined as the total internal energy of a system plus the product of pressure and volume[4]. When pressure and volume are constant, Enthalpy can be defined as the total heat content or sum of all thermodynamic energies within a system.   These energies include potential bond energies, potential nuclear energies as well as the sum of the kinetic energy of electrons. Chemists are unable to measure enthalpy because it is impossible to measure all of the energies in a system; however it is possible to determine the change in thermodynamic energy and thereby the change in enthalpy. The symbol associated with enthalpy is H and the symbol referring to a change in enthalpy is H.
Since H is defined as the change in thermal energy in a system and the 1st law of thermodynamics states that energy can neither be created nor destroyed, then the energy released by the surroundings when added to the energy lost by the system must equal 0 as seen in the equation below.
Esystem+ Esurroundings=0
Esystem= -Esurroundings
Since the change of internal energy of the system (Esystem) is also the change in enthalpy (H) and Heat (Q) is defined as the energy transferred to another body, such as the surroundings then this equation can be rewritten to:
Hsystem= -Qsurroundings
Calorimetry is the technological process used by chemists to measure heat transfer. A calorimeter is a device that insulates and isolates a chemical transformation inside water. A thermometer and stirrer are also used to measure the temperature change and ensure even heat distribution respectively. By using a calorimeter to measure the change in temperature of the surroundings of a chemical or physical reaction one can determine the amount...

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