Category Archives: JEE/NEET Chemistry

Periodic Table

The periodic table is a tool which helps us to study about 118 elements whether natural or synthetic and their compounds in a systematic way.

History of classification of elements : Till 18th century only a few elements and their compounds were known. so, it was easier to remember them. But, from the beginning of 19th century, the rate of discovery of elements and their compounds became fast and hence, the classification of elements became essential so that we can easily study about the properties of elements and their compounds.

Several chemists tried to classify elements. Some of them are as follows :

1. Doebereiner’s Triads : In 1817, a German chemist Johann Wolfgang Doebereiner presented the first classification of elements. In this classification, he prepared several groups of three elements called ‘Triads’ having similar properties. He told that the atomic weight of the middle element was the average of the rest two elements.

Ex : 1st Triads : Li(7), Na(23) & K(39) , 2nd Triad : Ca(40), Sr(87.5) & Ba(137) etc.

2. Newland’s Law of Octaves : In 1865, an English chemist John Alexander Newland  gave another idea of classifying elements. He showed that when elements are arranged in the increasing order of their atomic weights each 8th element has similar properties as the 1st one.  This is known as his law of octaves.

Element Li Be B C N O F
At wt 7 9 11 12 14 16 19
Element Na Mg Al Si P S Cl
At wt 23 24 27 29 31 32 35.5
Element K Ca          
At wt 39 40          

We can see that Li & Na have similar properties and so on.

3. Lothar Meyer’s Curve : In 1865, a German chemist Lothar Meyer plotted a graph between atomic volumes and atomic weights of elements. He found that elements with similar properties occupy the similar places on the curve. For example, the alkali metals lie on the top points on the curve.

4. Mendeleev’s Periodic Table : The most effective and systematic classification of elements was presented by a Russian chemist Dmitri Ivanovich Mendeleev. He proposed a law which is known as Mendeleev’s periodic law stated as follows :

The properties of elements are periodic functions of their atomic weights. 

He arranged elements according to their increasing atomic weights and found that elements with similar properties are repeated after certain intervals. The key points related to his periodic table are as follows:

  1. There are nine vertical columns which are called groups. These groups are denoted as 0, I, II, III,…….VIII.
  2. Each group except 0 and VIII has been divided in two sub-groups A and B.
  3. There are 7 horizontal rows which are known as periods.

Importance of Mendeleev’s periodic table : The periodic table developed by Mendeleev made several contributions to the study of chemistry. Some of its important contributions are as follows :

  1. Systematic study of chemistry : This periodic table made the study of elements and their compounds easy and simple. One can easily know the elements with similar properties by the help of groups. Also the periodicity in the properties of elements can be understood through the arrangement of elements in groups and periods.
Updated on 31 July 2019…

 

Electro Magnetic Radiations(EMR)

Electromagnetic radiations(EMR) : Charged particles on moving with acceleration produce alternating electrical and magnetic fields which transmit in the form of waves and are called electromagnetic radiations or electromagnetic waves. James Maxwell was the first scientist who described EMR in 1870.

Properties of EMR : The properties of electromagnetic radiations are as follows :

1. The electric and magnetic fields generated by oscillating charged particles are perpendicular to each other and also perpendicular to the direction of propagation.

2. EMR do not require any medium to travel i.e. they can travel even in vacuum.

3. Electromagnetic radiations constitute spectrum which has been divided into different regions and Each EMR lies in a particular region.

4. Electromagnetic radiations are expressed by the help of their characteristic properties as follows :

Wavelength : The distance between two consecutive crests and troughs is called the wavelength. It’s denoted by λ(Lambda) and its unit is ‘m’.

Frequency : The number of waves passing through a point per second is called frequency. It generally denoted by ν. Its unit is Hz or s

1.

Wave number : The number of wavelengths per unit of length is called the wave number. It is denoted by .The unit of wave number is m-1.

Key point : If the velocity of light is c, wavelength is λ and frequency is ν; then c = νλ

Electromagnetic spectrum : When different types of electromagnetic radiations are arranged according to their decreasing frequencies or increasing wavelengths, then the arrangement is called the electromagnetic spectrum.

Atomic Models

After the discoveries of electron, proton and neutron, scientists made efforts to understand their arrangement inside an atom. The arrangement of subatomic particles inside an atom is called an atomic model.

Thomson model : The British physicist J J Thomson presented his atomic model in 1898 which is also called watermelon model or raisin pudding model. According to this model :

1. The positive charge is uniformly distributed inside an atom.

2. The negative charge i. e. electrons are distributed in such a way that the atom becomes neutral.

Limitations of Thomson model : Thomson model was able to describe the neutrality of an atom but it didn’t answer many other questions like –

Why do only electrons involve in a chemical reaction?

Why are there electric and magnetic fields of an atom?

Rutherford’s nuclear atomic model : Rutherford presented his model in 1911 on the basis of his alpha scattering experiment.

Experiment : In this experiment a very thin (about 100 nm) gold foil was taken and it was bombarded with α particles. Gold foil was surrounded by a fluorescent screen of ZnS. A tiny flash of light is observed when α particles strike the screen.

Observations : In the experiment Rutherford made following observations :

I. Maximum α-particles pass through the foil undeflected.

II. A few α-particles are deflected with small angles.

III. A very few α-particles (1 in 20000) bounced back.

Conclusions : On the basis of above observations, Rutherford made some conclusions given below :

I. Most of α-particles pass without any deflection. It means that maximum space inside an atom is vacant.

II. Few α-particles are deflected. It means that there is a small positive charge in the centre of an atom and α-particles face repulsion.

Bohr’s atomic model for Hydrogen: In 1913, Neils Bohr presented his model for Hydrogen atom on the basis of his research and experiments. The postulates of his atomic model are as follows:

1. The electron moves on a circular path called an orbit. Orbits can also be called stationary states or energy states because they have fixed energy. All orbits are concentric and have fixed radius with nucleus as the centre.

2. In an orbit, the energy of an electron is fixed. But, when it jumps from the lower energy state to the higher one, it absorbs energy. It loses energy when it falls from the higher energy state to the lower one.

Quantum mechnical model : To answer many questions which were not entertained by Bohr’s model, another model was presnted by Shrodinger which is called Quantum mechanical model. The postulates of this model were as follows :

1. Electrons show dual nature i.e. particle and wave nature.

2. To find the correct location and velocity of an electron simultaneously is not possible.

3.

Carboxylic Acids

Carboxylic acids are very important organic compounds. They contain carboxyl functional group(-COOH). The carboxyl name has been derived from carbonyl(-CO) and hydroxyl(-OH). Carboxylic acids may be aliphatic or aromatic on the basis of the group alkyl or aryl attached to the carbon atom of the carboxyl group.

Fatty acids : Aliphatic carboxylic acids from C12 to C18 are called fatty acids.

Nomenclature : I. Common names :

  1. To get common names of carboxylic acids, we add a suffix -ic acid in the names obtained by Greek or Latin words for their natural sources. Ex : HCOOH is called formic acid because it was first obtained from red ants and the latin word for ants is formica. Some more examples are : CH3COOH –  Acetic Acid/ C3H7COOH-Butyric Acid

II. IUPAC names :

  1. In IUPAC system, we replace -e by -oic acid at the end of an alkane for aliphatic carboxylic acids.
  2. The numbering is started from the carboxylic carbon in the parent chain.
  3. To name a carboxylic acid having more than one carboxyl group -e of alkane is retained at the end and the number of carboxyl groups is expressed by prefixes like di, tri, tetra etc.
  4. The position of -COOH group is indicated by numerals 1,2,3…

Structure of the carboxyl group : The bonds to the carboxyl carbon in a carboxylic acid are coplanar. The angle between the bonds is 1200. The resonance structures of the carboxyl group is as follows:

Preparation of carboxylic acids :

1.From primary alcohols : When primary alcohols are oxidised by KMnO4 in neutral, acidic or alkaline medium or K2Cr2O7 and CrO3 in acidic, carboxylic acids are produced. Ex :

2.From primary aldehydes : When aldehydes are oxidised by mild oxidising agents, they give carboxylic acids.

3.From alkylbenzenes : Alkylbenzenes give aromatic carboxylic acids on oxidation by chromic acid or acidic or alkaline KMnO4. But, only primary and secondary alkyl groups are oxidised by this manner. Tertiary groups are not affected.

 

Radioactivity

Some substances which emit radiation on their own are called radioactive substances and such an event is called radioactivity.

The event of radioactivity was observed by the chemist Henri Becquerel.

Generally, radioactive substances emit three types of radiations or rays –

Alpha rays : Particles in alpha rays are similar to He nucleus with positive charge.

Beta rays : Beta particles are negatively charged and they are similar to electrons.

Gamma rays : Gamma rays are similar to X-rays.

Penetrating power : The penetrating power of beta rays is 100 times more than that of alpha rays and the penetrating power of gamma rays is 1000 times more than that of alpha rays.

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 :

 

Crystal Field Theory

Coordination compounds have magnetic and optical properties. They also show colour. To explain these properties shown by coordination compounds H Bethe presented his Crystal Field Theory in 1929.

The main points of this theory are as follows:

1. The nature of bonding in the complexes is electrostatic i.e. the metal ligand bond is supposed to be ionic.

2.Anionic ligands are supposed negative points or point charges & neutral ligands are taken as point dipoles.

3.CFT is based on the concept of splitting of d-orbitals.

Partition of d-orbitals: The five d-orbitals are divided into two sets: t2g (dxy, dyz and dxz) and  eg(dx2-y2 and dz2)

I.The crystal field splitting in octahedral coordination entities: In an octahedral coordination entity, there is repulsion between the electrons of d-orbitals of the central atom or ion and the electrons of the ligands.

The repulsion is more when d-orbitals are directed towards ligands,  than when they are away from ligands. Since, eorbitals which are along axes i.e. in the direction of ligands face more repulsion which finally leads the raising of energy of orbitals and the energy of t2g orbitals is lowered. Due to this, d-orbitals are splitted as follows:

.

.

The splitting of degenerate orbitals due to the presence of ligands is called crystal field splitting.

Crystal field splitting energy or crystal field stablising energy(CFSE): The energy required for the separation of d-orbitals is called CFSE. It is denoted by ∆o.

Mole Concept

Mole : One mole is the amount of a substance which contains 6.022 x 1023 constituent particles of the substance. These particles may be atoms, molecules or ions.

Or

One mole can also be defined as the amount of a substance which contains as many particles as the number of C-12 atoms in its 12 g mass.

The number 6.022 x 1023  is called Avogadro number or constant and it is denoted by N or NA i.e. NA = 6.022 x 1023

Atomic Mass Unit(amu) : In Chemistry, we have to make calculations with extremely small and big numbers especially the mass of an atom.

Molar Mass : The mass of one mole of a substance is called its molar mass.

Ex: Molar mass of CaCO3       = 100g

Molar mass of Na atom  =  23 g

Molar mass of Naion    = 23 g

Percentage Composition: As we know that each element of a compound holds a fixed percentage by mass.

% of an element = Mass of an element × 100/Molecular or Formula mass of the compound

Empirical formula : The empirical formula of a compound represents simplest whole number ratio of atoms present in it. Ex: The empirical formula of Benzene(C6H6) is CH.

Molecular formula : The molecular formula of a compound represents the actual number of atoms present in the compound. Ex: The molecular formula for Benzene is C6H6.

Key Point : Molar mass = Empirical formula mass x n

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Stoichiometry : The study of calculations on the basis of chemical reactions is called stoichiometry.

We can get a lot of  information from an equation of a chemical reaction. Consider a chemical equation or reaction :

2H2 (g)+  O2 (g)→2H2O (g)

We can make following observations:

I. 2 moles of Hreact with 1 mole of Oto produce 2 moles of water.

II. 2 molecules of H2 reacts with 1 molecule of Oto produce 2 molecules of water.

III. 44.8 L of Hreact with 22.4 L of Oto produce 22.4 L of water vapours.

IV. 4 g of Hreact with 32 g of Oto produce 36 g of water vapours.

Limiting Reagent : In a chemical reaction, the reactant which is present in the lesser amount is called the limiting reagent. In actual, a limiting reagent controls the reaction as no reaction takes place after it is consumed.

Ex : Let us take 2 g hydrogen and 20 g oxygen to form water.

According to the reaction  2H2 +  O2 →2H2O

We know that 4 g of hydrogen requires 32 g of oxygen to form water which means that 2 g of hydrogen will react with 16 g of oxygen. So, 4 g oxygen will be left unreacted. It’s obvious that hydrogen is the limiting reagent.

Phosphorus & Its Compounds

Allotropic forms : Phosphorus has several allotropes among which the following are important :

I. White phosphorus : It’s a translucent white waxy solid. Its properties are as follows:

1.It is poisonous.

2. It is insoluble in water but soluble in carbon disulphide.

3. It glows in dark and this characteristic is called chemiluminescence.

4. White phosphorus is more reactive than other allotropes in the solid state due to angular strain in P4 molecule in which the bond angle is 600 and so it is less stable.

II. Red Phosphorus : When white phosphorus is heated at the temperature level of 573K for several days, we get red phosphorus. It has following properties:

1. Red phosphorus is a solid with iron grey lustre.

2. It’s odourless and non poisonous.

3. It is non soluble in water as well as carbon disulphide.

4. It doesn’t glow in dark.

5. Red phosphorus is much less reactive than white phosphorus.

6. It is polymeric which contains 4 phosphorus atoms in a chain like structure.

II. Red Phosphorus : When red phosphorus is heated under high pressure, a series of phases of black phosphorus are formed. Its properties are as follows:

1. It has two forms α-black & β-black. α-black phosphorus is formed when red phosphorus is heated in a sealed tube at the temperature level of 803K. β-black phosphorus is formed by heating white phosphorus under high pressure at 473K.

2. It doesn’t burn in air upto 673K.

3. It sublimes in air .

Compounds of phosphorus :

1. PH3-Posphine : Phosphine (IUPAC name: Phosphane) is a poisonous gas which was discovered Gembre in 1783.

Preparation : When calcium phosphide is treated with water or dilute HCl, phosphine is produced.

Ca3P2 + 6H2O → 3Ca(OH)2 + 2PH3

Ca3P2 + 6HCl → 3CaCl2 + 2PH3

Nitrogen & Its Compounds

I. Dinitrogen

Nitrogen is a very important part of our atmosphere. It forms 78% of our atmosphere by volume and 75% by mass.

Preparation : We have two types of productions of Nitrogen:

Commercial production : The commercial production of Nitrogen is done by the liquifaction and fractional distillation of air.

Lab production : In lab Nitrogen can be produced by several methods-

I. When aqueous solution of ammonium chloride is treated with sodium nitrite.

NH4Cl(aq)+NaNO2(aq)→ N2(g) + 2H2O(l) + NaCl(aq)

II. Dinitrogen can also be obtained by the thermal decomposition of ammonium dicromate.

(NH4)2Cr2O7 →N2(g) + 2H2O(l) + Cr2O3

III. Thermal decomposition of sodium or barium azide also gives dinitrogen. Nitrogen obtained by this method is very pure.

Ba(N3)2 → Ba + 3N2

II. Ammonia()

Ammonia was detected by Priestley in 1774. It is generally formed by the bacterial decomposition of nitrogenous matter found in plants and animals. We can find it in a very less amount in air and soil.

Commercial production: Ammonia is produced by Haber’s process commercially. In this process we prepare a setup with following optimum conditions:

1. Pressure :200 × 105 Pa or 200 atm(Approx), 2. Temperature : About 700 K & 3. Catalyst : Iron oxide with small amount of K2O & Al2O3.

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Under these conditions ammonia is produced by the reaction : N2(g) + 3H2(g) → 2NH3(g)

The catalyst is used to increase the production rate of NH3.

III. Oxides of Nitrogen

Nitrogen reacts with oxygen to form different oxides with different oxidation state

1. N2O -Dinitrogen Oxide or Nitrous Oxide or Laughing Gas:

Oxidation state of Nitrogen: 1

Preparation: It can be produced by heating ammonium nitrate.

NH4NO3 N2O + 2H2O

Structure:

Properties : 1. Nitrous oxide is a colourless gas.

2. It is a has with sweet taste and pleasant odour.

3. It can create laughter when inhaling in a sufficient amount due to which it is called laughing gas.

4. It is soluble in cold water but not in hot water.

5. Nitrous oxide is heavier than air.

Uses : 1. As propellant, 2. As anaesthetic in minor surgical operations with oxygen

II. HNO3-Nitric Acid

Commercial Preparation(Ostwald Process):

The mixture of ammonia and air when passed over platinum gauze catalyst at 7500C-9000C, then ammonia is oxidised to nitric acid.

4NH3+5O2 – 4NO + 6H2O

By oxidising, the nitric oxide is converted to nitrogen dioxide.

2NO + O2 – 2 NO2

When nitrogen dioxide is cooled and absorbed in water, nitric acid is obtained.

3NO2 + H2O – 2HNO3 + NO

Lab Preparation: In the laboratory, nitric acid is formed by heating the mixture of KNO3 or NaNO3 and concentrated H2SO4 in a glass retort.

KNO3 + H2SO4 – KHSO4 + HNO3

NaNO3 + H2SO4 – NaHSO4 + HNO3

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