Delta G

Energy can be used to do work. Scientists call this useful kind of energy "free energy" and represent it with the letter "G" (named after the scientist Josiah Willard Gibbs, who first thought of this concept).

Free energy, that is energy available to do work, can be found, for example in the water trapped behind a high dam. Although no work is being done by the water when it is stationary, the moment it is allowed to move downwards and through an electrical turbine generator the "free energy" in the trapped water is released, and converted into the form of electricity.

Energy is also stored in the covalent bonds holding atoms together in molecules. This energy, like the water behind a dam, is not obvious when the molecules are not reacting, but when they collide with one another, break apart and then reform into new, different molecules, this "chemical reaction" is always accompanied by a change in the amount of free energy.

A change in the amount of free energy in the various molecules before and after the chemical reaction is called the "change in free energy" and is given the term "delta G" (usually the "delta" is written using the Greek letter which looks like a small triangle).

Bringing atoms together

For example, when molecular hydrogen (H₂) is reacted with molecular oxygen (O₂) to produce water, several things happen all at once.

• molecules of hydrogen crash into molecules of oxygen with great force
  
  
• the energy of this collision (mechanical energy) is used to tear apart the covalent bonds holding the hydrogen atoms together and the oxygen atoms together
  
  
• for a tiny fraction of time the molecules don't exist - but large amounts of the free energy stored in the original covalent bonds is given off
  
  
• almost instantly new molecules form. Hydrogen atoms combine with oxygen atoms to form a new molecule, water (H₂O)
  
  
• free energy is immediately reabsorbed to form the new covalent bonds that hold the hydrogen atoms to the oxygen atoms in the new water molecule.
  
  
• the difference in the amount of free energy given off as the original, reacting molecules fly apart, and the amount of free energy absorbed as the new covalent bonds form is left in the environment, often as light and/or heat. It could also be trapped and used to do something useful, such as move the engine in a car!
  
  

This whole process is usually represented by a chemical formula with describes the reaction and also gives the difference in free energy observed, thus:

[Note: free energy is measured in kilocalories (kcal)].

When the skeletal muscles in the human body contract, to lift a weight, for example, energy is take from the chemical bonds in a food molecule and converted into the work of raising the weight. Since this process is never 100% efficient, some heat is always given off, which is why the body gets warm during exercise.

The table below gives some examples of different covalent bonds and amount of energy found stored in each one.

There are several reasons why different covalent bonds have different amounts of stored energy, but one contributory factor is the difference in electronegativity between the atoms being bonded.

When the difference in elecronegativities is high, such as that between hydrogen and oxygen in the water molecule (H-O-H), then the bond energy also tends to be high (about 110 kcal in this case).

If there is no, or little difference in electronegativities between the atoms in a covalent bond, then the bond energy holding them together is lower. A bond between two carbon atoms, for example (C-C) is only 80 kcal. Or between hydrogen and carbon (two atoms with very similar electronegativities) the bond energy is about 98 kcal.

The larger the bond energy, the higher the amount of external energy it will take to pull the atoms apart, and thus the stronger the force holding the atoms together. Molecules, like water, where all the bond energies are high, are very stable molecules and very hard to break apart.

In most, natural, spontaneous chemical reactions, therefore, when molecules with low bond energies crash into one another it does not take much energy to rip them apart. However, when they reform into newer molecules, they tend to form those kinds of molecules that are more stable. These stable molecules have a much larger bond energies holding them together.

In chemical reactions is it easy to get molecular hydrogen to react with molecular oxygen (both molecules held together with lower energy covalent bonds) - all it takes is a spark to get it going!

But when the new molecules form, they consist of hydrogen bonded to oxygen in the form of water. The bond energies in a water molecule are much higher, and thus the molecule is much, much more stable and cannot easily be pulled apart again.