Chemical Kinetics

Chemical Kinetics involves the study of kinetics of chemical reactions. It basically deals with the speed of reaction - how to measure it and how it can be varied.

Importance

When it comes to entrance exams, Chemical Kinetics form a very small part of the questions asked. Not more than one question was asked in IIT-JEE/JEE Advcanced except for one year when it was 2 questions. The same applies for AIEEE/JEE Mains.

Despite having a very low weightage, it is important to concentrate on this chapter as most of the questions are easy and not too complicated, even for JEE Advanced. Chemical Kinetics is generally easy scoring if basics are understood.

Rate of Reaction

Consider the case of two reactions : rusting of iron and burning of coal explained in this video (Upto 01:00) to understand how the speed of reactions affect the nature of reaction

What do you mean by rate of chemical reactions? How is it measured?

Check this video out to find out.

The rate of a reaction can be talked in terms of the reactant or products.

Consider a reaction

!!\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ aA + bB -> cC + dD!!

Rate of disapperance of A =!!-(d[A])/dt!!

Rate of disapperance of B = !!-(d[B])/dt!!

Rate of apperance of C = !!(d[C])/dt!!

Rate of apperance of D = !!(d[D])/dt!!

We can write from the stoichiometry that

!!-(d[A])/dt xx 1/a = -(d[B])/dt xx 1/b = (d[C])/dt xx 1/c = (d[D])/dt xx 1/d!!

The Rate Equation or Rate Law

Rate equation or Rate Law is a relation connecting the rate of a reaction to the parameter used to measure the rate. This could be concentration for aqueous reactions and partial pressure for gaseous reactions.

The rate of a reaction is generally expressed as follows

!!Rate = k*[Reac\t\ant]^n!!

where k = rate constant or specific rate constant

n = order of reaction

[Reactant] = concentration of reactant

In general for a reaction !!A -> P!!

!!Rate = k*[A]^n!!

For!!A+B->P!!

!!Rate = k*[A]\^m* [B]^n!!

The powers in the rate equation is what we call order and is determined experimentally.

The rate of a reaction can be talked about in terms of both the reactants and products, but the rate law or rate equation contains only the reactants and not the products on the right hand side. So it is best to write the rate of reaction in terms of rate of disappearance of reactants when writing the rate law.

How we arrive at rate laws for various reactions will be discussed later.

Order and Molecularity

Order and molecularity are two important terms in kinetic studies of reaction.

What is order of a reaction? How is it important?

To find out see this video (From 07:07 till end)

What is molecularity of a reaction? How is it different from order?

Read this page to understand.

For some additional info on all that was covered till now check this out.

Half Life

The time taken for the concentration to fall to one half of its original value is called half life.

Zero order Reactions

Check out this page to learn more.

First Order Reactions

All radioactive reactions are first order in nature. To know more about such first order reactions check this page.

Second Order Reactions

Such reactions are also quite common and more about them can be found here

Check this page to find some common graphs of various order reactions

Factors affecting the rate of reaction

It is often believed that our lives have a lot of parallelism with science. This video talks about how to increase or decrease the rate of reactions.

A good explanation with real examples is given here (Upto 03:46)

Another video also shows how the various factors affect rate from the point of collission.

Arhennius Law and Collission Theory

The temperature dependence of rate of reaction is given in the form of an equation called Arhennius Law. One of the most common theories for reactions is collission theory

Check this page to get a clear idea on what collision theroy says and the significance of Arhennius Law.

Conclusion

We hope you know about rate of a reaction, how its is expressed and the factors that affect it. Also you are expected to know the different order reactions and their rate laws.

You can always check how much you know "Chemical Kinetics" by taking our 20 min test on eagerbug.com

Good Luck!!

Get it on Google Play