Th re are two laws of thermodynamics:

  1. First law of thermodynamics

It i also called the law of energy conservation. It states that energy can neither be created nor destroyed, it can only be transformed from one form to another. For example, electrical energy passes through a hot plate to produce he t energy. Or it can be transformed from potential to kinetic energy. For example a squirrel eats a nut and then uses this energy to climb a tree. But en gy can never be lost or created. Thus, the total amount of energy in the universe remains constant.

  1. Second law of thermodynamics

It tates that all objects in the universe tend to become more disordered an • that the total amount of disorder in the universe is continually in erasing. The measure of this degree of disorganization is called entropy. For example, natural gas is burnt in a stove. Potential chemical energy is stored in the bonds of the gas molecules. This energy is converted into light (the blue fia e) and heat. Some of the heat energy can be used to boil water on the stove.

S e energy is lost in the kitchen. This_ energy is no longer available to do work. Th s unusable energy increased entropy.


T e minimum amount of free energy to start a chemical reaction is called activation energy. Most chemical reactions require an input energy to start. For example a match is lit. Its heat energy is used to start wood burning in a fin place. Input energy must break existing chemical bonds in a chemical reaction. Then this energy form new bonds. This input energy is called activation e ergy in thermodynamics. There are two types of reactions•

(a Exergonic reaction: The reactions in which net release of energy take place is called exergonic reaction. The reactants contain more energy than the products. The amount of this excess energy is called free energy. This energy is greater than the activation energy required to initiate a reaction. It is released into the environment. These reactions occur spontaneously and are called exergonic.

(b) Endergonic reactions: The reactions which absorb energy are called endergonic reactions. The product contains more energy than the reactants. So they require a greater input of energy from the environment than is released. These reactions do not occur spontaneously. So they are called endergonic.




Fig: Activation energy

Reaction rate and catalysis

The amount of reactant substance converted to product substance in a given period of time is called reaction rate. The reaction rate of an exergonic reaction does not depend on the released energy. Its reaction rate depends on the amount of activation energy required for the starting of reaction. A few molecules succeed in overcoming the initial energy hurdle. Therefore, larger the activation energy of a chemical reaction, the more slowly the reaction occurs bonds are stressed, they may break more easily. The lowering of the activation of energy of a reaction is called catalysis.


Any substance that performs catalysis is called a catalyst. A catalyst is a substance that accelerates the rate of a chemical reaction. The reaction proceeds at a lower environmental temperature by decreasing the activation energy. But catalyst itself is not used up in the reaction. The catalysts are always enzymes in cells.


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