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.
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
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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