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Conventional Methods For Kinetic Study

 Conventional Methods For Kinetic Study

Conventional Methods For Kinetic Study
Conventional Methods For Kinetic Study

To get a better understanding of methods of kinetic studies and their classification have a look at a flow sheet diagram for better comprehension.

Conventional Methods For Kinetic Study

To find the concentration of reactants and products at any time 't' is our basic requirement. For that purpose, there are two methods:

i) Conventional method

ⅱ) Modern techniques

i) Conventional method

They are used for the study of slow reactions. Slow reactions take place at moderate rate. The methods used for the kinetic study of these reactions can be classified into two categories:

1) Direct method

2) Indirect method

1) Direct Method:

It is chemical method or discontinuous method. The concentration of reactant or product is determined directly. Samples are taken from reaction mixture at regular interval of time. The concentration of reactant or product is determined by reacting the sample with a standard. For example: precipitation or titration method.

2) Indirect method:

It is physical or continuous method. The concentration is determined indirectly. Progress of reaction is monitored by measuring a physical property of reactant or product at a regular time interval. The property which varies with concentration is used. Depending upon the physical properties there are different types of physical methods:

a) Polarimetry Method:

This method can be used as a physical method for kinetic study of those reactions which involve optically active reactants or product. Keep in mind that optically active compounds are those which rotate the plane of polarized (monochromatic light). The reactions which contain one or more optically active compounds can be monitored by polarimetry method.

Note:

+d dextrorotary = rotate polarized light to right

- l levorotatory = rotate polarized light to left

For example: hydrolyses of sucrose:

C12H22O11 + H2O          →          C6H12O6          +          C6H12O6

Sucrose                                   Glucose (+d)               Fructose (-l)

In start angle of rotation of sucrose in right is maximum. If one product is +d and other -l then, the product will have no effect on angle of rotation. The decrease in angle of rotation with time can be used to monitor the progress.

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b) Conductometry method:

As the name indicates in involves the measurement of conductance. It can be used as a physical method for the kinetic study of those reactions which proceed with change in no. of ions.

No. of ions     ∝  concentration

Conductance      no of ions

conductance      concentration

Change in concentration with time can be used to monitor the progress of reaction. For example:

CH3COOH     +         NaOH             CH3COO-Na+

Weak acid             Strong base           Weak salt

 

NaOH is strong base. So, at start conductance is high then it decreases with time. And at last, conductance of salts is less than base.

Similarly, we have other methods where a specific change in  particular parameter gives a method.

c) Dilatometry:

It is used for those reactions in which change of volume takes place.

d) Monometry:

It is utilized for change in pressure

e) Viscosity:

It is employed for change in viscosity

f) pHmetry:

Applicable for reactions in which a change in pH occurs. 

Next we will discussed the modern techniques or spectrophotometric method

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