Heat of Combustion for Magnesium

Heat of combustion for Magnesium.

Problem

What is the heat of combustion for magnesium?

Materials

  • MgO
  • 1.0 M HCl
  • Magnesium ribbon
  • Thermometer
  • Calorimeter
  • Graduated cylinder

Hazards

Hydrochloric acid (HCl) reacts vigorously or violently with most metals, bases and many other chemicals. Contact with the eyes or skin can cause serious permanent damage; concentrated solutions of this acid are extremely corrosive and very dilute solutions are mildly corrosive. 
HCl is toxic by inhalation – the concentrated solution releases dangerous quantities of hydrogen chloride vapour.1

Magnesium oxide (MgO) may cause irritation to the nasal passages and respiratory tract when inhaled. Inhalation can also cause a flu-like illness called metal fume fever. This 24- to 48-hour illness is characterized by chills, fever, aching muscles, dryness in the mouth and throat, and headache. When ingested, MgO is slowly absorbed and may cause rapid bowel evacuation. Contact with the skin and eyes may cause irritation.2

Magnesium (Mg) is a flammable solid and is dangerous when wet. It is highly reactive and may ignite spontaneously on contact with water, or other damp substances. Mg may cause abdominal pain and diarrhea when ingested. Particles of Mg embedded in the skin may cause eruptions or rashes, and molten magnesium can cause serious skin burns.3

Procedure

Reaction 1:

1. Approximately 0.2 g of MgO was massed, and approximately 20 mL of 1.0 M HCl was measured with the graduated cylinder.

2. The initial temperature of the reaction was taken by measuring the temperature of the HCl.

3. The MgO and then the HCL were poured into a calorimeter and reacted.

4. The final temperature of the reaction was recorded as the highest temperature the reaction attained while in the calorimeter.

Reaction 2:

1. Approximately 0.1 g of Mg ribbon was massed, and approximately 20 mL of 1.0 M HCl was measured with the graduated cylinder.

2. The initial temperature of the reaction was taken by measuring the temperature of the HCl.

3. The Mg was placed in the calorimeter and the HCl poured over it.

4. The final temperature of the reaction was recorded as the highest temperature the reaction attained while in the calorimeter.

Observations

Table 1: Quantitative Observations

Reaction 1

Mass MgO 0.213 g
Mass HCl 19.8 g
Initial Temperature 22.8°C
Final Temperature 29.0°C
Reaction 2 Mass Mg 0.102 g
Mass HCl 20.6 g
Initial Temperature 23.7°C
Final Temperature 41.8°C

Table 2: Qualitative Observations

Before During After
Reaction 1 - HCl is a clear, colourless liquid

- MgO is a white powder

- some fizzing and bubbles

- becomes cloudy

- a clear, colourless solution

- a few white bits remain

Reaction 2 - HCl is a clear, colourless liquid

- Mg is a shiny, malleable metal ribbon

- fizzed profusely for some time

- became cloudy

- a clear, colourless solution

- very small amount of Mg left unreacted

Calculations

See Appendix.

Conclusions

The two chemical equations in this lab and their heats of reactions can be manipulated and then added together algebraically to form our target equation,

MgO (s) + O2 (g) → MgO (s), which is the combustion of magnesium.4 In this way, it is possible to obtain the heat of reaction of magnesium, which is difficult to find experimentally. The value calculated from this lab for the combustion of magnesium was 275 kJ/mol. The accepted value is -601.6 kJ/mol.4 The percent error of this experiment was 54.3%, which signifies that there were several experimental errors throwing the data off.

One of the most significant experimental errors was the fact that the calorimeter used had no lid. This would allow the heat released by the reactions to escape instead of being measured, which affects the change in temperature calculated; this in turn affects the heat of reaction calculated, making the value lower than it should be. This problem could be corrected by adding a lid to the calorimeter, with a hole burned through it for the thermometer, making an airtight fit. Another experimental error found in the calculations of this lab was that the specific heat capacity of water was used as the value for the specific heat capacity of the 1.0 M HCl solution. This value is not accurate, even though the HCl solution was mostly water. The usage of this inaccurate value makes the heat of reaction calculated less accurate. To correct this, the exact specific heat capacity of the HCl solution should be found in the lab through extensive experiments.

References

1. n.a. (2003). Chemical Safety Data: Hydrochloric acid. Retrieved April 18, 2009, from Education and Training. Website:

http://cartwright.chem.ox.ac.uk/hsc /chemicals/hydrochloric_acid.html

2. n.a. (2004). Material Safety Data Sheet: Magnesium Oxide. Retrieved April 18, 2009, from Chamotte. Website:http://www.chamotte.co.za/MSDS/MSDS%20- %20MgO.pdf

3. n.a. (2001). Magnesium Metal, Powder. Retrieved April 18, 2009, from Mallinckrodt Chemicals. Website: http://www.jtbaker.com/msds/englishhtml/m0088.htm

4. van Kessel, Hans et al. (2003). Chemistry 12. Toronto: Nelson. p. 314.

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2 Responses to “Heat of Combustion for Magnesium”
  1. singingstar6 Says...

    On June 10, 2010 at 12:14 am

    Um..hi I can’t seem to find your sample calculations


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