QUANTUM DOTS

• Quantum dots are Semiconductor whose excitons are confined in all three spatial dimensions.
  
• Exciton is a bound state of an electron and an imaginary particle called an electron-hole.
  
• Being zero-dimensional, they have sharp density of states.
  
• Colour of quantum dots dictated by quantum confinement.
  
• Quantum confinement allows tailoring of band-gap of materials.

Quantized energy levels in a quantum dot have no net linear momentum. They do not require any momentum transfer. Transition probabilities are high. This explains the broad-band absorption nature of quantum dots.

Colour of Quantum dots

• Quantum dots of same material with different sizes emit light of different colours.
  
• Larger the dot, redder the fluorescence.
  
• Smaller the dot, bluer the it is.
  
• Larger dots with more closely spaced energy levels absorb photons containing less energy.

Inter band transitions control luminescent properties of quantum dots. Band gap is a function of size of the dots. Hence changing the size of quantum dots can tailor their luminescence. They are produced by colloidal synthesis, electrochemical techniques etc.They have applications as biotags, in diodes, biological sensors, solid-state quantum computation, quantum dot photovoltaic cells.

POLYMERS

Polymers are macro-sized molecules with relatively high molecular mass. They are formed by joining together large number of small molecules. The process of formation of polymers from their starting materials is called polymerisation and the small molecules which combine with each other are termed as monomers.

Polymers formed from one kind of monomers is called homopolymer while those formed from more than one kind of monomer units is called copolymer or mixed polymer.

Ethylene or ethene is the monomer unit of polyethylene. It is a homopolymer.
n CH2= CH2 = (-CH2CH2-)n

Nylon-66 is a copolymer made from Hexamethylene diamine and Adipic acid.
nH2N-(CH2)6-NH2 + nHOOC-(CH2)4-COOH =
- [NH-(CH2)6-NH-CO-(CH2)4-CO-]n + nH2O


Classification of Polymers.
Polymers are classified in different ways.

1. Classification based on source.On the basis of source polymers are classified into two types. They are natural polymers and synthetic polymers.

Natural polymers are obtained from nature. They are cellulose, starch, natural rubbers, proteins etc. Natural rubber is a polymer of isoprene. It is obtained from the latex of rubber tree. Polymers such as polysaccharides (starch, cellulose), proteins and nucleic acid which control various life processes in plants and animals are also called as biopolymers.

Synthetic polymers are artificially prepared in the laboratory. They are man-made polymers. Examples are polyethene, polyvinyl chloride (PVC), nylon, terylene, bakelite etc. They find diverse applications in man's daily life.

2. Classification based on Structure. Bases on structure, polymers are classified as:

Linear polymers.In these, the monomeric units are linked together to form long straight chains. They are closely packed and hence have high densities, high melting point and high tensile strength. eg; nylon, polyethene.

Branched chain polymers. The monomer units are linked together to form a main chain. Side chains of different lengths arises from the main chain thus forming branches. These polymers are irregularly packed and have low tensile strength and melting points. eg; amylopectin, glycogen.

Cross Linked polymers.In these the monomer units are linked together to form a three dimensional network. These cross-linked polymers are hard, rigid and brittle. eg; melamine, bakelite.

3. Classification Based on Synthesis. On this basis, polymers can be classified as addition polymers and condensation polymers.

Addition polymers.It is formed by the direct addition of repeated monomers. eg; polyethylene, polypropylene, PVC etc.

Condensation polymers.They are formed from two or more kind of monomers. Each monomer usually contains two functional groups. During the formation of condensation polymers simple molecules such as H2O is lost. eg; Nylon 66, Dacron.

THERMOCHEMISTRY

Thermochemistry is a branch of physical chemistry which deals with the energy changes accompanying chemical transformations. This is also known as chemical energetics and is based on the first law of thermodynamics.


Chemical reactions are accompanied by the evolution or absorption of heat.

Endothermic Reactions.

In these chemical reactions, heat is absorbed by the reactants. It is possible when the bond energy of reactants is greater than that of the products.
At constant pressure, Hp >HR
At constant volume, Ep>ER

Chemical equation can be written as;
C(s) + H2O(g) = CO(g) + H2O(g) ; Enthalpy change= +31.4 kcal


Exothermic Reactions.

In these chemical reactions heat is evolved as a product of the reaction. It occurs when the bond energy of the reactants is less than that of the products.
At constant pressure, Hp < HR
At constant volume, Ep< ER

Chemical equation can be written as;
NaOH(aq) +HCl (aq) = NaCl(aq) +H 2O(l); Enthalpy change = -13.7 kcal

ENTROPY

Entropy is a thermodynamic state quantity which is a measure of randomness or disorder of the molecules of the system.
Entropy is represented by the symbol 'S'. The entropy is a state function. It depends only on the initial and final states of the system. It is expresed in terms of calories per degree, i.e; cal K^-1

Change in entropy = S final - S initial.
When S final > S initial, the change in entropy is positive.

For a chemical reaction, S = Sproducts - Sreactants

Spontaneity in terms of Entropy Change.

For a spontaneous process in an isolated system, the change in entropy is positive. If the system is not isolated, the entropy change of both the system and the surroundings are taken into consideration. The total entropy change (S total) is the sum of the change in entropy of the system (S system) and the change in entropy of the surroundings (S surroundings).

S total = S system + S surroundings

For a spontaneous process, S total must be positive. During spontaneous process, the entropy of the system goes on increasing and becomes a maximum till no more increase in entropy of the system is possible. The system attains an equilibrium. i.e; Change in entropy = 0 (at equilibrium for an isolated system)
If S total is negative, the direct process is non-spontaneous and the reverse process is spontaneous.

PHOTOCHEMISTRY

Photochemistry is the study of reaction of molecules by absorbing radiation from the electromagnetic spectrum in the U.V and visible region and becoming electronically active. The photo physical phenomenon are fluorescence and phosphorescence.

Fluorescence
When a compound absorbs light radiation, its electrons get excited to a higher energy level. The excited electrons comes to the ground state either directly or in steps with the emission of light energy. When this emission of light is instantaneous (1o^-8 sec) , the phenomenon is known as fluorescence.
i.e; When an illuminating system emits light of wavelength different from that of absorbed light, the phenomenon is known as fluorescence and it takes place as soon as the light is absorbed and ceases as soon as the light is stopped.

• Molecules having conjugated double bond or pi-bonds give fluorescence.
• Some electron donating groups such as -OH, -NH2 etc enhances fluorescence.
• Electron withdrawing groups such as -COOH, -NO etc decreases fluorescence.
• Carboxylic groups and aromatic rings decreases fluorescence.
• Certain organic chelating agents increases fluorescence.
• Polarity of the solvent also affects fluorescence.
• As viscosity of the solvent increases, the fluorescence decreases.
• Neutral or alkaline solution of aniline exhibits fluorescence in the visible region, but in acid solution, fluorescence disappears in the visible region ad appears in the U.V region.

Phosphorescence.

When a compound absorbs light energy, the electron goes to a higher energy level from the ground state. The electron then returns to the normal position by the emission of light energy. When this emission of light is observed after some time (10^-3 sec), it is known as phosphorescence.This is usually shown by solid compounds and is also termed as slow fluorescence.

i.e; The molecules with relatively stable excited state may undergo transition to a metastable triplet state and after some time returns to the ground state by the emission of U.V or visible light. This phenomenon is known as phosphorescence.

• Phosphorescence is obtained nicely at room temperature.
• The polarity of the solvent affects phosphorescence.
• Only those molecules which absorbs U.V or visible light shows phosphorescence.
• When increased phosphorescence is required, compounds containing heavy atoms are usually incorporated into the solution.
• Organic compounds containing conjugated ring systems produce phosphorescence intensely.

SPECTROSCOPY

Spectroscopy is the study of the interaction between radiation and matter as a function of wavelength. It is also referred to as interactions with particle radiation. Particle radiation is the radiation of energy by means of fast moving sub-atomic particles known as a particle beam.


The term spectrum refers to the bands into which electromagnetic radiation can be split or resolved. By the study of spectrum, matter and energy is investigated. The study of spectroscopy deals with emission as well as absorption spectra.



Emission spectrum is produced by the emission of radiant energy by an excited atom. When an atom is thermally or electrically excited, electrons in the ground state is promoted to metastable states. When electrons from the metastable state jump to the lower energy state, some energy is released as radiation which is analysed with the help of a spectroscope.



Absorption spectrum is produced by the absorption of radiation of a certain wave length which characterise a particular functional group or a copmound. After absorption the transmitted light is analysed by a spectrometer. Dark pattern of lines corresponding to the wavelengths absorbed is obtained which is the absorption spectrum.


The types of spectroscopy depends on the physical quantity measured, ie; normaly the intensity of energy absorbed or produced.

• Electromagnetic spectroscopy involves interaction of matter with electromagnetic radiation.



• Electron spectroscopy involves interactions with electron beams.



• Mass spectroscopy involves interaction of charged species with magnetic or electric field.



• Acoustic spectroscopy involves the frequency of sound.



• Mechanical spectroscopy involves the frequency of an external mechanical stress.



• Dielectric spectroscopy involves the frequency of an external electric field.

PHOTOELECTRIC EFFECT

The phenomenon of ejection of electrons from the surface of some metals like Cs, K and Rb when light of a certain frequency strikes on it is called photoelectric effect. The emitted electrons are called photoelectrons.
Metals having very low ionization energies exhibit photoelectric effect with visible light. Other metals show this effect with more energetic radiations such as U.V light.

. Photoelectric effect is instantaneous.
· For each metal, there is a characteristic minimum frequency called the threshold frequency below which the photoelectric effect does not occur.
· The kinetic energy of the ejected electron is proportional to the frequency of incident radiation and is independent of its intensity.

If hu is the energy of the striking photon and hu0 is the minimum energy required to eject an electron then the excess energy hu-hu0 is transformed to the photo electron as kinetic energy, 1/2mv2 where m is the mass of electron and v is the velocity. Thus,
hu-hu0 = ½ mv2 = K.E of electron.
This is the Einstein’s photoelectric equation.

ELECTROMAGNETIC RADIATION

Electromagnetic radiation is associated with oscillating electric and magnetic fields which are perpendicular to each other. It is a form of radiant energy which propagates through space in the form of waves. All electromagnetic radiations can travel through vacuum with the velocity of light. Its energy increases with its frequency and decreases with its wave length.

Electromagnetic Spectrum.
The arrangement of different electromagnetic radiations in the increasing order of their wave lengths or in the decreasing order of their frequencies is known as electromagnetic spectrum.

Low energy
Low frequency /Radio / Microwaves/IR waves/Visiblelight/UV waves/X rays/Gamma rays / High energy
High frequency


Visible region consist only a small portion of the total spectrum. Its wave length ranges between 3800 A* (Violet) to 7600 A* (red).

Particle nature of Electromagnetic radiation.
Phenomenon such as diffraction and interference can be explained on the basis of wave nature of electromagnetic radiation. But the phenomenon of photoelectric effect and black body radiation cannot be explained on the basis of wave theory. These properties can be explained on the basis of particle nature of electromagnetic radiation.

ORGANIC CHEMISTRY

Organic Chemistry is the chemistry of hydrocarbons and their derivatives. Since carbon is an essential constituent of all organic compounds, organic chemistry is defined as the chemistry of carbon compounds. But, carbon compounds such as CO, CO2, carbonates, bicarbonates, metal carbides, metal cyanide, etc. are considered as inorganic because of their properties.

The reason for the existence of large number of organic compounds are:-

• Catenation - the self linking property of carbon atoms.


• Isomerism - the existence of different compounds having the same molecular formula.

Hybridisation in carbon.

In the excited state, carbon atom has four unpaired electrons to form four covalent bonds. The four valence orbitals are different, one being an s-orbital and the other three p-orbitals. These four orbitals (2s and three 2p) mix up to produce equivalent orbitals.

The process of mixing up of atomic orbitals of almost equal energies to get an equal number of orbitals of identical shapes and equal energies is known as hybridisation.

Carbon atom can have 3 types of hybridisation : - sp3, sp2 and sp.

sp3 hybridisation.

This is known as tetrahedral hybridisation. The four sp3 hybridised orbitals of carbon atom are directed to the four corners of a regular tetrahedron with an angle of 109*18' between two hybridised orbitals. The orbital has 25% s- character. eg:- methane, ethane.

sp2 hybridisation.

This is known as trigonal hybridisation. The three sp2 hybridised orbitals of carbon atom are directed to the corners of an equilateral triangle with an angle of 120*. The hybrid orbitals has 33.3% s- character. eg:- ethylene.

sp hybridisation.

This is known as diagonal hybridisation. The two sp hybridised orbitals of carbon atom are directed along a line with an angle of 180* to each other. The orbitals has 50% s- character. eg:- acetylene.

Bond length and bond energy are influenced by the type of hybridisation. As the s-character of the hybrid orbitals increases the electronegativity of the atom increases and hence the bond length decreases. Hence the bond energy increases. Therefore, sp hybrid orbitals has the shortest bond length with the highest bond energy.

NUCLEAR CHEMISTRY

Nuclear Chemistry is concerned with the study of atomic nuclei and the natural and artificial changes in it. In nuclear reactions, the elements are transformed into new elements and a large amount of energy is released.

An example of nuclear reactions are given below.
14N7 + 1n0 = 14C 6 + 1p1
This can be simplified as below.
14N7 (n, p) 14C6.

Natural Radioactivity.
This was discovered by the French scientist Henri Becquerel in 1896. All elements with atomic numbers higher than 83 is radioactive.
The spontaneous emission of radiations by atomic nuclei resulting in their disintegration is called natural radioactivity. There are three kinds of radiations:- alpha, beta and gamma.

• Alpha radiations are nuclei of helium atom. Its velocity is nearly 1/10th of light velocity. Of the three radiations it has the least penetrating power. It has the highest ionising power.
• Beta radiations consist of highly energetic electrons. Their speed is almost 90% that of light. Their penetrating power is almost 100 times that of alpha radiations. Their ionising power is nearly 1/100th of that of alpha radiation. They produce fluorescence on ZnS screen.
• Gamma radiations are electromagnetic radiations. Their penetrating power is immensely high. They have no ionising power. They produce weak fluorescence on ZnS screen.

Cause Of Radioactivity

An atom is radioactivity because of its unstable nucleus. In the nucleus there is a ratio between the neutrons and the protons, called the n/p ratio which determines the stability of the nucleus.

If there are too many neutrons, a neutron will be converted to a proton and an electron. The electron will be ejected as beta particle. If there are too many protons, either a helium nucleus is ejected or a positron is ejected.

Artificial Transmutation.

The conversion of one element into another is known as transmutation.

Artificial transmutation is carried out by bombarding an element with projectiles such as protons, neutrons, alpha particles etc. Neutrons are the best projectiles because of its neutral charge. It does not experience any repulsion from the nucleus. Accelerators such as cyclotrons are used to energise the projectiles.

Artificial Radioactivity.

The process of making a stable element radioactive by bombarding it with projectiles is called artificial radioactivity.

The new atoms formed may be stable or unstable.

27Al13 + 4He2 = 30P15 + 1n0

30P15 = 30Si14 + 0e1

Mass Defect and Binding Energy.

The difference between the sum of the masses of the nucleons and the actual mass of the nucleus is called the mass defect. The energy equivalent to mass defect is called binding energy. Higher the value of binding energy per nucleon, the more stable will be the nucleus.

ATOM

The fundamental unit of a chemical substance is called an atom. The word is derived from the Greek atomos, meaning ''uncuttable''. Atoms are extremely small neutral particles.

John Dalton in 1804 suggested the atomic theory of matter which indicates the existence of atoms. He suggested that atoms are extremely small indivisible particles. But atoms are no longer considered as indivisible. They are composed of subatomic particles like electrons, protons, neutrons, etc.

In an atom, a small, heavy nucleus is surrounded by a relatively large, light cloud of electrons. The nucleus consist of protons and neutrons which are called the nucleons. They are made up of even smaller particles. i.e; the elementary particles.

The elementary particles are fundamental objects which do not have a measurable internal structure. They are classified according to their spin as,

• FERMIONS (half integer spin)
• BOSONS (integer spin)

1. Fermions

They are particles with a half integral spin and anti-symmetric wave functions such as protons and electrons. They carry charge and spin angular momentum. They are classified as,

• QUARKS
• LEPTONS

Quarks

Quarks are one of the two basic constituents of matter. These are of six types:- up, down, top, bottom, charm, strange. The various types of quarks combine together in a specific way to form protons and neutrons.

Proton = 2 up quarks + 1 down quark

Neutron = 1 up quark + 2 down quarks

The antiparticles of quarks are known as anti quarks. Groups of quarks are called hadrons.

Gluons helps quarks to bind together and are indirectly responsible for binding of protons and neutrons in the atomic nucleus.

Leptons

It is the second basic constituent of matter which include the electrons and neutrino. Their respective antiparticles are known as anti leptons. They are of six types indicated below with their corresponding antiparticle.

• Electron and Positron
• Electron neutrino and Electron antineutrino
• Muon and Antimuon
• Muon neutrino and Muon antineutrino
• Tau lepton and Antitauon
• Tau neutrino and Tau antineutrino

2. Bosons

They are particles with integral spin and symmetric wave functions such as photons. They are classified as :

• Photon (spin=1)
• W boson (spin=1)
• Z boson (spin=1)
• Gluon (spin=1)
• Graviton (spin=2, existence not confirmed)
• Higgs boson (spin=0, existence not confirmed)

Mastering Chemistry.

A Chemistry student need to visualize chemical reactions at the molecular level. Chemistry is molecular but, at the same time it is quantitative. To master this subject, BE AN ACTIVE LEARNER.

• Ask questions
• Seek information of the slightest detail from many sources
• Indulge yourself in efficient study groups
• Prepare brief notes
• Write down all the equations for quick reference
• Work out many problems
• Imagine molecules
• Draw pictures to show reactions at the molecular level

When you have mastered these points, you will find it easy to learn Chemistry. It is definitly a UNIQUE SUBJECT.

Importance of Chemistry.

Chemistry is a developed branch of science that deals with the composition, structure, and properties of matter. These aspects are understood by studing the basic constituents of matter i.e; atoms and molecules.

Chemistry overlaps other branches of science like physics, biology, agriculture, medicine, pharmacy, geology, engineering, material science and nanotechnology. It is a mature science. Yet it is always presenting its exciting new discoveries, challenges and very influential applications.

Study of this field helps us to understand the behaviour of a substance, the cause of this behaviour, an application for this and so on.