Hope this Helps:
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.
There are many forms of energy:•electrical
•mechanical
•chemical
but all forms are ultimately converted into heat. So it is convenient for biologists to measure energy in units of heat. The unit we shall use most often is the kilocalorie (kcal): the amount of heat needed to warm 1 liter of water 1 degree Celsius.
Link to discussion of the international system of units used in scientific work.
The kilocalorie is also the unit used to describe the energy content of foods. It is the "Calorie" used on food labels.
It takes a net of 118 kcal to decompose 2 moles of H2O into its elements. Actually it takes more than 118 kcal to decompose the water into its atoms, but some of the energy is given back as the atoms immediately bond together to form molecules of hydrogen and oxygen.
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-O 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.
Where has the energy gone?
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.
The conversion of free energy to work is never 100% efficient. As you contract your muscles, much of the free energy of your fuel is given off as heat. It is no longer free; there is no way you can harness the warmth of your muscles to accomplish biologically useful work.
A change in free energy is depicted by the letter G preceded by the Greek Delta (Δ).
By convention, we indicate the storage of free energy with a plus sign. So, our reaction is expressed:
2H2O -> 2H2 + O2, Δ G = +118 kcal.
You may have had a chemistry professor ignite a mixture of hydrogen and oxygen. It not, simply accept my word that the result is a dramatic explosion. The equation for this chemical reaction is the reverse of the one we have been studying and is expressed as
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.)
We express the fact that energy came out of the reacting system by putting a minus sign before Δ G.
2H2 + O2 -> 2H2O, Δ G = -118 kcal.
These chemical reactions may not seem very "biological" to you, but in fact, they are good models for the reactions at the very heart of life itself.
