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Heat Changes During Chemical Reactions, Enthalpy of reaction , deltaH
    Revised August 22, 2012 all rights reserved Dr. Walt Volland

Energy changes in chemical reactions result from the breaking and forming of bonds.

The energy needed to break a bond is called the bond-dissociation energy, DE. In a reaction if the energy needed to break bonds is less than the energy produced by the bonds formed, the reaction will give off energy.

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.




On the other hand the relatively low bond dissociation energy for oxygen is one reason why so many minerals are combinations of metals and oxygen. Oxygen atoms form a bond with metals easily. We can see this when magnesium and oxygen react

An example is topaz Al2SiO4F2 (OH)2 which is 43% oxygenAnother is the mineral gypsum CaSO4• 2 H2O which is 55.76 % oxygen. click to see more mineral data


Covalent bond

Bond DE   kcal/mol

Bond DE kjoules /mol

Covalent bond

Bond DE kcal/mol

Bond DE kjoules /mol


226 strong bond

945 strong bond








36.8 very weak

154. very weak





































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.

teflon ball and stick


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.

animation exothermic


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
cold-pac-one use disposable





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, deltaH.

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.

deltaH = (deltaH for bonds formed in products) + (deltaH for bonds broken in reactants)


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

  H2(g) + F2(g) ---> 2 HF(g)

deltaH = 2 mol x (H:F bond energy) + 1 mol x (H:H bond energy) + 1 mol x ( F:F bond energy)

deltaH = 2 mol x (-135.2 kcal /mol) +1 mol x (103 kcal /mol) + 1 mol x (36 kcal/mol)
deltaH = 2 x (-135.2 kcal /mol) + 1 x (103 kcal /mol) + 1 (36 kcal/mol)

deltaH =  -270.4 kcal + 103 kcal + 36 kcal  = -131.4 kcal

EXOTHERMIC REACTION: the minus sign means energy is lost to the outside

The reaction releases energy. It is exothermic -131.4 kcal

Online Introductory Chemistry

Dr. Walt Volland, All rights reserved, 2001-2012
last modified August 22, 2012