CHEMICAL KINETICS

DEFINITION:
Branch of physical chemistry that deals with the rate of reaction, factor affecting the rate of reaction and mechanism of reaction is called chemical kinetics.

RATE OF REACTION:

Change in concentration of reactant and product is per unit time is called rate of reaction.

EXPLANATION:-
When a reaction take place then the reactants change into product so with time concentration of reactant go on decreasing and the concentration of product go on increasing.Thus rate of reaction is decrease in concentration of reactant and increase in concentration of product per unit time. As according to law of mass action rate of reaction is directly proportional to concentration of reactants, so with time rate of reaction go in decreasing by decrease in concentration of reactants.

MATHEMATICAL EXPRESSION OF RATE OF REACTION:-

Mathematically rate of reaction is expressed as:

Rate of reaction equal to change in concentration per unit time.

If we take small interval of time represented by dt and small change of concentration in it represented by dx then:

Rate of reaction=dx/dt.

UNIT OF RATE OF REACTION:-
Unit of rate of reaction is equal to unit of concentration per unit time.

Thus unit of rate of reaction is mol/dm^3/sec.

RATE CONSTANT OF REACTION:-

Suppose a reaction in which a reactant R changes to product P.According to law of mass action rate of reaction is directly proportional to concentration of reactant so the equation will be:

dx/dt = K[R].

Where K is constant called Rate and equation is called rate equation.

SPECIFIC RATE CONSTANT:-

Rate constant K is called specific rate constant if the concentration of each reactants is unity means 1mol/dm^3.

CHARACTER OF SPECIFIC RATE CONSTANT:-

1)It is constant at constant temperature.

2)It changes by the change of temperature.

3)It does not depend on the concentration of reactants, So by changing the concentration of reactants value of rate constant does not change.

VELOCITY OR SPEED OF REACTION:-

Rate of reaction at any time is called velocity or speen of reaction.

TYPES OF REACTION ON THE BASIS ON SPEEN OR VELOCITY OF REACTION:-

There are three types of reaction on the basis on speed or velocity of reaction.

EXTEMELT FAST REACTION:-

Reaction which complete in very short time are called extremely fast reactions.

EXAMPLE:when hydrochloric acid (HCl) react with the sodium hydroxide then sodium chloride ( comman salt) and water will be form and this reaction is very fast reaction, this reaction take 0.000001 sec to be complete, means that the velocity of this reaction is very high.

EXTREMELY SLOW REACTIONS:-

Those reactions which take very long time to complete are called extremely slow reactions.

EXAMPLE:rusting of iron, reaction under earth crust like formation of coal, diamond, natural gas, prtrolium etc.These reactions proceed at very low speed and take very long time to complete like months, years, hundered years and thousand years to complete.

3)MODERATE REACTION:-

Reaction which take moderate time to complete are called moderate reactions.

EXAMPLE: Reaction of organic compounds and non polar covelant compounds.The reaction of formation of ester is the moderate reaction because it take neigther more and nor less time to complete.

MATHODS OF DETERMINING THE RATE OF REACTION:-

PHYSICAL METHOD:-

In physical mathods changes in some physical properties of reactants or products are measure at a particular time.

Some physical mathods are given below:

REFRACTOMETRIC METHOD:-

In this method change of refractive index is measured at a particular time.

SPECTROSCOPIC METHOD:-

In this method absoption ultraviolet or infra red radiations is measured in a particular time.

CALORIMETEIC METHOD:-

In this method change in intensity of colour is measured at a particular time.

CONDUCTIVITY METHOD:-

In this method change in electrical conductivity is measured at a particular time.

PH METHOD:-

In this method change in PH value is measured at a particular time.

POLARIMETRIC METHOD:-

In this method change in optical rotation to the plane polarized light is measured at a particular time.

CHEMICAL METHODS:-

In chemical methods is stopped at a regular interval of time by chilling the reaction vessel at ice bath or adding some specific chemical.Then a smaple is taken from vessel and is titrated  against a suitable reagent and the amount of reactants or products is calculated from observed data.

EXAMPLE:In hydrolysis of ethyl acetate by the reaction:

CH3COOC2H5+H2O<===> CH3COOH+C2H5OH.

Reaction is stopped by chilling the reaction flas and sample is taken out which is titrated against standard solution of NaOH by usinh indicator.From the reading obtained amount of acetic acid formed is calculated.

FACTOR AFFECTING THE RATE OF CHEMICAL REACTION:-

Following affect the rate of reactions.

CONCENTRATION OF REACTANT:-

Rate of reaction is directly proportional to thd concentration of reactants.Thus by increasing concentration of reactants rate of reaction also increases.This is because number of reactant molecules increases so the collisoins between them increase and more molecules gain activation energy and change into product and vice versa.

NATURE OF REACTANTS:-

On the basis of nature of reactants, lower activation energy of reactant more is the rate of reaction.

TEMPERATURE:-

By increasing temperature rate of rate anf also increase due to increase in kinetic energy of reactants molecules.

SURFACE AREA OF REACTANT:-

 catalyst and othet factor affect the rate of reaction.