Bond dissociation energy, DE, is the amount of energy needed to break a covalent bond.

The bond formation energy is the amount of energy released when a covalent bond is formed. These energies are different for each combination of bonds. The weakest bonds have the smallest values. The numbers here are in kilocalories per mol and kilojoules per mol. The smaller the bond energy the more reactive and weaker the bond.

Notice the high bond energy, 945 kjoules per mol, for the nitrogen triple bond in N₂.   The bond energy for oxygen, O₂ , is 496 kjoules per mol which is almost half of the nitrogen bond energy.

Nitrogen does not react as readily as oxygen, O₂. Oxygen has a bond energy that is almost half the nitrogen bond energy. The high bond dissociation energy for nitrogen is one reason why it is the dominant gas in our atmosphere.

Covalent bond

Bond DE   kcal/mol

Bond DE kjoules /mol

Covalent bond

Bond DE kcal/mol

Bond DE kjoules /mol

N:::N

226 strong bond

945 strong bond

O::O

118.4

496.

N::N

100

418

F:F

36.8 very weak

154. very weak

H:H

103

431

Cl:Cl

58

242.

H:F

135.2

565.9

Br:Br

46.2

193.

H:Cl

102.

427.

I:I

35.6

149.

H:Br

86.8

363.

C:H

98.8

413.

H:I

70.6

296.

C:C

83.0

347.

S:S

63.6

266.

C:F

116.

486.

In the table you can see why oxygen in the air reacts to "burn" substances rather than nitrogen. The bond energy for oxygen is very low. The triple bond in N₂ is almost twice as strong.

This means it is easier to break the bond in oxygen than it is to break the bond on N₂. The weaker the bond the more reactive the bond. 

Weak bonds are easily broken.  Strong bonds are more stable. Teflon contains C:C and C:F bonds. The C:F bonds are very strong.   The C:F bonds physically protect the C:C bond in the polymer molecule.  The C:F bonds wrap the carbon skeleton and shield the C:C bonds from attack. The C:F bonds are hard to break. This is why teflon on cookware doesn't react with food.  In fact teflon is relatively inert and impervious to reactions.

Exothermic reactions release energy. The energy released by the bonds formed in products is greater than the energy needed to to break bonds in reactants.

Endothermic reactions need energy from the outside.

The energy required to break the old bonds in reactants is more than the energy released by the new bonds formed in products.  Cold packs use an endothermic reaction to cool the surroundings and absorb energy. It involves s a dissolv. A common practice is to simply dissolve ammonium nitrate in water. This is not a true reaction because making  solution is a physical change, click to see details of a cold pac patent

Every reaction can be written in the reverse sense. The energy changes for the forward and reverse reactions have the same numerical value but opposite signs.

The energy change that accompanies a reaction is called the heat of reaction. This is also called the enthalpy of reaction. The symbol for enthalpy is "H". A change in enthalpy is a delta H, H.

The sign of the enthalpy change is negative (-) for exothermic reactions. The reaction loses energy to the outside surroundings. This means the molecules, atoms and bonds "lost" energy to the surroundings.

Endothermic reactions have a positive (+) sign for the enthalpy change. The reaction gains energy from the outside surroundings. This means the molecules, atoms and bonds "gain" energy from their surroundings.

The Law of Conservation of energy:

The law of conservation of energy says the amount of energy in the universe is constant. Energy is only transfered in chemical reactions .

This means the amount of energy that exists today is essentially the same amount of energy that existed thousands and millions of years ago. The distribution of energy is changing. The pockets of high energy intensity like stars are gradually cooling off. Billions and billions of years from now the hot spots will be cold. The universe will have a nice uniform temperature. When all the energy in the universe is uniformly spread around the average temperature will be 1 or 2 degrees Kelvin. That is -270 ^oC. Some people call this "heat' death. Pretty cold, huh?

Covalent bonds store energy. This is a form of potential energy. The energy change for a reaction equals the energy released when bonds are formed in products plus the energy needed to break bonds in reactants. The total is negative for exothermic reactions and positive for endothermic reactions.

Example:

What is the energy change for the reaction between hydrogen and fluorine? This was considered by NASA as a fuel system for rocket boosters.

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