Category Archives: 10+2 Chemistry

Surface Chemistry : Colloids

A heterogeneous solution in which the diameter of the solute particles is in the range 1nm to 1000nm is called a colloid.

The solute and solvent are called dispersed phase and dispersion medium respectively in case of a colloid.

◆ Methods of preparation of colloids : Colloids are prepared by following methods-

1. Chemical methods : There are several methods to prepare colloids like oxidation, reduction, hydrolysis, double decomposition etc. All these methods can be called condensation methods because molecules aggregate together to form sols.

I. Double decomposition : As2O3 + 3H2S  → As2S3 (Sol)+ 3H2O

II. Hydrolysis : FeCl3 + 3H2O → Fe(OH)3 (Sol)+ 3HCl

III. Oxidation : SO2 + 2H2S → 3S(Sol) + 2H2O

IV. Reduction : 2 AuCl3 + 3 HCHO + 3H2O  → 2Au(Sol) + 3HCOOH

2. Electrical Disintegration or Bredig’s Arc Method : This method is used to prepare sols of metals like gold, silver, platinum etc. The colloid is formed by the involvement of both dispersion and condensation.

Process : In this method an electric arc is struck between metal electrodes immersed in the dispersion medium. The metal is first vaporised due to intense heat and then condenses to form parties of colloidal size. This, a colloid is prepared.

3. Peptisation : The process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of an electrolyte is called peptisation.

The electrolyte used for this purpose is called the peptising agent.

Process : In this process the precipitate adsorbs one of the ions of the electrolyte on its surface. It causes the development of positive or negative charge on precipitates due to which precipitate particles break up into smaller particles of colloidal size. Thus, a colloid is formed.

Purification of colloidal solution : While forming a colloidal solution some impurities enter into it. Sometimes electrolytes are in the excess. Due to these impurities a colloidal particles may coagulate and the solution may be spoiled.

Properties of colloids : 1. Colligative properties :

2.Charge on colloidal particles : There is charge on each particle of the dispersed phase in a colloidal solution. The reason behind it are as follows:

I. Frictional electrification :  One the reasons responsible for charge on the particles of the dispersed phase is the rubbing of the particles of the dispersed phase with those of the dispersion medium.

II. Dissociation of molecules :

III. Selective adsorption of ions :

 

The Nitrogen Family

Elements in 15th group are known as ‘The Nitrogen Family’. The family includes the following elements:

Elements

Symbol State

Metallic Properties

Nitrogen

N Gas Non-metal
Phosphorus P Solid

Non-metal

Arsenic

As Solid Metalloid
Antimony Sb Solid

Metalloid

Bismuth

Bi Solid Metal

Occcurence : Nitrogen – Nitrogen is found in the molecular form in the atmosphere. It comprises 75% by mass and 78% by volume of the atmosphere. It is also found in Earth’s crust in the form of Chile Saltpetre(Sodium Nitrate : NaNO3) and Indian Saltpetre(Potassium Nitrate-KNO3). In the form of protein, Nitrogen is also available in plants and animals.

Phosphorus : Minerals of the apatite family are main source of phosphorus as flourapatite : Ca9(PO4)6.CaX2 where X =F, Cl and Br. In the animal and plant matter nitrogen is an essential component. It is also found in bones of animals. As phosphoproteins it is available in milk and eggs.

Arsenic, Antimony & Bismuth : They are found in sulphide minerals.

Properties of 15th group elements :

Atomic properties :

● Electronic configuration : The electronic configuration of the outermost shell is ns2np3. Due to half filled p orbital these elements are quite stable.

Atomic & ionic radio :

Reactivity towards oxygen : Group 15 elements form oxides by reacting with oxygen of the form E2O3, E2O4 and E2O5 .Ex: N2O3, N2O4, P2O5, As2O5 etc.

Key Points : 1. Among oxides the acidic nature increases with increase in the oxidation state.

2. The acidic nature also increases with increase in the percentage of oxygen.

3. In the group, the acidic nature decrease with increase in atomic number due to increase in the metallic character.

E2O3 Type Oxides of

Nature

Nitrogen & Phosphorus

Purely acidic

Arsenic & Antimony

Amphoteric

Bismuth

Basic

4. The stability of oxides decreases down the group.

Reactivity towards Hydrogen : Group 15 elements react with Hydrogen to form hydrides of the form EH3. Ex : NH3 (Ammonia), PH3 (Phosphine), BiH3 (Bismuthine) etc.

Key Points : 1. The thermal stability decreases down the group because the tendency to form covalent bond decreases as the size of atoms increases which leads to increase in the metallic character.

Metallurgy

Metals are very important for us. Many useful things like utensils, different types of tools, vehicles, tanks etc are made up of metals. But, metals are not easily available. Only a few metals are found in free state. Maximum metals are found in the combined form i.e. mineral in earth’s crust.

Ore : A mineral from which the extraction of a metal is feasible and economical is called an ore.

Abundance of metals : Aluminium is the most abundant element among metals. It is 8.3% by weight in earth’s crust. The main ores of some important metals are as follows:

Metal

Ores

Composition

Aluminium

Bauxite

Kaolinite

AlOx(OH)3-2x Where 0<x<1 </x<1

Al(OH)4.Si2O5

 

Iron

Haematite

Magnetite

Siderite

Iron pyrite

Fe2O3

Fe3O4

FeCO3

FeS2

Copper

Copper pyrite

Malachite

Cuprite

Copper Glance

CuFeS2

CuCO3 . Cu(OH)2

Cu2O

Cu2S

Zinc

Zinc Blende or Sphalerite

Calamine

Zincite

ZnS

ZnCO3

ZnO

Metallurgy : The whole technological and scientific process of the isolation of a metal from its ore is called metallurgy.

Gangue : An ore contains several unwanted impurities with a metal which are called gangue.

Concentration of ores : The process of removing guange from an ore is called the concentration of ore.

There are different methods of the concentration of the ores of different metals. Some of them are as follows :

1. Hydraulic washing : This method is based on the difference in specific gravities of the ore and the gangue particles.

Procedure : In this method, water with high speed is drawn on ore particles due to which lighter gangue particles flow with water and ore particles are left behind.

2. Magnetic separation method : This method is based on the differences in magnetic properties of the ore components.

Procedure : In this method, the powdered ore is placed over a conveyer belt which passes over a magnetic roller. The ore particles fall near the roller due to the attraction and gangue particles fall far from the roller.

Refining : Metals obtained from the extraction process are not pure. They still have some impurities. The process of removing these impurities is called refining. For different metals, different refining processes are used.

I. Distillation(Zn/Hg) : The metals with low boiling points are refined by distillation. In this method, the metal with impurities is heated, due to the low boiling point metal vaporizes and it is collected in a flask and impurities are left behind.

II. Liquation(Sn) : This method is suitable for metals having low melting points. In this method, the metal with impurities is heated, due to the low melting point metal vaporizes and it is collected in a flask and impurities are left behind due to their high melting point.

 

Solutions

A solution is a homogeneous mixture of two or more substances.

The substance with largest amount is called solvent and other substances with lesser amounts are called solute.

Binary solution : A solution having only two components is called a binary solution.

Types of binary solutions : On the basis of the states of solvent, there are three types of solutions as follows:

1. Solid solutions : A solution in which the solvent is a solid is called a solid solution.

2. Liquid solutions : A solution in which the solvent is a liquid is called a liquid solution.

3. Gaseous solutions : A solution in which the solvent is a gas is called a gaseous solution.

Concentration : The amount of solute present in a solution is called the concentration of the solution.

◆ Methods of expressing concentration :

1. Strength : The amount of the solute in grams present in one litre of a solution is called strength.

i.e. Strength = Mass of solute / Volume of solution(in L)

2. Mass Percent : The mass percent of the solute is expressed as follows :

Mass % = Mass of solute×100 / Mass of the solution

3. Volume Percent : The volume percent of a solution is defined as follows:

Volume % = Volume of the solute × 100/ Volume of the solution

4. Mole Fraction : If a binary solution has two components A and B having nA and nB moles then their mole fraction is denoted by xA & xB respectively where

xA = nA/(nA + nB) &  xB = nB/(nA + nB)

Key Point : xA + xB = 1

5. Molality : The molality of a solution is defined as the moles of solute per kilogram of solvent i. e.

Molality = Moles of solute / Mass of solvent(In Kg)

Key Point : Molality is denoted by ‘m’.

6. Molarity : The molarity of a solution is defined as the number of moles per litre of the solution i. e.

Molarity = Moles of the solute /Volume of the solution(In Litre)

Key Point : I. Molarity is denoted by ‘M’.

II. If the molarity of a solution changes from M1 to M2 and it’s volume changes from V1 to V2 then M1V1 = M2V2

III. Molality doesn’t change with change in temperature because it depends upon mass whereas molarity is affected by temperature because of its dependence on volume.

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