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

( 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 for calorimeter)
• 400mL beaker (transportation of acid)

We began this investigation by suiting up in lab aprons and goggles, we then gathered our materials, found a lab station and got to work. We decided to start with the magnesium in hydrochloric acid first, we measured out 198.5 L of HCl and put it in the foam-cup calorimeter and took and initial temperature reading.  We then selected a piece of magnesium ribbon and found its mass: 0.01g.  This piece was placed in the calorimeter and the lid was shut immediately to prevent heat from escaping.  We “swirled” the liquid mixture in the calorimeter to ensure reaction, and waited for a temperature change.  After a few moments, the final temperature was recorded and ΔT determined.  This process was then repeated.  On the second trial, the Mg ribbon did not completely dissolve and the results were thrown out.  The third trial (referred to as the second in the following analysis due to the exclusion of the previous one) was successful, and measurements can be seen below.

We then moved onto the second reaction using magnesium oxide and hydrochloric acid in the fume hood.  We measured 200.1 mL of HCl and placed it in the calorimeter, and initial temperature reading was taken.  Next, we measured 1.07 g of magnesium oxide, using a balance in the fume hood,  added it to the HCl in the calorimeter, and shut the lid quickly to conserve heat.  This mixture was  “swirled” and allowed a few moments to react.  The final temperature was recorded and ΔT determined.  This process was repeated successfully and the measurements can be seen below.

Results and Analysis

 Table 1:Results of Magnesium and Hydrochloric acid 

Table 2: Results of Magnesium oxide and Hydrochloric acid

Throughout this investigation data was collected and placed on the above tables to be used in the calculations below.  

Calculations:

Given:

H2 + 1/2O2 → H2O        ΔH = -285.8 kJ/mol

Mg + 2HCl → MgCl2 + H2

Trial 1:(not used)

.01g. Mgx   1mol Mg    = .0004 mol Mg                        198.5 L = 198.5g.

                 24.305 g. Mg

qsur = (198.5 g.)(4.184)(.4⋅C) = 332.2J x   1kJ    = .3322kJ

   g.⋅C                             1000J

qrxn = -.3322kJ → ΔH =    -.3322 kJ   = -830.5 kJ/mol Mg

.0004 mol Mg

Trial 2:

.05g. Mg x   1mol Mg   = .0021 mol Mg                        199.4 L = 199.4g.

      24.305g. Mg

qsur = (199.4)(4.184)( 1.1⋅C) = 917.7J x   1kJ    = .9177kJ

           g.⋅C                            1000J

qrxn = -.9177kJ → ΔH =    -.9177kJ     = -437 kJ/mol Mg

.0021 mol Mg

MgO + 2HCl → MgCl2 + H2O 

Trial 1:

1.07g. MgO x  1mol MgO   = .0265 mol MgO                200.1 L = 200.1 g.

          40.304g.MgO

qsur = (200.1g.)(4.184J )(4.3⋅C) =3600.04J x   1 kJ  _        = 3.600kJ        

                    g. ⋅C                                1000 J        

qrxn = -3.600  → ΔH =     -3.600kJ     = -135.8 kJ/mol MgO

        .0265 mol MgO

Trial 2:(not used)

1.04g. MgO x   1mol MgO  = .0258 mol MgO                199.5 L = 199.5 g.

          40.304g.MgO

qsur = (199.5g.)(4.184)(3.2⋅C) = 2671.1J x   1 kJ   = 2.6711 kJ

  g.⋅C                                1000 J

qrxn = -2.6711kJ → ΔH =    -2.6711kJ     = -103.5 kJ/mol MgO

 .0258 mol MgO

Heat Summation:

2(H2 + 1/2O2       → H2O)                (ΔH = -285.8 kJ/mol)2 =  -571.6

2(MgCl2 + H2O → MgO + 2HCl )        (ΔH =  135.8 kJ/mol)2  =  271.6

2(Mg + 2HCl    → MgCl2 + H2 )        (ΔH =  - 437  kJ/mol)2  = -874              

         2Mg + O2 → 2MgO                        ΔH =  -1174 kJ/mol O2     =

-1174 kJ/mol O2 = -587 kJ/mol Mg

2

Percent Error:

MgO % error = | -120 - (-135.8)| x100 = 13.2 %

         -120

Mg % error  = | -440 - (-437)| x100 = .682 %

       -440

Mg + O2 % error = | -601.8 - (-587)| x 100 = 2.46 %

   -601.8

Discussion

After collection and organization the data was interpreted.  Through the uses of Hess’s Law of heat summation, we were capable of deriving the change in enthalpy of a reaction from alternative reactions.  Though this process proved fairly successful in this investigation a margin of error can/was found using the “given answers” provided by our instructor.  Using the following givens we were capable of calculating our percent error at different stages in the investigation as well as our final answer:        

Mg + 1/2O2        ΔH = -601.8 kJ/mol Mg

We found our change in enthalpy for magnesium oxide had a 13.2% error, while our change in enthalpy for magnesium had only a .682% error.  Our final answer was fairly accurate, only having a 2.46% error.  These errors may be accounted for by inaccurate measurements, mathematical mistakes, incomplete reactions, poor heat collection, incorrect recording of data, and poorly calibrated tools.  To avoid such error one should label all materials, check each tool before use, take extra care in reading and recording of measurements, double check all calculations, and most of all be patient, labs take time and a rushed procedure leads to inaccurate data and incorrect analysis.  

Sources Cited

Cartwright, H.(last revised 2002, November 5). Chemical and other Safety Information. The Physical and Theoretical Chemistry Laboratory, Oxford University. [online].<http://physchem.oxac.uk/MSDS/IO/iodine.html> (accessed 2002, December, 4)

LeMay, H.E.,Beall, H., Roblee, K.M., and Brower, D.C. (1996).Chemistry: Connections to our changing world, Laboratory Manual. Upper Saddle River, NJ: Prentice Hall.