Experiment No. 2: Coupled Reactions
Institute of Chemistry, University of the Philippines, Diliman, Quezon City 1101 Philippines
Results and Discussions
In this experiment, two setups involving magnesium pieces were executed in succession; the first one had ignited pieces of magnesium left burning between two slabs of dry ice with vent holes; the second involved lighting up magnesium strips openly in the air. The following table summarizes the observations obtained from both setups:
Table 1. Observations from the two setups
The slabs of ice lit up while the magnesium strips burned between them. Inspecting the setup after the strips ceased ignition, black and white materials were formed in the location where the Mg strips were burned. 2
The magnesium strips turned white when the setup ceased burning.
In Setup 1, the formation of matter after the experiment can be explained by the following exothermic reaction: Mg(s) + CO2(s) MgO(s) + C(s) 
Here, the black matter that formed was the solid carbon, while the white one being the magnesium oxide.
The reaction that follows summarizes what happened in Setup 2: Mg(s) + O2(s) MgO(s)  The white product that was observed is magnesium oxide.
There are three ways to obtain a product from a nonspontaneous reaction: (1) Changing reaction conditions to make the given reaction spontaneous and (2) through electrolysis. The third is exhibited by this experiment: by “coupling” two reactions. A coupled reaction is a system wherein a nonspontaneous reaction is combined with a spontaneous one in order to obtain a product from the latter process.
The nonspontaneous process in the experiment is the breakdown of CO2 to oxygen gas and solid carbon. By adding up the ΔG° values of both reactions, a negative value will result, indicating a spontaneous reaction.
ΔG° for MgO(s) : -569.4 kJ...
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