Chemical Reactions and Energy

[Bernard NebelKeymaster]

A most basic organic chemical reaction is the burning of carbon (charcoal is almost pure carbon). It is modeled by attaching the two oxygen atoms of a molecule of oxygen gas to an atom of carbon with double covalent bonds.

C.        +          O₂         —>        CO₂.           +       energy

               C      +             O=O      —->     O=C=O         +       energy    

Carbon.            Oxygen  yields.  Carbon dioxide          energy 

Stress the energy that is given off by this burning charcoal or any other organic compound. The energy is experienced by the heat and flame of a charcoal fire, but cannot be modeled with the kits.  

Where hydrogen atoms occur in an organic molecule, the burning (oxidation) includes the hydrogen atoms attaching, two to each oxygen atom, to make a water molecule. For example:

CH₄        +         2O₂          —>       CO₂     +    2H₂O   +   energy

  Methane            oxygen          carbon dioxide     water       energy                       

          Natural gas

Encourage kids to model additional (more complex) reactions, e.g., the oxidation of sugar (glucose).                   

C₆H₁₂O₆    +    6O₂   —>   6CO₂   +.  6H₂O.    +     energy

Note that a chemical reaction can be written and modeled in a backwards direction as well as the forwards direction shown. The convention is to write a chemical reaction in the direction that it goes spontaneously (by itself). The rule is: A chemical reaction goes by itself in a direction that releases energy. The release of energy is most commonly observed as a release of heat and sometimes light, i.e., the flame of burning. 

Yes, a reaction may be “pushed” in the opposite direction but this requires the input of energy. Energy-wise chemical reactions are like the bolder rolling down a hill. In rolling down the hill, high potential energy is covered to kinetic energy (heat). Yes, the bolder can be moved back up the hill but only with the input of energy from another source. This release of or requirement of energy by chemical reactions cannot be overemphasized. 

A simple reaction that can be used to demonstrate this is he following. Hydrogen gas (H₂) is highly flammable, explosively so. This is to say, the chemical reaction of hydrogen gas combining with oxygen is like the bolder rolling down the hill, it releases energy. 

           2H₂    +        O₂        —>        2H₂O   +   Energy

Hydrogen gas       Oxygen                  water

On the other hand, an electric current conducted through water drives the reaction in the opposite direction (pushes the bolder back up the hill).

Electrical current   +   2H₂O  —>     2H₂.     +.     O₂

This reaction is known as the electrolysis of water. It is an easy reaction to demonstrate as shown in the following video. 

When enough gas has accumulated in the tubes (about half the tubes) they can be tested by emptying the remaining water and inserting a match with the flame blown out but still with a glowing ember. In hydrogen, you will hear a pop as the hydrogen ignites and burns with a “mini explosion”. In oxygen the glowing match will burst into flame. 

Summarizing, here is the rule that will enable you to predict whether any organic chemical reaction (one involving the carbon-containing molecules) will yield energy and go spontaneously, or require energy input. If  C—C,  C—H, and/or O = O bonds are broken to yield carbon dioxide and/or water, the reaction will give off energy and go spontaneously. Conversely, if  C = O bonds of carbon dioxide and/or the O—H bonds of water are broken to make to make product, energy input will be required.

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