Group 13

Group Members: Min Woo Cho, Michael Lee, Ji Soo Kim

AP CHEM from Ji Soo Kim on Vimeo.

Bourne Identity

Combustion of Magnesium in Carbon Dioxide

Chemistry Background Information
Magnesium is combusted in excess of CO2. CO2 is sublimed from the dry ice. It is exothermic reaction.
File:Carbon dioxide pressure-temperature phase diagram.svg
File:Carbon dioxide pressure-temperature phase diagram.svg

This is the phase diagram of CO2. At pressure higher than 1 atm, carbon dioxide will exist as a solid. So, dry ice (solid CO2) forms when excess CO2 gas is forced into a container. Since the number of moles for CO2 increase and the volume is kept the same, the pressure increaases and forms solid CO2. When it is exposed to the room pressure, it will sublimate because CO2 exists as gas at 1 atm.

2 Mg(s) + CO2 → 2 MgO(s) + C(s)
2 Mg(s) + 3CO2 → 2MgCO3(s) + C(s) are the reaction that is happening.
MgO and MgCO3 are the white residue after the reaction and the black substance is carbon that is formed from both reactions.

Both of the reactions above are likely to happen and we cannot distinguish the solids because MgO and MgCO3 are both white.

For our lab, we used 3 grams of Mg instead of four becuase the reaction was bright enough with only 2 grams of Mg. Since molar mass of Mg is 24.3g, we used .125 moles of Mg. Since the reaction occurs in excess CO2, instead of O2, and the reaction releases brighter light than the regular combustion reaction of Mg, second reaction occurs more than the first reaction. To approximate the number of moles consumed in the reaction, we use the second reaction. The mole ratio of Mg and CO2 is 2:3, so, .1875 mole of CO2 is consumed. So, (.1875 mol CO2)*(44.0 g/molCO2)= 8.25 g of CO2 is consumed.

0 kJ/mol
CO2 (g)
-393.5 kJ/mol
MgO (s)
-601.8 kJ/mol
C (s)
0 kJ/mol
MgCO3 -1111.69 KJ/mol

To calculate the enthalpy for the first reaction, we use the values given above.
Hf reaction = (2 mol)(-601.8 kJ/mol) + (1 mol)(0) - (2 mol)(0) - (1mol)(-393.5 kJ/mol)= -810.1KJ/mol
For the Second reaction
Hf reaction = (2 mol)(-1111.69 kJ/mol) + (3 mol)(0) - (2 mol)(0) - (3mol)(-393.5 kJ/mol)= -1042.88 KJ/mol

The higher absolute value of the second reaction explains why brighter light is emitted when Mg goes through combustion reaction in CO2.

Since the Hf is negative in this reaction, we can conclude that the reaction is exothermic. The large amount of energy that is released by the reaction is shown in the bright light and the temperature which goes up to 2000 degrees Celcius.

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