Determining the change in enthalpy for the combustion of Mg using solution calorimetry and heat summation.

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Period 1 C.P. Chem

March 7,2003

Determining the change in enthalpy for the combustion of Mg using solution calorimetry and heat summation.

Abstract

        Hess’s law of heat summation states that the value of ΔH for a reaction is the same whether it occurs directly or as a series of steps.  This principle was used to determine the change in enthalpy for a highly exothermic reaction, the combustion of magnesium metal.  Enthalpy changes for the reactions of Mg in HCl (aq) and MgO (s) in HCl (aq) were determined experimentally, then added to that for the combustion of hydrogen gas to arrive at a value of

–587 kJ/mol Mg.  Compared with the accepted value of –601.8 kJ/mol Mg, our experimental error was 2.46%.

Introduction        

In this investigation the change in enthalpy will be determined from the following equation: 2Mg + O2  2MgO, but in an indirect manner.  Magnesium metal burns with a bright extremely hot flame to produce magnesium oxide. It would be difficult to measure the heat of the reaction since the reaction is rapid and occurs at a high temperature (LeMay et al, 1996).  So, to determine the change in enthalpy we will employ Hess’s Law of heat summation: It states that the value of ΔH for a reaction is the same whether it occurs directly or as a series of steps (LeMay et al, 1996).  We will perform the two following reactions: Mg + 2HCl  MgCl2 + H2 and

MgO + 2HCl  MgCl2 + H2O, determine their enthalpy changes (ΔHs), and they will then be “added” to that of a given equation, the combustion of water, H2 + 1/2  O2   H2O   ΔH=-285.5 kJ/mol.

In this investigation we will be working with potentially dangerous chemicals and safety precautions must be made.  Magnesium oxide is a respiratory and eye irritant, the dust must not be inhaled and all work with MgO should be conducted in the fume hood

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( Cartwright, 2002).  Hydrochloric acid is extremely corrosive, inhalation of the vapor can cause serious injury, ingestion could be fatal, and the liquid can cause sever damage to the skin and eyes; when working with HCl splash goggles and gloves should be worn, and work should be conducted in a well ventilated area (Cartwright, 2002).    

Materials and Methods

  • Goggles
  • Lab apron
  • Magnesium oxide MgO - 1g.
  • Lab balance
  • 100mL graduated cylinder
  • Hydrochloric acid HCl - 200mL/.5M


  • 2 plastic foam cups
  • thermometer
  • cover for cup
  • Magnesium (Mg) ribbon
  • 600mL beaker (base ...

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