Thursday, 27 September 2012

Origin of Charges on Colloidal Particles (COLLOIDS)...!

Origin Of Charges On Colloidal Particles (Colloids)

  • As we know that in Colloids the Dispersed phase remains suspended in Dispersion medium.
  • The Colloidal system is a two phase system and very stable.
  • It does not grow in size or lead to precipitation.
  • This remarkable stability of Colloids is due to the presence of a small quantity of electrolyte in the medium and presence of charge on the surface of colloids.
  • The colloidal particles carry an electric charge on it either positive or negative.
  • Due to the presence of similar type of charges, the colloidal particles repel one another and do not combine together to form larger particles.
  • This keeps them dispersed in the medium and the colloids remains stable.
  • The type of charge present on the Colloidal particles can be detected by Electrophoresis experiment.

The various factors responsible for the origin of charges on the
colloidal particles are as follows :-

1) Dissociation of Molecules :-
The charge on the colloidal particles can be generated by the dissociation of molecules.

For eg :-
Soaps are sodium salts of fatty acids. In solutions they dissociate to give rise to ions as shown in the reaction below :-
R-COONa -----> RCOO- + Na+

The RCOO ions consists of two parts,long hydrocarbon chain ' R ' which is hydrophobic i.e water repelling and the polar group i.e ' COO ' which is hydrophilic i.e water loving.
As they form ions their hydrocarbon chain points at centre and COO remain outward.
This gives the colloids extra stability.

2) Medium :-
The charge present on the particles might be due to the medium in which they are present.

For eg:- Consider Proteins,
a) Amino acids are the building blocks of Proteins.

b) Amino acids have two functional groups i.e., Primary Amine & Carboxylic Acid.

c) In Acidic medium ---->
The Primary Amine i.e the NH2 group accepts a proton from an acid and becomes positively(+) charged.
H2N - RCOOH --(H+)--> NH3 -RCOOH

Thus In Acidic Medium the proteins will move towards Negative(-) electrode.

d) In Alkaline medium ---->
The Carboxylic Acid i.e the R-COOH group loses a proton from COOH group and becomes Negatively(-) charged.

RCOO + H2H2N - RCOOH --(OH-)--> H2N - O

Thus In Alkaline Medium the proteins will move towards Positive(+) electrode.

3) Surface Adsorption :-
The Colloidal particles have large surface area and are capable of adsorbing. Sometimes it happens that Certain kinds of ions from Dispersion medium get adsorbed on the colloidal particles and this can be the reason for the charge present on colloidal particles.

For eg:- Consider that,
AgI ' Sol ' is prepared by the reaction between Silver Nitrate
(AgNO3) and Potassium Iodide (KI).
Now, If excess of KI is added to AgNO3 , AgI sol absorbs I- ions preferably,forming a Negative(-) colloid, Whereas, If excess of AgNO3 is added to KI, then AgI sol absorb Ag+ ions preferably,forming a Positive(+) colloid.

4) Frictional Electrification :-
Sometimes it happens that the dispersed phase particles i.e the Colloidal particles gets rubbed with the particles of the dispersion medium giving rise to friction and this could be the reason for the charge present on the colloidal particles.

Friday, 14 September 2012

What are Colloids, their Properties and their Types..?

There are three major groups of mixtures,they are :-

a) Solution :-
·        A solution is a homogeneous mixture of two or more components.
·        The dissolving agent is called as solvent where as the substance which is dissolved is called as solute.
·        The components of a solution are unevenly distributed and cannot be seperated.
·        The components of a solution are smaller in size up to 10-9 m .
Eg. Sugar and water

b) Suspension :-
·        A suspension is a heterogeneous mixture containing solid particles that are sufficiently larger than the particles found in the solutions and have a average size up to 10-7 m .
·        The Components of a suspension can be evenly distributed by mechanical means, like shaking,stirring etc., but inspite of this the components settle down and seperate from one another.
Eg. Oil and Water.

c) Colloids :-
·        The Colloids were first discovered in 1860.
·        The credit for the discovery of Colloids can be given to a Scottish Scientist, Thomas Graham.
·        A COLLOID is a substance which is microscopically dispersed (widely spread) evenly throughout another substance.
·        Colloids when mixed remain evenly distributed without settling forming a mixture known as Colloidal Dispersion.
·        Colloids are not visible with our naked eyes, but they can be seen under microscope.
·        The Particles of colloids are intermediate in size between those found in solutions and suspensions.
·        Colloidal particles have an average size of 10-9 to 10-7 m .
·        The Colloidal particles form Dispersed phase and the medium used for this is called as Dispersion medium.
·        Dispersing medium is the external phase and is found in the greater extent in the colloid whereas the Dispersed phase is the internal phase and is found in the lesser extent.
·        A colloidal system can exist in any of the three forms solid, liquid or gaseous.
Eg :- Milk

Properties of Colloids

1) Tyndall Effect :-
A British Physicist John Tyndall in 1869 first observed this phenomenon and hence it is known as Tyndall Effect.
He observed that when a beam of light is passed through a colloidal solution, the light is scattered by the particles present in the colloidal solution.
This effect is often used as a measure of the existence of a colloid.

2) Brownian Movement :-
The Molecules of the Dispersion medium constantly collide with the colloidal particles thereby passing Kinetic Energy to them.
This phenomenon results into zigzag movement of the colloidal particles.
This zigzag movement of the colloidal particles is known as Brownian movement.

3) Electrophoresis :-
The movement of the colloidal particles under the influence of an applied electric potential is called as Electrophoresis.

4) Electro-Osmosis :-
When the movement of the Colloidal particles is prevented by some suitable means, it is observed that the Dispersion medium itself begins to move under the influence of an electric field. This phenomenon is known as Electro osmosis.

Types of Colloids

1) Sol :-In Sol the Dispersing medium is the Liquid whereas the Dispersed phase is the solid.
Eg :- Paint,Ink,Detergent etc.

2) Gel :- In Gels the Dispersing medium is the Solid whereas the Dispersed phase is the Liquid.
Eg :- Butter,Jelly etc.

3) Emulsion :- In the Emulsions both the Dispersing medium and the Dispersed phase are the liquids.
Eg :- Cometic lotions,Lubricants etc.

4) Foams :- In Foams the Dispersing medium is the Liquid whereas the Dispersed phase is the Gas.
Eg :- Shaving lather,whipped cream etc.

5) Aerosols :-In Aerosols the Dispersing medium is the Gas whereas the Dispersed phase is the Liquid.
Eg :- Insecticide sprays,Smog,Cloud,Fog etc.

Saturday, 1 September 2012

Ideal Gas & Ideal Gas Law

What is an Ideal Gas ????

An Ideal Gas or a Perfect Gas is the one which is composed of a set of randomly-moving, non-interacting point particles.
An Ideal Gas obeys Boyle's Law and Charle's Law.
Practically ideal gas does not exist,But many common gases exhibit behaviour very close to that of an Ideal Gas at ambient temperature and pressure.

Ideal Gas Law :-

A law relating the pressure, temperature , and volume of an ideal gas is called as Ideal Gas Law . It was first stated by a Physicist and Engineer Benoit Paul Emile Clapeyron in 1834. The ideal gas law predicts the state of a gas at a given pressure and temperature.

Mathematically the Ideal Gas Law can be stated as :-
PV = nRT
P is the absolute pressure ,
V is the volume of the vessel ,
n is the number of moles of gas ,
R is the Universal Gas constant, and in accordance with it's name it is same for all gases &
T is the absolute temperature in Kelvin.

Ideal Gas Law is derived as follows :-

According to Boyle's Law for a given mass of an ideal gas,the product of pressure and volume is constant,i.e.,
P x V = constant

Also, according to Charle's Law for a given mass of an ideal gas, the ratio of the volume to temperature is constant,i.e.,
V/T = constant

Combining the above two laws,we get,
P x V/T = constant

The constant in the above equation is denoted by R and is known as Universal Gas Constant.

Thus we can say,
Now, for 'n' kilomoles of an Ideal Gas,the above equation can be written as ,
PV = nRT
This above equation is the ultimate mathematical form of Ideal Gas Law.

The Ideal Gas Law states 3 facts :-
i) Volume of a gas is directly proportional to the number of moles
ii) Volume of a gas is directly proportional to absolute temperature
iii) Volume of a gas is inversely proportional to the pressure.