CHEMICAL KINETICS
  
 
 
CHEMICAL KINETICS
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The branch of chemistry, which deals with the "RATE" and the SPEED at which a chemical reaction occurs, is called "CHEMICAL KINETICS". The study of chemical kinetics, therefore, includes the rate of a chemical reaction and also the factors that influence or alter or control the rate of chemical reactions. In chemical kinetics we study how molecules react, bond breaking and new bond formation.
RATE OF A CHEMICAL REACTION
  
"It is defined as the quantity of a reactant consumed or the
quantity of a product formed in unit time."
In other words:
  The conversion of the number of moles of reactants into products in unit time. The rate of reaction is not constant through out the activity but decreases with time due to decrease in the concentration of reactants.

MATHEMATICALLY:
VELOCITY OF REACTION
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Since the rate of reaction is not constant through out the reaction, therefore, we can not determine the uniform rate of reaction precisely.

Thus velocity of reaction may be defined as the rate of reaction at a particular given moment i.e. at a specific time.

If we consider a very small interval of time dt in which the change in concentration dx is taken to be nearly constant, then velocity of reaction is given by:
Velocity of reaction is actually the instantaneous rate of reaction.
RATE EXPRESSION & RATE CONSTANT
 
Consider a general reaction:
According to the law of mass action, rate of reaction is directly proportional to active mass, hence for the above reaction:
This expression is called rate expression and K is called rate constant or velocity constant.
CHARACTERISTICS OF RATE CONSTANT
  
(i) It has a fixed value at a particular temperature.
(ii) Value of K varies with temperature.
(iii) Value of K remains unaltered with the change in concentration of reactants.
ORDER OF REACTION
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"The order of reaction is defined as the sum of all the exponents
of the reactants involved in the rate equation."
It should be noted down that all the molecules shown in a chemical equation do not determine the value of order of reaction but only those molecules whose concentrations are changed are included in the determination the order of a reaction.
In other words:

"The number of reacting molecules whose concentration alters as a
result of chemical reaction is termed as the order of reaction."

For example:                                                                                                                                                                  
                              2NO + O2 ® 2NO2                
                            dx/dt = k[NO]2[O2] , the reaction is of third order as 2 + 1 = 3
For a reaction maximum order is three and the minimum is zero.
FIRST ORDER REACTIONS
  
The reaction in which only one molecule undergoes a chemical change is called first order reactions. Example:
                     N2O5 ® 2NO2 + ½ O2
SECOND ORDER REACTIONS
  
The reaction in which two molecules undergo a chemical change is called second order reactions. Example:
                     2CH3CHO ® 2CH4 + 2 CO
THIRD ORDER REACTIONS
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The reaction in which only three molecules undergo a chemical change is called third order reactions. Example:
                     2NO + O2 ® 2NO2
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