Many of you may have seen this experiment in school—when you mix plain sugar with sulfuric acid, a violent reaction occurs that leaves you with graphite and a lot of steam, but what’s really happening? First, let’s see what the reaction looks like:
When acid is added to the sugar, the sugar turns black, expands and bubbles while letting off a great deal of steam; however, if both the sugar and acid were at room temperature, where did all of this newfound heat come from? The answer is all in the chemistry of the situation.
The reaction in the video is called an exothermic (from Latin, ex meaning out, and thermein to heat) reaction. More precisely it is a dehydration reaction (a class of elimination reactions). The formula here is:
C12H22O11 (Sugar) + H2SO4 (Sulfuric Acid) → 12C (Graphite) + 11H2O (Water) + A mix of water and acid
As you can see, the reaction is called a dehydration reaction because water is lost (by the sugar), and to balance the equation it must be transferred (the acid is hydrated—“dehydration reaction” is a bit of a misnomer since even though the sugar is dehydrated, that water is “gained” by the acid and doesn’t just disappear). This water transfer is what creates the heat in the reaction. The water and the acid create very strong hydrogen bonds, which in turn give off energy in the form of heat. When bonds break and form energy is often transferred, some bonds are easier to form than others, so often the excess energy that doesn’t go into making bonds, gives off heat energy.
In this reaction when hydrogen and oxygen are taken away from sugar, all we have left is carbon (the black stuff). The water is separated out and escapes mostly as vapor, and like most reactions, not everything is able to react, so we often have unreacted acid evaporating with the water (and making this a very dangerous experiment, have a teacher show you if you are curious, or just enjoy the video).
This lab is often demonstrated in school because it is a good introduction to many general chemistry topics including bond energies, acids, and more.
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