A Simple Guide on Chemical Kinetics

Chemical kinetic mainly deals with the rate at which reactions take place. There are three major factors that affect the rate at which reactions take place; these are: catalyst, temperature, and concentration of reactants. We observe that in chemical reactions of any kind, as the reaction takes place, the reactant concentration reduces, and the product concentration increases. The rate of reaction takes into account the rate at which reactants are consumed or the rate at which a reaction produces a product. To understand Chemical Kinetics, we need to understand some basic concepts like average rate of reaction, instantaneous rate and order of reactions.

Average rate of reaction

The rate of reaction observed during a particular fixed time period is called the average rate of reaction. If we take into consideration a reaction

          R→P

Where R is the reactants and P is the product. Then Average rate of reaction becomes:

= decrease in the concentration of reactants/ time taken

 =∆R/∆t

Or

=increase in product concentration/ time taken

=∆P/∆t

Instantaneous rate of reaction

When there is a change in concentration of either products or reactants at a particular instant of time is called the instantaneous rate of reaction. That is, it is the rate of the reaction at particular period of time. Mathematically it can be represented as follows:

Average rate of reaction =∆R/∆t

When ∆t=0, Instantaneous rate of reaction =-∆[R]/∆t=∆P/∆t

Rate Law

It is also known as rate equation and is an equation that relates concentration of reactants in a reaction and the rate of reaction. It is represented as follows:

aA+aB—- cC+dD

Where a,b,c and are coefficients of reactants , the rate is directly proportional to [A]^x[B]^y or

                                               Rate= k [A]^x[B]^y

Where k is the rate constant and x,y are partial orders of reactants.

Order of a reaction

Order of reaction refers to the total concentration of the reactants. This is obtained by adding the powers of reactants concentration. Based on these reactions there may be zero order, first order, second order or third order reaction. The order of reaction can also be a fraction

Zero order reaction– In this the order of reaction is zero, that is there is no change in the concentration of reactants before and after reaction. An example would be the Haber reaction.

First order reaction- In this type of reaction the order of reaction is one, that is there is change only in one reactant’s concentration and the rate reaction is dependent on the concentration of that reactant. An example would be Hydrolysis of aspirin.

Second order reaction- In this the order of reaction is two, that is the rate of reaction is dependent on concentration of two reactants. An example would be decomposition of nitrogen dioxide.

Third order reaction- In the order of reaction is three, that is rate of reaction is dependent on three of the reacting molecules. An example of such a reaction would be the reaction between nitric oxide and chloride.

Influences on rate of reaction

We observe that there are three main factors on which rate of reaction depends on temperature, catalysts and concentration of reactants. When temperature rises reactants have more energy and move quickly, this results in an increase in rate of reaction. Thus, temperature is directly proportional to rate of reaction. A catalyst increases the rate of reaction without getting used up in the reaction. Thus, the rate of reaction increases. Again, concentration of reactants is directly proportional to rate of reaction. The rate of reaction increases with an increase in the rate of concentration of reactants.

Conclusion

Chemical Kinetics is the study of the rate of reactions, rearrangement of atoms, formation of intermediates, and influence of various variables involved in a reaction. The rate of a reaction is the speed at which the reactants react to give products. This rate of reaction is dependent upon three main factors; catalyst, temperature and concentration of reactants. All these factors are directly proportional to the rate of reaction. We also understood that Rate law is also known as rate equation and is an equation that relates concentration of reactants in a reaction and the rate of reaction. Order of reaction is the sum of concentration of reactants. Based on these reactions are categorized into zero order, first order, second order and third order reactions