EXOTHERMIC AND ENDOTHERMIC REACTIONS (article)

Understanding the concept of exothermic and endothermic reactions gives student more insight on various aspect of chemistry such as; Thermochemistry, geochemistry, biochemistry, rate of chemical reactions, chemical equilibrium etc. In this chapter we are going to discuss this reactions separately.


EXOTHERMIC REACTION

Exothermic reaction can be defined as a reaction that gives out heat to the surroundings.

Food for taught: Does that mean mean this reaction does not absorb heat? 

This is the reason why students do not have a good understanding of exothermic reaction. For any reaction to take place, there is always loss and gain of energy. In an exothermic system, the heat needed for the reaction to proceed is less than the heat given out during the reaction.

Therefore, we define exothermic reaction as a reaction that gives out more heat than it absorb.

For example, during the production of ammonia, it has been observed that the reaction is exothermic, that is the energy released is more than the energy applied. This can be explained using the equation below. The reaction below is taking place at a temperature of about 500^oc


N2     +      3H2   ⇆   2NH3           ᐃH= -Ve


As observed from the equation above, 4 moles of reactant is produced 2 moles of product, that is the amount of energy released in the reaction is higher than the energy absorb and also the product formed.

Other examples of exothermic reaction include:

• Reaction of sulphur dioxide with oxygen (contact process).
• conversion of liquid to ice.

In conclusion, we say that for a reaction to be exothermic, the amount of energy absorbed must be less than the amount of energy released and also it must have a negative change in enthalpy.

ENDOTHERMIC REACTION 

Endothermic reaction is a reaction that absorb heat from the surrounding. This does not mean that heat is not giving out during the reaction, that is there is always loss and gain of energy during an endothermic reaction.


So, we define endothermic reaction as a reaction that absorb more heat than it gives out.

For example, the melting of ice to liquid is an endothermic reaction because molecules in the lattice structure of ice which is in a fixed position have been made mobile that is energy has been applied to make the molecules mobile.

in addition, for an endothermic reaction, change in enthalpy is positive. Other examples of endothermic reactions include;

• decomposition of calcium carbonate
• decomposition of phosphorus pentachloride etc

The following point should be noted for proper understanding

• exothermic reaction has negative enthalpy change.
• endothermic reaction has a positive change in enthalpy.
• there is loss and gain of heat in both exothermic and endothermic reaction, but the heat giving out in an exothermic reaction is higher than the heat absorb while the heat absorb by an endothermic reaction is higher than the heat evolve.

NOTE: That the reverse of an exothermic reaction will give an endothermic reaction, while the reverse of an endothermic reaction will give an exothermic reaction.