Author Archive: Diwakar Verma

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

The Nitrogen Family

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

Element Atomic Number Electronic Configuration Character
N 7 1s22s22p3 Non-metal
P 15 1s22s22p63s23p3 Non-metal
As 33 1s22s22p63s23p6 3d104s24p3 Metalloid
Sb 51 1s22s22p63s23p6 3d104s24p64d105s25p3 Metalloid
Bi 81 1s22s22p63s23p6 3d104s24p64d10 4f145s25p65d106s26p3 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.

Reactivity towards oxygen : Group 15 elements form oxides by reacting with oxygen of the form E2O3, E2O4 and E2O.Ex: N2O3, N2O4, P2O5, As2Oetc. 

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.

 

Classification of Crystalline Solids

In this topic, we have to discuss the types of crystalline solids. Crystalline solids are classified into four categories as follows:

1. Molecular solids: Solids whose constituent particles are molecules are called molecular solids.

Molecular solids are of three types:

I. Non-polar molecular solids: A solid in which the constituent particles are held by weak dispersion forces or London forces are called non-polar molecular solids.

Ex: H2, O2, N2, Noble gases like He, Ne, Ar, Kr etc

II. Polar molecular solids: A solid in which the constituent particles are held by dipole-dipole forces is called a polar solid.

Ex: HCl, NH3, SO2 etc

III. Hydrogen bonded molecular solids: A solid in which the constituent particles are held by Hydrogen bond is called a hydrogen bonded molecular solids.

Ex: Ice, HF etc

2. Covalent or network solids: A solid in which the constituent particles form a huge network of very strong covalent bonds is called a covalent or network solid.

Ex: Diamond, Graphite, Silica, Silicon Carbide etc.

3. Ionic solids: A solid whose constituent particles are ions is called an ionic solid.

Ex: NaCl, MgCl2 etc

4. Metallic solids: A solid whose constituent particles are atoms of a metal is called a metallic solid.

Ex: Fe, Cu, Zn etc.

Trigonometric Equations

Trigonometric equation: An equation having trigonometric functions is called a trigonometric equation.

Ex : sin θ = 1

sin θ + cos θ =1

tan θ = 1

Trigonometric identity : A trigonometric identity is also an equation which gets satisfied by any value of the unknown quantity.

Ex : sin2θ + cos2θ=1

Key Point: A trigonometric equation is satisfied by finite or infinite specific values of the unknown quantity not by any value of the unknown quantity.

Solution of a trigonometric equation: There are two types of solutions of a trigonometric equation:

I. Principal solution: A solution in which the values of the unknown quantity belong to the interval [0,2π] is called a principal solution.

II. General solution: A solution in which there are infinite values of the unknown quantity is called a general solution.

Important results :

I. sin θ = 0 ⇒ θ = nπ           II. cos θ = 0 ⇒ θ = (2n+1)π/2             III. tan θ = 0 ⇒ θ = nπ

Amines

When one or more hydrogen atoms of ammonia molecule are replaced by alkyl or aryl group amines are formed. These are also called derivatives of ammonia.

Classification of amines: Amines can be classified into three categories:

I.Primary amines: If one hydrogen atom is replaced by R or Ar, we get a primary amine.

II.Secondary amines: If two hydrogen atoms are replaced by alkyl or aryl group we get a secondary amine.

III.Tertiary amines: If three hydrogen atoms are replaced by alkyl or aryl group, we get a tertiary amine.

Alcohols, Phenols & Ethers

Intro: When a hydrogen atom from a hydrocarbon whether aliphatic or aromatic is replaced by -OH group alcohols and phenols are formed.

Ex:

But, when a hydrogen atom from a hydrocarbon whether aliphatic or aromatic is replaced by alkoxy or aryloxy group(R-O/Ar-O) alcohols and phenols are formed.

Ex:

Classification

Haloalkanes & Haloarenes

Intro: When hydrogen atoms are replaced by halogens in hydrocarbons whether aliphatic or aromatic, alkyl halide (haloalkanes) and aryl halide(haloarenes) are formed.

Difference between haloalkanes and haloarenes:

1.In haloalkanes, halogens are attached to sp3 hybridised carbon atoms of alkyl group.

However, in haloarenes halogens are attached to sp2 hybridised carbon atoms of aryl group.

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