Bond Energy

2H₂O -> 2H₂ + O₂

• it takes energy to break chemical bonds
• energy is released as chemical bonds form
• but in this case the energy released is less than the energy needed.

Bond Energy

For any particular chemical bond, say the covalent bond between hydrogen and oxygen, the amount of energy it takes to break that bond is exactly the same as the amount of energy released when the bond is formed. This value is called the bond energy.

• electrical
• mechanical
• chemical

The kilocalorie is also the unit used to describe the energy content of foods. It is the "Calorie" used on food labels.

Let's look at the numbers.

• The bond energy of the H-O bond is 110 kcal.
• The bond energy of H-H bonds is 103 kcal.
• The bond energy of the O=O bonds is 116 kcal.
• The decomposition of 2 molecules of water requires breaking 4 H-H bonds and thus the input of 440 kcal.
• The formation of 2 moles of hydrogen yields 206 kcal (2 x 103).
• The formation of 1 mole of oxygen yields 116 kcal.
• The difference between
  • the energy released (206 + 116 = 322 kcal) and
  • the energy consumed (4 x 110 = 440 kcal)
• gives the net energy consumed - 118 kcal.

It is now chemical energy stored in the bonds of the hydrogen and oxygen molecules. The energy stored in this reaction is called free energy because it is still available to do work. It is useful to have a symbol for free energy, and we shall use the letter G (in honor of Josiah Willard Gibbs who developed the concept of free energy).

What is free energy?

It is energy that can be harnessed to do work. The water stored behind a dam has free energy. When allowed to fall through a turbine, it can generate electricity (another form of free energy).

But for biologists, the most interesting form of free energy is the energy stored in chemical bonds. It, too, can be harnessed to do work. When you lift a weight, you are using the free energy stored in the bonds of food molecules to run a machine - your skeletal muscles.

A change in free energy is depicted by the letter G preceded by the Greek Delta. Because this symbol is not available in HTML, I shall simply use the word Delta.

2H₂O -> 2H₂ + O₂, Delta G = +118 kcal.

2H₂ + O₂ -> 2H₂O

And, as the explosion suggests, this time a release of energy occurs. In fact, the free energy change is once again 118 kcal. This is because it took only 322 kcal to break the H-H and O=O bonds, and 440 kcal were liberated by the 4 moles of H-O bonds that were formed. (The igniting spark provided the initial input of energy; the surplus from the reaction then provided what was needed to get all the other molecules to react.)

2H₂ + O₂ -> 2H₂O, Delta G = -118 kcal.

Cellular respiration

C₆H₁₂O₆ + 6O₂ -> 6CO₂ + 6 H₂O, Delta G = -686 kcal.

Photosynthesis

More on Bond Energies

There are several contributing factors but usually the most important is the difference in the electronegativity of the two atoms bonding together. Thus the energies of bonds between atoms of substantially different electronegativities tend to be high, e.g., the 110 kcal of the H-O bond. Another important example is the bonds between oxygen and carbon atoms in carbon dioxide, CO₂. The carbon atom shares two pairs of electrons with each of the oxygen atoms, and each of these double bonds has a bond energy of 187 kcal (or about 93 kcal for each shared pair of electrons). The larger the bond energy, the more energy is needed to break the bond. Thus bonds between atoms of differing electronegativities are apt to be very strong and stable.

On the other hand, the energies of bonds between atoms of similar electronegativity tend to be smaller. Of course, where the atoms are the same (e.g., O=O, C-C) there is no electronegativity difference and we are not surprised to find lower bond energy values. Each of the shared electron pairs in the oxygen molecule is worth 58 kcal for a total of 116 in each molecule, as we saw above. The bond energy of the C-C bond is 80 kcal. There is a slight difference in electronegativity between carbon and hydrogen, and a bond energy of 98 kcal. The bonds with smaller bond energies are, by definition, easier to break. Thus these bonds are weaker and less stable.

We should also note that the energy needed to break a particular bond, e.g., between carbon and oxygen, may also be influenced by the nature of the other atoms attached to the ones we are interested in. Thus the C=O double bond in carbon dioxide (O=C=O) has a bond energy of 187 kcal, whereas when this bond is found as part of a larger molecule, the value is closer to 170 kcal. Because of these variations, we speak of average bond energies. This table gives average bond energies for some of the bonds that are always being broken and formed in biochemical processes.