Chemical Reactions - Part III

3. Energy changes in a chemical reaction
The chemical reaction on a microscopic level is nothing but breaking and making bonds between the reactant compounds involved in the reaction. It has been observed that a stable compound has mass (or energy) less than the sum of masses of each of the atoms forming the compound. The missing mass goes to forming the bonds between the atoms. The missing mass is called the binding energy[1] of the compound. All chemical reactions proceed in the direction of attaining greater binding energy. Greater the binding energy, more difficult it is to break the compound and hence it is more stable. If you plot energy (or mass) scale vertically you will see that a compound will be lower on the same scale. Lower down the compound is, tighter is the binding.   

The adjacent figure is not drawn to scale but depicts what happens when a free Oxygen atom combines with free Hydrogen atoms.   We have studied in Physics (see the chapter on “Work, Energy and Power”) that the unit of energy is joule. Bond energy is measured in kilo-joules (kJ) per mole of the substance.  Mole is a measure of the mass involved in chemical reactions, since it is not possible to calculate energy changes on atomic levels. We will study more about moles later in the chapter.

1kJ  = 1000joules

There are two basic type of reactions, when energy changes are considered. They are exothermic reactions and endothermic reactions. Exothermic reactions release energy as the reaction proceeds. For endothermic reactions, the energy has to be supplied to make the reaction possible. Thus, exothermic reactions will occur spontaneously when the reaction substances are brought close together. In case of endothermic reactions, the spontaneity is not present as heat has to be supplied. Examples given below will make these concepts clear. It has to be noted here that heat changes in reactions are generally available in standard books and tables; these are internationally accepted and used.

Examples of exothermic reactions :

1.         H   +   H           H₂        +    435.8 kJ

When two H mole atoms come together to form a H₂ molecule (1mole of molecule), 435.8kJ per mole of energy is released.

2.         C   +   H₂           CO₂     + 395 kJ     

When 1 mole of C and  1 mole of H₂  interact, they produce 1 mole of CO₂  molecule and give off 395 kJ of energy. This equation shows what happens when coal (carbon) burns in air, coal gives off energy as heat. It is because of this reaction, coal is used as a heating substances in many instances.

Examples of endothermic reactions :

1.         N₂        +          O₂        +          heat            2NO

If 1 mole of N₂ molecule reacts with 1 mole of O₂ molecule then heat of 184 kJ has to be supplied to initiate the reaction to give 1 mole of NO molecule.  This means that the bonds between N    N and O    O are so strong that they do not break easily. N₂  has triple covalent bond between the two N atoms. O₂ has a double covalent bond. Thus energy has to be put into the reaction to break the strong bonds. Thus the reaction is a good example of an endothermic reaction.

2.         CaCO₃     +     heat            CaO     +     CO₂

The amount of heat given out or absorbed in a chemical reaction is called the heat of reaction.

There is another definition that comes into use while discussing chemical reactions. This is heat of formation. The heat of formation is a little different from of heat of reaction. Heat of formation comes into picture when the basic compounds or molecules are forming.

The amount of heat given out or absorbed when one mole of the constituent atoms is reacting to form 1 mole of the molecule, is called the heat of formation. The following examples will show how to calculate heat of formation of a compound.

1.      Let us calculate the heat of formation for H₂O

The reaction involved is  :  2H₂     +       O₂          2 H₂O   +    heat

2 moles of hydrogen molecule is interacting with 1 mole of oxygen molecule to give 2 moles of water molecule

Two   H    H   bonds are being broken =  2 x 435.6  kJ/mole is given out.
One   O    O bond is being broken   = 494.6 kJ/mole is given out.

The bond energy of the O    H  bond in H₂O = 462.2 kJ/mole.
A water molecule has 2 O    H bonds, 
hence the bond energy stored in 1 mole of H₂O molecule is = 2 x 462.2 kJ/mole  =  924.4 kJ/mole

If we put in all the energies of bond formation in the equation above, we will see that the left hand side of the equation adds up to 1365.8 kJ. The right hand side of the equation comes to be 924.4 kJ. The heat released for making 2 moles of H₂O molecule is 483 kJ. Thus for making 1 mole of H₂O molecule, 241.5 kJ/mole is released. This is the heat of formation of water molecule.

2.   Let us calculate the heat of formation for  NaCl.

The reaction involved is :                2Na     +   Cl₂      2NaCl    +   heat

Bond energies involved are Na  Na  = 109 kJ/mole,  Cl    Cl   =   239 kJ/mole,

Na   Cl  =   769.1 kJ/mole.

Besides bond breaking, Na atoms will also give off their excess electron. The energy absorbed for ionization of Na to Na⁺ = 493.3 kJ/mole. Similarly the energy released by Cl atoms to become Cl^- negative ions is equal to 348.2 kJ/mole.

Thus the energy consideration on the left hand side of the equation for bond breaking and ionizing processes is

2 x 109  +  239 + 2 x 493.3 -  2 x 348.2   =   747.2 kJ

The right side of the equation has energy 2 x 769.1 = 1538.2 kJ
The total energy comes out to be 1538.2-747.2 = 791 kJ for making 2 moles of NaCl. For making one mole of NaCl the energy will be half of this = 395.5 kJ/mole.

Thus the heat of formation of NaCl is 395.5 kJ/mole.

Another term used for indicating heat involved in a chemical reaction is the heat of combustion. The heat released on completely burning 1 mole of a compound or substance in air (or oxygen) is known as the heat of combustion of the compound or substance. If the heat of combustion is high, the compound or the substance is a good material for burning. For example coal is a good substance for burning because of the following reaction :

C   +   H₂         CO₂     +    395 kJ

[1] Einstein’s famous equation E = mc², the missing mass becomes the binding energy of the compound or the molecule.

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